Why is hydrophobic effect important

The image above indicates that when the hydrophobes come together, they will have less contact with water. They interact with a total of 16 water molecules before they come together and only 10 atoms after they interact. When a hydrophobe is dropped in an aqueous medium, hydrogen bonds between water molecules will be broken to make room for the hydrophobe; however, water molecules do not react with hydrophobe.

This is considered an endothermic reaction, because when bonds are broken heat is put into the system. Water molecules that are distorted by the presence of the hydrophobe will make new hydrogen bonds and form an ice-like cage structure called a clathrate cage around the hydrophobe.

According to the Gibbs Energy formula. The mixing hydrophobes and water molecules is not spontaneous; however, hydrophobic interactions between hydrophobes are spontaneous. Hydrophobic interactions are relatively stronger than other weak intermolecular forces i. In this case, the anion and cation were both found to be enriched in the collapsed state of the polymer. At higher salt concns.

Finally, in a third case, it was found that salts which interacted in an intermediate fashion with the polymer e. These results provide a detailed, mol. It also helps explain the circumstances under which guanidinium salts can act as powerful and versatile protein denaturants.

Okur, Halil I. Ions differ in their ability to salt out proteins from soln. Although it has been clear that the Hofmeister series is intimately connected to ion hydration in homogeneous and heterogeneous environments and to ion pairing, its mol. This situation could have been summarized as follows: Many chemists used the Hofmeister series as a mantra to put a label on ion specific behavior in various environments, rather than to reach a mol.

In this Feature Article the cationic and anionic Hofmeister series can now be rationalized primarily in terms of specific interactions of salt ions with the backbone and charged side chain groups at the protein surface in soln. At the same time, the authors demonstrate the limitations of sepg.

Hofmeister effects into independent cationic and anionic contributions due to the electroneutrality condition, as well as specific ion pairing, leading to interactions of ions of opposite polarity. Finally, the authors outline the route beyond Hofmeister chem. National Academy of Sciences. We report exptl. All three osmolytes stabilize collapsed conformations of the ELP and reduce the lower crit. LSCT linearly with osmolyte concn.

As expected from conventional preferential solvation arguments, betaine and glycine both increase the surface tension at the air-water interface. TMAO, however, reduces the surface tension. Atomically detailed mol. To investigate alternative mechanisms for osmolyte effects, we performed FTIR expts. These expts. Glycine also caused a red shift in the OH stretch region, whereas betaine minimally impacted this region. Thus, the effects of osmolytes on the OH spectrum appear uncorrelated with their effects upon hydrophobic collapse.

Similarly, MD simulations suggested that TMAO disrupts the water structure to the least extent, whereas glycine exerts the greatest influence on the water structure. These results suggest that TMAO stabilizes collapsed conformations via a mechanism that is distinct from glycine and betaine. In particular, we propose that TMAO stabilizes proteins by acting as a surfactant for the heterogeneous surfaces of folded proteins. Paterova, Jana; Rembert, Kelvin B. Ion-specific effects on salting-in and salting-out of proteins, protein denaturation, as well as enzymic activity are typically rationalized in terms of the Hofmeister series.

Here, we demonstrate by means of NMR spectroscopy and mol. Using triglycine as a model system, we show that the Hofmeister series for anions changes from a direct to a reversed series upon uncapping the N-terminus. Weakly hydrated anions, such as iodide and thiocyanate, interact with the peptide bond, while strongly hydrated anions like sulfate are repelled from it.

In contrast, reversed order in interactions of anions is obsd. These results demonstrate that the specific chem. The present study thus provides a mol. It also provides a route for tuning these interactions by titrn.

Differential scanning calorimetry was employed to investigate the temp. The different salts affected the phase sepn. It was further obsd. The order of how the salts affect the phase sepn. These observations are discussed in relation to existing models of how the different nature of the ion and polymer hydration can lead to effective attractive and repulsive ion-polymer interactions depending of the exact chem. It is suggested that the previous confusion about the Hofmeister effect is due to a misleading conceptual picture of how polymer hydration is affected by the presence of ions.

