Solvation of Hydrophobic Amino Acid Side Chains and Peptide Backbone in Aqueous Glycine Betaine and Trimethylamine N-oxide Solutions

Abstract

There is a large class of small, water-soluble molecules broadly referred to as osmolytes that may stabilize or destabilize biomacromolecular structures. For example, trimethylamine N-oxide (TMAO) and glycine betaine are prominent stabilizers of proteins, while urea is a strong denaturant. Although these osmolytes have been employed in protein studies for more than 70 years, the mechanisms of their action are still largely unknown. One reason for this deficiency is the lack of direct thermodynamic data that can be used to quantify solute-solvent interactions versus solute-osmolyte interactions. In this work, we use high precision densimetry and ultrasonic velocimetry to examine the solvation properties of amino acid side chains and the peptide group in binary mixtures of water and TMAO or glycine betaine. Specifically, we report the partial molar volume, V°, and adiabatic compressibility, Ks°, of N-acetyl amino acid amides (alanine, valine, leucine, isoleucine, phenylalanine) and oligoglycines (Gly)1-5 in binary mixtures containing 0 to 4 M TMAO or glycine betaine. We use our volumetric results to evaluate the osmolyte-dependent group contributions of amino acid side chains and the peptide group. We analyze these osmolyte-dependent group numbers to evaluate the binding constants and elementary changes in volume and compressibility accompanying the replacement of water of hydration in the vicinity of the solutes with a TMAO or glycine betaine molecule. We compare these data with similar results previously obtained in our laboratory for the interactions of urea with protein groups. In general, we discuss the implications of our results for elucidating the mechanism of stabilization/destabilization of protein structures by osmolytes.