Solvation and conformational effects in aqueous solutions of biopolymer analogues.

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Biopolymer conformational transitions play a fundamental role in life processes. These transitions are triggered and controlled by subtle changes in the ‘solvent medium’ and are therefore likely to depend on the hydration states of the biopolymer and the soluble small molecule species involved. Three distinct types of hydration behaviour common to biopolymer systems are discussed: (1) Ionic hydration is of an electrostatic nature, i.e. long range and radial but specific effects exist in the manner in which water molecules are oriented about different ions and this may well be the origin of the various lyotropic series of ion specific effects which are so common in colloid, polymer and biochemistry. (2) The term ‘specific hydration’ is used to describe the direct interaction by hydrogen bonding between water and polar sites on organic molecules capable of acting as proton donors or acceptors. Since the hydrogen bond potential is very orientation specific, it follows that ‘specific hydration’ effects depend sensitively on the detailed stereochemistry of the molecular hydration sites, i.e. their distances of separation and mutual orientations. Thus in cases where molecules can exist in several anomeric or diastereoisomeric forms, or where different conformational states can arise by rotation about carbon-carbon bonds, specific hydration interactions may significantly affect the conformational energy minimum such that solvent dependent conformational states may exist. Similarly the solvent will influence the positions of the equilibria between various isomeric states, e.g. of sugars. (3) Hydrophobic hydration probably arises from the reorientation of water molecules in the vicinity of an apolar molecule or residue, such that the OH vectors are not directed towards the apolar moiety. This is an entropically unfavourable process which can be partly reversed by the association of two (or more) such hydrophobically hydrated residues. In this way some of the perturbed water can relax back to its normal bulk state in which more molecular orientations are possible. A reassessment of the hydrophobic interaction shows, however, that contrary to currently held views, the potential well due to two alkyl groups in aqueous solution is shallower than it is for two similar groupsin vacuo, and also that the entropy gain from the pair interaction is not as large as has been believed. These findings necessitate a reappraisal of the molecular details of the hydrophobic interaction.