It is concluded that the Hofmeister effects, in the present case, can be described by a balance between the effective interactions governed by the asym. At room temp. In water, I solns. I soln. In solns. The lower crit. MeOH via cloud-point and microcalorimetric measurements. The soln. Flory-Huggins ternary soln. Colloid Interface Sci. Elsevier Ltd.

The different explanations for the cononsolvency available in the literature are introduced. Crosslinker d. Microgels provide advantages to study cononsolvency by en'abling a broader exptl. Furthermore, multi-sensitive microgels can be prepd. A primary thermodn. Through use of recently detd. Conformational transitions of bio macromols. Though computer simulations are ideally suited to investigate such phenomena, in conventional setups the excess of one cosolvent close to the solute leads to depletion elsewhere, requiring very large simulation domains to avoid system size effects.

We, here, propose an approach to overcome this depletion effect, which combines the adaptive resoln. In AdResS, a small all-atom region, contg.

The particle exchange would be almost impossible had they been performed in an all-atom setup of a dense mol. As a first application of the method, we study the concn.

Nature Communications , 5 , pp. Water and alc. The intriguing behavior of solvent mixts. It is a widespread phenomenon, relevant for many formulation steps in the physicochem. Here, by using a combination of simulations and theory, we presented a simple and universal treatment that requires only the preferential interaction of one of the cosolvents with the polymer.

The results showed striking quant. Stabilization of macromol. However recent theor. Based on Mol. Dynamics simulations we have characterized the mechanism through which urea stabilizes the collapsed state of PNiPAM in water. We instead find that with increasing urea, solvation of the unfolded state is entropically penalized over solvation of the folded state, thereby shifting the folding equil. The unfavorable entropy contribution to the excess chem. These energy fluctuations are particularly relevant at low urea concns.

We further find indications that urea increases the entropy of the globular state. Elsevier B. The expts. The conformation change of the hydrogel is caused by a conformation change of the single polymer chains in its backbone. The latter was studied by massively parallel mol. In the simulations the mean radius of gyration of the polymer chains was monitored. When the temp.

We performed mol. We have reproduced recent measurements of the transition temp. The constraints of the dihedral dynamics require elevated temps. This result is confirmed by calcns. We present mol. The simulation trajectories exhibit the reversible conformational transitions between swollen and collapsed chain conformations, which has rarely been reported in previous simulation studies, with the overall transition occurring at different temps.

The inconsistency of the transition temps. Instead of evaluating av. The simulation trajectories are analyzed in terms of the radius of gyration, intrachain distances, hydrophobic contacts, and chain-water and intrachain hydrogen bonding. In particular, the formation of stable intrachain hydrogen bonds is a signature of the tightly collapsed chain conformations that persist, once formed, for the entire simulation duration.

LCST at K and a conformational transition of single chains at the same temp. The results of many simulation studies suggest that std. We show by performing long mol. The results show further that the relaxation times of single-chain simulations are considerably longer than anticipated. Conformational transitions of single polymers can therefore not necessarily be used as surrogates for a real phase transition.

Thermoresponsive polymer architectures have become integral building blocks of 'smart' functional materials in modern applications. For a large range of developments, e. In order to gain insights into the nanoscale structure and binding details in such systems, we here employ mol. We study a single polymer chain and explore the influence of its elongation, stereochem.

While we find that the excess adsorption generally increases with the size of the solute, the temp. We elucidate the mol. We find that the preferential binding of methanol with PNIPAm side chains, bridging distal monomers along the polymer backbone, results in increased organization. Our findings reveal that the collapse of PNIPAm is dominated by enthalpic interactions and that the std.

Though urea is commonly used to denature proteins, the mol. Previous mol. The simulations capture the exptl. We find that the denaturation is driven by favorable direct interaction of urea with the protein through both electrostatic and van der Waals forces and quantify their contribution. Though the magnitude of direct electrostatic interaction of urea is larger than van der Waals, the difference between unfolded and folded ensembles is dominated by the van der Waals interaction.

