Configurationally-Coupled Protonation of Polyproline-7.

Structure and dynamics regulate protein function, but much less is known about how biomolecule-solvent interactions affect the structure-function relationship. Even less is known about the thermodynamics of biomolecule-solvent interactions and how such interactions influence conformational entropy. When transferred from propanol into 40:60 propanol:water under acidic conditions, a remarkably slow protonation reaction coupled with the conversion of the polyproline-I helix (PPI, having all cis-configured peptide bonds) into polyproline-II (PPII, all trans) helix is observed in this work. Kinetics and equilibrium measurements as a function of temperature allow determination of the thermochemistry and insight into how proton transfer is regulated in this system. For the proton-transfer process, PPI(+)(PrOH) + H3O(+) → PPII(2+)(PrOH/aq) + H2O, we determine ΔG = -20 ± 19 kJ·mol(-1), ΔH = -75 ± 14 kJ·mol(-1), and ΔS= -188 ± 48 J·mol(-1)·K(-1) for the overall reaction, and values of ΔG(⧧) = 91 ± 3 kJ·mol(-1), ΔH(⧧) = 84 ± 9 kJ·mol(-1), and ΔS(⧧) = -23 ± 31 J·mol(-1)·K(-1) for the transition state. For a minor process, PPI(+)(PrOH) → PPII(+)(PrOH/aq) without protonation, we determine ΔG = -9 ± 20 kJ·mol(-1), ΔH = 64 ± 14 kJ·mol(-1), and ΔS= 247 ± 50 J·mol(-1)·K(-1). This thermochemistry yields ΔG = -10 ± 29 kJ·mol(-1), ΔH = -139 ± 20 kJ·mol(-1), and ΔS= -435 ± 70 J·mol(-1)·K(-1) for PPII(+)(PrOH/aq) + H3O(+) → PPII(2+)(PrOH/aq) +H2O. The extraordinarily slow proton transfer appears to be an outcome of configurational coupling through a PPI-like transition state.