Conformations of peptides in solution by nuclear magnetic resonance spectroscopy. Part IV. Conformations of valinomycin determined from homoallylic proton coupling across peptide bonds

Abstract

100 MHz Proton magnetic resonance measurements have been made on valinomycin in the complexed and uncomplexed form in polar [CD₃OD, (CD₃)₂N·CDO] and non-polar (CDCl₃) solutions. A small five-bond long-range coupling was observed between α-CH groups across the peptide bonds of the L-Lac-L-Val and D-Hylv-D-Val molecular fragments. Observation of ⁵J(HH) enables the L- and D-Val α-CH proton signals to be assigned conveniently. Analysis of ⁵J(HH) in terms of homoallylic coupling provides information on peptide conformational angles ϕ and ψ, i.e. for valinomycin ψ(L-Lac), ϕ(L-Val), ψ(D-Hylv), ϕ(D-Val). The results for both complexed and uncomplexed valinomycin are compared with those of recent crystal structure analyses and with conformational models of these systems in polar and non-polar solvents.