Validation of the "Stoichiometric Hydration Ice-Bridge Model" Provides Method To Predict Protein Folding Energetics

The hydration of macromolecules plays a critical role in powering functional macromolecular shape changes to open/close cellular pores, clip protein chains, and perform a myriad of other functions critical to life. This study of collagen "bound water" offers insight into these processes by quantifying protein hydration energetics powered by ice-bridge formation on protein backbones. Quantification of ice-bridge properties on rat tail tendon is uniquely possible because the elevated stability of mammalian collagen at laboratory temperature provides 100% at-rest in vitro occupancy of backbone tripeptide units serving as ice nucleation sites. Full occupancy p_o=100% provides stoichiometric hydration ice-bridge counts to calculate enthalpy using known properties of the collagen molecule and bulk water-ice. SHIM theory predicts the specific enthalpy of native collagen melting, ΔH_m = 70.31 J/g-collagen, in close agreement with experimental measures. This provides, for the first time, a molecular definition of protein-bound water. We used measures of peak temperature of melting T_m to calculate specific entropy of melting ΔS_m = ΔH_m/T_m. These measurements show 100% of collagen entropy of melting ΔS_m = 0.2084 J/g_c°K in native collagen results from restrictions of the first monolayer water mobility. Penetration of acetate ion from acetic acid used by many to obtain independent tropocollagen molecules increased ΔS_m = 0.2251 J/g_c°K by altering the molecular first monolayer to form a hydration slurry. The somewhat speculative character of this first paper on collagen ice-bridges has been validated by successful extension of SHIM theory to ectotherm collagen and globular proteins, where interesting changes of partial occupancy, p_o<100%, manipulate functional protein conformations. These papers will be published shortly. Significance. This study of the energetics of water interactions with collagen provides the calculational foundation to quantify water interactions with macromolecules in general. Preliminary studies of globular proteins, DNA, RNA, and cellulose show similar but incomplete ice-bridge formation that makes bridge-quantification on those molecules more difficult. The maximized content of ice-bridges on the ice-nucleation sites of mammalian collagen appears to be unique. The melting and refreezing of ice-bridges on macromolecular backbones provide the molecular mechanism to convert thermal energy into shape change-induced mechanical movements. The molecular constants and associated calculational methodology developed here should allow for more rapid progress in understanding the response of macromolecules to water and important cosolutes such as glucose. The role of glucose in collagen hydration slurry formation clearly plays an important role in progressive organ changes observed in patients with diabetes. This ensures that both basic science and clinical science will find uses for SHIM theory.