Discolsing the role of hydrophobic interactions through aggregation of non-polar solutes in water: a molecular dynamics study

CHARA OSVALDO

Perugia

Congreso; Frontiers in Water Biophysics 2012; 2012

Although it is well known that water dynamics is crucial for protein folding, itsprecise contribution for the stability of the protein is still unknown. What iswell known is that the structure and dynamics of water constitute the basis ofthe so-call hydrophobic interactions. These interactions are not onlyenthalpically-driven by the hydrophobic residues of the protein but alsoentropically-driven by the water molecules in the neighbourhood of theprotein. The complexity derived from the important number of interactionsbetween protein residues and the water molecules induces additionaldifficulty to disclose the contribution of these interactions in the stability of aprotein. This is why we studied hydrophobic interactions in a simple systemcomposed only by a non-polar solute, Lennard-Jones (LJ) particles, andwater SPC/E [1] under different pressure and temperature conditions bymeans of molecular dynamics simulations.Previously we demonstrated that water is structurally altered by ahydrophobic surface even in a distances greater than the thickness of theplasma membrane (80 Å) at different temperatures [2]. It is commonlyaccepted that liquid water structure can be understood in terms of twoclosely interweaved components: a tetrahedral and hexagonal structures,characterized by a low density and high density arrangement. Based on thecalculus of radial distribution function and a definition of a new orderparameter we determined a crossover point in structural dominance [3]. Atroom temperature this point in the range of 1-2 kbar, pressure at which mostof the ‘anomalous’ properties of water vanish.We here show that 1) temperature triggers the aggregation of non-polarsolutes in water within a certain range and 2) pressure inducesdisaggregation of the clusters. An equilibrium curve defining the coexistencebetween the non-agregated (soluble) and aggregated (insoluble) binarysystem of non-polar solutes and water was extracted. The critical pressureobtained from this curve is 986 bar. The results are in agreement with previous experimental reports describing hydrocarbon in water mixturestransitions as well as protein unfolding.We concluded that protein unfolding induced by pressure augmentationcould be explained by the reduction in the number of hydrophobicinteractions determined by the water hydrogen-bonds network distortion.References1. H. J. C. Berendsen, J. R. Grigera and T. P. Straatsma, J. Phys. Chem. 91,6269–6271 (1987).2. O. Chara, A. N. McCarthy, C. G. Ferrara, E. R. Caffarena and J. R.Grigera, Physica A, 388, 4551-4559 (2009).3. O. Chara, A. N. McCarthy and J. R. Grigera, Phys. Lett. A. 375, 572–576(2011).