Conformational Equilibria in Polypeptides. I. Determination of AccurateJHCCoupling Constants in Antamanide by 2D NMR Multiplet Simulation

High-precision heteronuclear three-bond coupling constants including³J(H^N, C′),³J(H^N, C^β),³J(H^α,C^γ), and³J(H^β, C′) are determined for the nonproline residues in uniformly¹³C-enrichedantamanide,cyclo-(-Val¹-Pro²-Pro³-Ala⁴-Phe⁵-Phe⁶-Pro⁷-Pro⁸-Phe⁹- Phe¹⁰-), using quantitative numerical 2D NMR spectrum evaluation based on the product-operator formalism. The experimental basis comprises two-dimensional¹H,¹³C-heteronuclear relayed E.COSY spectra J. M. Schmidt, R. R. Ernst, S. Aimoto, and M. Kainosho,J. Biomol. NMR6,95 (1995)], the multiplet patterns of which are subjected to iterative least-squares 2D multiplet-simulation procedures. Accuracy and precision of the spectrum fit are assessed byFstatistics and analysis of variances (ANOVA) leading to confidence intervals for the optimized spin-system parameters. The long-range J coupling constants obtained and their standard deviations provide the experimental foundation for a later detailed analysis of φ and χ₁dihedral-angle equilibrium conformations contributing to the flexible peptide structure.