We also find that hydrogen bonding of urea to the peptide backbone does not play a dominant role in denaturation. The unfolded ensemble sampled depends on urea concn. The m-value is predicted to increase with temp. Cosolvent Effects on Protein Stability Annu. Cosolvents that shift the equil. Urea is a widely used denaturant in protein folding studies, and the mol.

Here we review recent exptl. Urea has been shown to denature proteins through a direct mechanism, by interacting favorably with the peptide backbone as well as the amino acid side chains. In contrast, the mol. Recent studies have established the strong interaction of TMAO with water. Detailed mol. We present the development of a model for TMAO that is consistent with exptl.

Osmolytes affect hydrophobic collapse and protein folding equil. The underlying mechanisms are, however, not well understood. We report large-scale conformational sampling of two hydrophobic polymers with secondary and tertiary amide side chains using extensive mol.

The underlying mechanism is rooted in opposing entropic driving forces: while urea screens the hydrophobic macromol.

Only at sufficiently high urea concns. The observations provide a new angle on the interplay between side chain chem. It is found that urea influences the phase transition of the hydrogels in opposite ways: lowering the lower crit.

The self-diffusion coeff. Moreover, the enhanced pos. Solution Chem. Underlying assumptions were examd. As a result, it was possible to improve upon the R. Pierotti corresponding anal. Potential energy nonadditivity should create an orientational bias for mols.

Some specific conclusions were drawn about the solvation mode for the nonpolar rigid-sphere solute. Theory of the Hydrophobic Effect J. A microscopic theory is developed which can describe many of the structural and thermodn.

The theory is based on an integral equation for the pair correlation functions assocd. It requires as input the exptl. O-O correlation function for pure liq. The theory is tested by computing thermodn. The predictions of both the Henry's Law const. The calcn. The correlation functions can be used to predict the soly.

The calcns. The effects of solvent environments on the conformations of small chain mols. Hydrocarbon solvents as well as water tend to reduce the spatial extension of the chain mols. However, calcns. A mol. Because primitive hydrophobic effects can be tied to the probability of observing a mol. The modeled distribution then yields the probability that no solvent centers are found in the cavity vol. This model is shown to account quant.

The connection of information theory to statistical thermodn. The simplicity and flexibility of the approach suggest that it should permit applications to conformational equil. American Physical Society. Hydrophobic hydration plays a crucial role in self-assembly processes over multiple length scales, from the microscopic origins of inert gas soly.

Many theor. This Colloquium revisits the scaled particle theory proposed 30 years ago by Stillinger, adopts a practical generalization, and considers the implications for hydrophobic hydration in light of current understanding. The generalization is based upon identifying a mol. It will be demonstrated that the generalized theory correctly reproduces many of the anomalous thermodn. The model considered here serves as a ref. The results are discussed in terms of length scales assocd.

In particular, 1st there is a discussion of the microscopic-macroscopic joining radius identified by the theory; then follows a discussion of the Tolman length that describes curvature corrections to a surface area model of hydrophobic hydration free energies and the length scales on which entropy convergence of hydration free energies are expected.

We develop a unified and generally applicable theory of solvation of small and large apolar species in water. In the former, hydrogen bonding of water is hindered yet persists near the solutes. In the latter, hydrogen bonding is depleted, leading to drying of extended apolar surfaces, large forces of attraction, and hysteresis on mesoscopic length scales.

The crossover occurs on nanometer length scales, when the local concn. Our theory for the crossover has implications concerning the stability of protein assemblies and protein folding. We have calcd. These results were obtained using umbrella sampling of ensembles with fixed, ambient temp. For the same water models, we have also calcd. Analogous calcns. For both water and the LJ fluid at the conditions we have simulated, extrapolation of our results suggests that the planar interface between coexisting liq.

