The helix is the most common secondary structural motif in proteins, and it continues to attract much theoretical and experimental interest. We are using oligomers of the strongly helix forming residue ?-aminoisobutyric acid (Aib) as models of helical structure and dynamics. Oligomers of this achiral residue are known to form left- and right-handed 310 helices. The rates of interconversion between enantiomeric left- and right-handed helices can be measured using dynamic NMR techniques (Hummel et al., Angew. Chem. Int. Ed. 26, 1150 (1987)). We have synthesized hexameric and octameric oligomers of AIB, each 13C-labeled at a single residue. We have measured the enantiomerization kinetics for these organic-soluble peptides in CD222. Within the assumption of a fully cooperative enantiomerization transition, the octamer and hexamer show a similar ΔH‡ of around 30 kJ/mol. However, the octamer has a larger, more negative ΔS‡ than the hexamer. The thermodynamic activation parameters for the conformational enantiomerization dynamics of these peptides will be discussed.
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