Configurational assignment of D- and L-isovalines in intact, natural, and synthetic peptides by 2D-NMR spectroscopy.

We investigated, by means of 2D-NMR, the naturally occurring and chemically synthesized 16-mer integramides A and B, which belong to a group of bioactive, fungal peptides (peptaibiotics), that are characterized by an abundance of Aib as well as D- and L-Iva residues. The chemical shifts of the C(alpha)-alkyl groups in the Iva enantiomers depend on the alpha-C-atom configuration and on the helical screw sense of the peptides, the latter determined by CD. In the full-length, right-handed helical integramides, as well as in the partial sequences exploited for their total chemical syntheses, the gamma-Me H-atoms of the Et side chain of the D-Iva residues located near the C-terminus are significantly more shielded (delta 0.95 ppm). The opposite behavior is observed for the left-handed, synthetic, intermediate Z-Aib-L-Hyp-L-Iva(14)-D-Iva(15)-O(t)Bu. Here, the gamma-Me H-atoms of L-Iva(14) are more shielded (0.838 ppm) than those of D-Iva(15) (0.905 ppm). The chemical-shift difference between the diastereotopic beta-CH(2) H-atoms of the Iva side chains in the right-handed helical peptides is much larger for D-Iva than for L-Iva. For D-Iva(14/15), the values range from 0.38 to 0.63 ppm, whereas, for D-Iva(1), the value is in the range of 0.26-0.31 ppm. In each case, the difference is always larger for the d-Iva than for the l-Iva residues (which is always tetrapeptide. Overall, our method enables the nondestructive assignment of the configuration of each Iva residue in peptides of known helical screw sense.