Structure and energetics of the hydrogen-bonded backbone in protein folding.

We seek to understand the link between protein thermodynamics and protein structure in molecular detail. A classical approach to this problem involves assessing changes in protein stability resulting from added cosolvents. Under any given conditions, protein molecules in aqueous buffer are in equilibrium between unfolded and folded states, U(nfolded) <==> N(ative). Addition of organic osmolytes, small uncharged compounds found throughout nature, shift this equilibrium. Urea, a denaturing osmolyte, shifts the equilibrium toward U; trimethylamine N-oxide (TMAO), a protecting osmolyte, shifts the equilibrium toward N. Using the Tanford Transfer Model, the thermodynamic response to many such osmolytes has been dissected into groupwise free energy contributions. It is found that the energetics involving backbone hydrogen bonding controls these shifts in protein stability almost entirely, with osmolyte cosolvents simply dialing between solvent-backbone versus backbone-backbone hydrogen bonds, as a function of solvent quality. This reciprocal relationship establishes the essential link between protein thermodynamics and the protein's hydrogen-bonded backbone structure.