Chiral Configuration of the Hydration Layers of D- and L-Alanine in Water Implied from Dilution Calorimetry

The size and configuration of the hydration layer of solutes play a major role in their thermodynamic features. With respect to amino acids in water, a series of indirect evidence strongly suggest that their hydration layer acquires a chiral configuration induced by their chiral centers. Such a chiral hydration may act as a recognition factor in the various biochemical interactions, but information on it remains rather scarce. In this study, we determined by dilution microcalorimetry the fraction of the hydration energy invested in the chiral distortion of the hydration layer surrounding D- and I-alanine in water. The results indicate that in dilute solutions, a multilayered chiral hydration surrounds each of these solutes and amounts to over 100 water molecules. In concentrated solutions, the immediate chiral hydration layer decreases to similar to 30 water molecules. The energy invested in the induction of the chiral twist in the hydration layer is predominantly attributed to T Delta S, the energy associated with "configurational entropy,'' which amounts to only several cal/mol, about a thousandth of the total energy of the hydration shell. Chirality 22:587-592, 2010. (C) 2009 Wiley-Liss, Inc.