Our work characterizes the effects of solvent viscosity, polarity, and hydrogen-bonding capacity towards determining the rates of helical conformational exchange of a dissolved solute probe. We employ Z-Aib6-β-Ala-OMe, unique for its ability to undergo isoenergetic conformational exchange between left- and right-handed helices and millisecond activated barrier crossings in its electronic ground state. We use 1H and 13C NMR spectroscopy to determine the rates of conformational exchange using line-shape analysis. We vary solvent viscosities by using a series of solvents, such as methanol-d4 and dichloromethane-d2, and by varying sample temperature. Obtained rate constants are analyzed with Eyring treatments to characterize transition state thermodynamics. According to the results obtained from the transition state thermodynamics, ΔG‡ is lower for MeOD solvent than for CD2Cl2, suggesting that the MeOD may preferentially stabilize the transition state as compared to CD2Cl2 possibly due to specific hydrogen bonding interactions. We interpret the kinetic data with theories that admit an explicit role for solvent viscosity, such as Kramers’ theory. This work provides novel insight into solvent influence in helical exchange dynamics in a unique synthetic polyamide system.

Solvent mediation of unimolecular helical exchange dynamics in the synthetic helical peptide Z-Aib6-β-Ala-OMe

253rd National Meeting (2017)

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