We expect this correspondence to be a general result for fluids at thermodn. The solvation free energies we have computed for water and the LJ fluid cross over at microscopic solute sizes from a dependence on solute vol.

B , , ]. The authors have studied the effect of weak solute-solvent attractions on the solvation of nonpolar mols. With a reasonable strength of alkane-H2O interactions, an accurate prediction of the alkane-H2O interfacial tension is obtained.

As previously established for solutes with no attractive interactions with H2O, the free energy of solvation scales with vol.

The crossover to the latter regime occurs on a mol. It is assocd. In the absence of attractions, this interface typically lies more than one solvent mol. With the addn. For attractive force strengths typical of alkane-H2O interactions, the drying interface adjacent to a large hydrophobic solute remains largely intact, but is moved into contact with the solute surface.

This effect results from the soft modes characterizing fluctuations of liq. Attractive interactions are of almost no consequence to the temp. We study the static and dynamic properties of the water-d. With the help of extensive mol. For purely repulsive solutes, the squared width of the interface increases linearly with the logarithm of the solute size, as predicted by capillary-wave theory.

The apparent interfacial tension extd. The characteristic length of local d. Probed locally, the interfacial d. These fluctuations result in transitions between locally wet and dry regions that are slow on a mol. We probe the effects of solute length scale, attractions, and hydrostatic pressure on hydrophobic hydration shells using extensive mol.

The hydration shell compressibility and water fluctuations both display a nonmonotonic dependence on solute size, with a min. These results and calcns.

More importantly, the nonmonotonicity implies a solute curvature-dependent pressure sensitivity for interactions between hydrophobic solutes. Hydrophobicity is often characterized macroscopically by the droplet contact angle.

Successful theories predict a drying transition leading to a vapor-like region near large hard-sphere solutes and interfaces. Adding attractions wets the interface with local d. Here the authors present extensive mol. The water d. But the probability of cavity formation or the free energy of binding of hydrophobic solutes to interfaces correlates quant.

Specifically, the probability of cavity formation is enhanced in the vicinity of hydrophobic surfaces, and water-water correlations correspondingly display characteristics similar to those near a vapor-liq. Hydrophilic surfaces suppress cavity formation and reduce the water-water correlation length. The results suggest a potentially robust approach for characterizing hydrophobicity of more complex and heterogeneous surfaces of proteins and biomols. Approaches to quantify wetting at the macroscale do not translate to the nanoscale, highlighting the need for new methods for characterizing hydrophobicity at the small scale.

We use extensive mol. For homogeneous SAMs, new pressure-dependent anal. Heterogeneous surfaces show an interesting context dependence - adding a single -OH group in a -CH3 terminated SAM has a more dramatic effect on water in the vicinity compared to that of a single -CH3 group in an -OH background.

We also present preliminary results to map hydrophobicity of protein surfaces by monitoring local d. These mol. At least for one protein, hydrophobin-II, we show that the hydrophobicity map is different from that suggested by a commonly used hydropathy scale. Our results are obtained with a biased sampling of coarse-grained densities that is easily combined with mol. Our principal result is that the probability for d. Specifically, the probability of d.

In contrast, we find that the statistics of water d. Patel, Amish J. Interfaces are a most common motif in complex systems. To understand how the presence of interfaces affects hydrophobic phenomena, we use mol. The solutes range in size from subnanometer to a few nanometers. The interfaces are self-assembled monolayers with a range of chemistries, from hydrophilic to hydrophobic. We show that the driving force for assembly in the vicinity of a hydrophobic surface is weaker than that in bulk water and decreases with increasing temp.

We explain these distinct features in terms bf 'an interplay between interfacial fluctuations and excluded vol. Our results suggest a catalytic role for hydrophobic interfaces in the unfolding of proteins, for example, in the interior of chaperonins and in amyloid formation.

