Nature of collagen–water hydration forces: a problem in water structure

Abstract

The detailed make up of the total hydration force, F TOT, or its osmotic pressure equivalent, π, for the collagen–water system has been revealed by Raman spectroscopy, thermodynamics, and statistical mechanics. π is composed of three terms: (1) the pure water internal pressure, p i H 2O , (2) a collagen–water stochastic electric field term, p i FIELD, and (3) a thermal term, nRT ST/( V− V 0)= ρRT ST/MW H 2O . V− V 0 is the water volume, ρ is the density of pure water at the structural temperature, T ST, and MW H 2O is the molecular weight. An electric field modified, van der Waal’s type, equation of state results, namely, ( π+ p i ′) ( V− V 0)= nRT ST. p i ′ is the total internal pressure including the electric field, namely, p i ′≡ p i H 2O − p i FIELD. p i H 2O ≡(∂ E/∂ V) T =[ βT ST/ κ T − P], where β and κ T are the expansion coefficient and compressibility of pure water. The structural temperature, T ST, of the hydration water, was obtained from Raman OH-stretching component intensity ratios, K(T)≡(I 3250 cm −1 /I 3400 cm −1 ) , measured for collagen, but using the known T dependence of K( T) for pure water [1]. P= P external=1 atm. The electric field contribution, p i FIELD, to the total internal pressure, p i ′, comes from the negative V derivative of the stochastic electric field energy, U E. However, the sign of − p i H 2O , alone, determines the overall hydrophobicity or hydrophilicity; changing from (+), hydrophobic, positive force, for low T ST; to (−), hydrophilic, negative force, for high T ST; with increasing interaxial separation, X, in Å. T ST is ≈220–230 K for the driest collagen and the smallest X≈13 Å, implying, from Raman measurements of supercooled water [2], a dominance of pentagon-based, hydrophobic, clathrate-like rings of H-bonded water molecules in excellent agreement with X-ray measurements, although some hydrophilic interactions remain. T ST for the wettest collagen, X≈18 Å, is ≈296 K ( T i=293 K, is the constant experimental temperature) which signals the dominance of hydrophilic forces with high water content or large X. An excellent fit to the experimentally measured osmotic pressure was obtained for the collagen–water system using only well-known, short-range forces.