title: Configurationally-Coupled Protonation of Polyproline-7.
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 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.
authors.affiliation: †Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States.
authors.forename: Liuqing
authors.initials: L
authors.lastname: Shi
authors.affiliation: ‡Department of Chemistry, Moravian College, Bethlehem, Pennsylvania 18018, United States.
authors.forename: Alison E
authors.initials: AE
authors.lastname: Holliday
authors.affiliation: †Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States.
authors.forename: Neelam
authors.initials: N
authors.lastname: Khanal
authors.forename: David H
authors.initials: DH
authors.lastname: Russell
authors.affiliation: †Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States.
authors.forename: David E
authors.initials: DE
authors.lastname: Clemmer
mesh_terms: D000433:1-Propanol / Q000737:chemistry; D019277:Entropy; D007700:Kinetics; D008958:Models, Molecular; D010455:Peptides / Q000737:chemistry*; D017433:Protein Structure, Secondary; D011522:Protons; D012997:Solvents / Q000737:chemistry; D014867:Water / Q000737:chemistry
chemical_list: D010455:Peptides; D011522:Protons; D012997:Solvents; D014867:Water; C011083:polyproline; D000433:1-Propanol