We present a coarse-grained lattice model of solvation thermodn. Through comparison with mol. Our model is intermediate in detail and complexity between implicit-solvent models and explicit-water simulations. Macroscopic characterizations of hydrophobicity e. Theory and state-of-the-art simulations suggest that water d. Fluctuations affect conformational equil. Enhanced fluctuations are reflected in enhanced cavity formation, more favorable binding of hydrophobic solutes, increased compressibility of hydration water, and enhanced water-water correlations at hydrophobic surfaces.

These d. They highlight that the hydrophobicity of a group is context dependent and is significantly affected by its environment e. The ability to include information about hydration water in mapping hydrophobicity is expected to significantly impact our understanding of protein-protein interactions as well as improve drug design and discovery methods and biosepn.

Density Fluctuations in Liquid Water Phys. The d. It is found that the occurrence of low- and high-d. The spatial extent of d.

Water near extended hydrophobic surfaces is like that at a liq. Here we use mol. Consequently, water near these surfaces is sensitive to subtle changes in surface conformation, topol. Our work also resolves the long-standing puzzle of why some biol. Dissecting Hydrophobic Hydration and Association J. The authors use appropriately defined short-ranged ref.

Water tends to form ordered cages around the non-polar molecule and this leads to a decrease in entropy. The enthalpy of transfer, however, is now positive unfavourable. Because the temperature dependence of entropy and enthalpy are not the same, there is some temperature at which the hydrophobic effect is strongest, and the effect decreases at temperatures above and below this temperature. The decrease in the strength of the hydrophobic effect with decreasing temperatures is probably the major cause of cold-denaturation in proteins.

The contribution of the hydrophobic effect to globular protein stability has been estimated empirically both by measuring the thermodynamics of transfer of model compounds e. The number arrived at is usually given as a function of the change in the solvent accessible non-polar surface area upon going from the unfolded to the folded state.

The model compound studies predict that the hydrophobic effect of exposing one buried methylene group to bulk water is 0. Addressing this question requires an accurate characterization of the conformational space of a protein in water to study the number of hydrogen bonds formed by water molecules in proximity of its surface with respect to the bulk. To attack this problem, we exploited the opportunities offered by the case of yeast frataxin Fig. As shown by the pioneering work of Privalov, the thermodynamic analysis of cold denaturation, compared to that of the thermal denaturation, offers unique advantages for understanding the molecular determinants of hydrophobicity 19 , Indeed, it is generally recognised that while thermal denaturation is the consequence of a temperature-induced increase in conformational fluctuations, cold denaturation is a consequence of an enthalpy gain of the solvent 19 , Not many atomistic details are known, however, about the structural consequences of this gain of entropy or enthalpy in the hot and cold denatured states, respectively.

Thus, modelling the cold and hot denatured states of yeast frataxin in the absence of any additional agent should highlight the different roles played by the protein and solvent at different temperatures. Nine microstates are shown representing the local and global minima. Sixteen microstates are shown representing the local and global minima. In the following we employ the chemical shifts measured in cold and hot denaturation conditions 15 , 16 , 17 together with replica-averaged metadynamics RAM simulations 21 , 22 see Materials and Methods to elucidate at atomic resolution the structure and the dynamics of the cold denatured state CDS and hot denatured state HDS.

In the RAM simulations the experimental information provided by NMR chemical shifts is incorporated in terms of structural restraints 22 and at the same time the sampling of the conformational space is enhanced by metadynamics This overall approach makes it possible to simultaneously change the force field used in the molecular dynamics simulations to improve their agreement with the experimental data in the spirit of the maximum entropy principle 24 and to significantly decrease the computational resources required to obtain convergence in the sampling.

From the analysis of the RAM simulations at convergence Fig. S1 we conclude that the two denatured states exhibit qualitatively different behaviours, as the regions of conformational space that they sample are essentially non-overlapping Fig. The two ensembles are also different in terms of their radii of gyration R g , with 1. The average fraction of native contacts 29 , Q, is 0. To investigate these differences, we first calculated the number of hydrogen bonds in the three states.

