Heat capacity of hydrogen-bonded networks: an alternative view of protein folding thermodynamics

Large changes in heat capacity (deltaCp) have long been regarded as the characteristic thermodynamic signature of hydrophobic interactions. However, similar effects arise quite generally in order-disorder transitions in homogeneous systems, particularly those comprising hydrogen-bonded networks, and this may have significance for our understanding of protein folding and other biomolecular processes. The positive deltaCp associated with unfolding of globular proteins in water, thought to be due to hydrophobic interactions, is also typical of the values found for the melting of crystalline solids, where the effect is greatest for the melting of polar compounds, including pure water. This suggests an alternative model of protein folding based on the thermodynamics of phase transitions in hydrogen-bonded networks. Folded proteins may be viewed as islands of cooperatively-ordered hydrogen-bonded structure, floating in an aqueous network of less-well-ordered H-bonds in which the degree of hydrogen bonding decreases with increasing temperature. The enthalpy of melting of the protein consequently increases with temperature. A simple algebraic model, based on the overall number of protein and solvent hydrogen bonds in folded and unfolded states, shows how deltaCp from this source could match the hydrophobic contribution. This confirms the growing view that the thermodynamics of protein folding, and other interactions in aqueous systems, are best described in terms of a mixture of polar and non-polar effects in which no one contribution is necessarily dominant.