Configurationally-Coupled Protonation of Polyproline-7.
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abstract
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 G = -20 19 kJmol(-1), H = -75 14 kJmol(-1), and S= -188 48 Jmol(-1)K(-1) for the overall reaction, and values of G() = 91 3 kJmol(-1), H() = 84 9 kJmol(-1), and S() = -23 31 Jmol(-1)K(-1) for the transition state. For a minor process, PPI(+)(PrOH) PPII(+)(PrOH/aq) without protonation, we determine G = -9 20 kJmol(-1), H = 64 14 kJmol(-1), and S= 247 50 Jmol(-1)K(-1). This thermochemistry yields G = -10 29 kJmol(-1), H = -139 20 kJmol(-1), and S= -435 70 Jmol(-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.