In increasing the temperature, bulk water molecules BWM form decreasing numbers of hydrogen bonds 31 , 32 , 33 , in the current case by going from 3. Remarkably, these numbers are rather well preserved also for water molecules in the first shell around the protein interface water molecules, IWM, Fig. In this interface region the number of water-water hydrogen bonds per molecule drops Fig. In essence, while the overall number of hydrogen bonds formed by a water molecule is relatively constant for water molecules in the bulk and at the interface 10 , the protein itself respond to the change in this number with temperature by redistributing its protein-protein Fig.

Our results indicate that water forms 83 such hydrogen bonds with the protein, thus stabilising its CDS Fig. The number of hydrogen bonds is calculated for bulk water molecules BWM, farther than 0.

In our calculations, we found that bulk and interface water molecules form the same number of hydrogen bonds at the three temperatures. Two hydrogen bonds formed by a hydrophobic residue are shown in particular, for which only the backbone amide and carbonyl groups can form hydrogen bonds; these two water-protein hydrogen bonds are highlighted in blue.

The number of protein-protein hydrogen bonds is shown in yellow, and that of protein-water hydrogen bonds in blue. The degree of order of the water molecules near the protein correspond to the extent to which they form hydrogen bonds with the protein, which is large at cold denaturing conditions, small under folding conditions, and intermediate at hot denaturing conditions.

We obtained additional insight into the factors that determine the cold and hot denaturation processes by calculating the van der Waals and the Coulomb energies of the protein-water system Fig. While the protein-protein energy is minimized in the folded state, the protein-water energy is strengthened under cold denaturation conditions, and is slightly weakened in hot denaturation conditions with respect to the NS.

These results suggest that the CDS is stabilized by interactions with the solvent and as a consequence the protein is more expanded. In the NS the balance of the interactions is moved towards the protein-protein interactions and indeed the protein is structured.

Finally the HDS exhibits an intermediate behaviour where the protein conserves some residual structure and remains more compact. A qualitative analysis of the role entropy is obtained by analysing the average orientations of water molecules with respect to the protein as a function of their distances from the protein surface in the CDS, the HDS and the NS Fig.

A value of zero indicates that on average a water molecule at a given distance does not have any preferred orientation with respect to the protein and so its rotational entropy is the same as in bulk water. Positive and negative values report about an anisotropic distribution of the solvent with respect to the protein and correspond to an entropy loss with respect to the bulk; these curves take also into account the number of water molecules at a given distance Fig.

Up to 0. This distance corresponds to the first two shells of water around the protein Fig. In the denatured states this behaviour is slightly more pronounced than in the NS, due to the more exposed surfaces and as a consequence the larger number of water molecules that are oriented.

The integral of the absolute value of the curves of Fig. The NS shows the least water entropy loss, indicating that in the native state the hydrophobic forces, here interpreted essentially in terms of loss of rotational freedom, are minimized.

The structural analysis described above characterizes the entropic and energetic contributions to the folding process of frataxin as a function of the temperature Fig. At the same time, the protein unfolds by gaining energy by interacting more strongly with the solvent at the expense of protein-protein interactions Fig. In parallel, the protein unfolds, losing energy both due to loss of protein-protein and protein-water interactions and gaining entropy by accessing a larger conformational space Fig.

It is also remarkable that both at low and at high temperatures the number of hydrogen bonds per surface water molecule does not depend on the free energy of the conformation in the ensemble Fig. S6 , although the number of hydrogen bonds depend on the identity of the amino acid and to the degree of solvent exposure Figs S7—S9. This result is particularly important because it shows that the number of number of hydrogen bonds per surface water molecule is a property robust with respect to possible inaccuracies in the simulations, as for example a slightly overestimated compactness of the protein.

While these results are consistent with previous observations about the relevance of protein-water interactions in driving cold denaturation 9 , 10 , 16 , 34 , 35 , here we show explicitly that the number of hydrogen bonds is the same for bulk and interface water molecules at all the relevant temperatures.

This phenomenon could determine the properties of the folded and unfolded states, as also recently suggested using a two-dimensional model The differences observed for the residual structure of the two denatured state ensembles together with the differences observed in the protein-water interactions offer the opportunity to investigate the consequences of the denaturing conditions on the protein folding process.

S10 show that the hot and cold denaturation pathways are characterized by two structurally different transition states, which provide a structural perspective on the folding reaction. Also, the fraction of native contacts Q is 0. The structural ensembles shown correspond the most populated microstates in the two denatured states, the two transition states and the native state. The contact map of the cold transition state upper half and the cold denatured states lower half is shown on the left, the contact map for the native state in the middle, and the contact map of the hot transition state upper half and the hot denatured states lower half on the right.

At low temperature the data are more linearly correlated than at high temperature, corresponding to a higher degree of structure in the cold transition state than in the hot transition state, as reported by the corresponding values of Q insets.

The anti-Hammond effect occurs when the transition state moves towards the product as the reactant becomes more stable. To provide a structural rationalisation of this behaviour, it may be considered that, because the CDS is very heterogeneous, the free energy well associated with it is shallower than that of the HDS as a function of Q Fig. Thus, the intersection between the free energy wells of the native and denatured states, which identifies the transition state of the reaction, may shift towards the native conformation, resulting in an apparent anti-Hammond effect.

The increase of residual structure in the denatured state from the CDS to the HTS shifts the folding mechanisms from nucleation-condensation 38 where the protein folds as a single cooperative unit to diffusion-collision 39 where the protein folds in contiguous blocks.

At low temperature, with a weakly structured CDS, there is a strong correlation 0. The results that we have presented provide an atomistic representation of the seminal observations by Privalov about the favourable hydration of protein upon cold denaturation 19 , We have shown how a major role in determining the state of yeast frataxin at different temperatures is played by the competition between water and protein molecules for forming hydrogen bonds with the protein hydrogen bonding donors and acceptors.

Our results indicate that this protein interacts differently with water molecules at low and high temperatures, resulting in a more expanded and less structured denatured state at low temperatures, and in a more compact and structured denatured state at high temperatures.

Upon increasing the temperature from cold to native conditions, as the number of protein-water hydrogen bonds decreases, the protein can form stable secondary and tertiary structures. Then, when the temperature is raised from native to hot denaturing conditions, thermal fluctuations destabilise the native fold, but at the same time a further decrease in the number of protein-water hydrogen bonds results in the conservation of many of secondary structure elements and in a relatively compact denatured state.

These observations may also explain the finding of an increase in compactness and secondary structure content for denatured and for intrinsically disordered proteins upon increasing temperatures 30 , 40 , Since intrinsically disordered proteins are richer in polar groups than globular proteins, by increasing the temperature, the loss of protein-water hydrogen bonds is rescued by protein-protein hydrogen bonds resulting in more compact conformations than those observed by standard proteins of similar length 30 , 40 , In summary, we have shown that the analysis of the hydrogen bonds between water and protein molecules can provide a detailed characterisation of the hydrophobic effect in protein folding.

Our results suggest that proteins behave rather closely according to the classic description of small hydrophobic particles 1 , 42 , 43 , 44 , 45 , as we show that interface water molecules do not lose hydrogen bonds with respect to bulk water Fig.

As a consequence of this result, as the temperature is varied the conformation of the protein should adapt to this requirement which causes it to unfold both a low and at high temperatures. Molecular dynamics simulations of yeast frataxin were performed using the Amber03W force field 46 with the TIP4P05 water model Van der Waals and Coulomb interactions were implemented with a cutoff at 0. All simulations were carried out in the canonical ensemble at constant volume and by thermosetting the system using a stochastic velocity rescaling RAM simulations 21 , 22 were performed using chemical shifts BMRB depositions and as replica-averaged restraints 21 , 22 and bias-exchange metadynamics 54 following a protocol recently introduced to sample the denatured state of ACBP