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 Ca-alkyl groups in the Iva enantiomers depend on the a-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 g-Me H-atoms of the Et side chain of the D-Iva residues located near the C-terminus are significantly more shielded (d<0.90 ppm) than those of the L-Iva residues (d>0.95 ppm). The opposite behavior is observed for the left-handed, synthetic, intermediate Z-Aib-L-Hyp-L-Iva14-D-Iva15-OtBu. Here, the g-Me H-atoms of L-Iva14 are more shielded (0.838 ppm) than those of D-Iva15 (0.905 ppm). The chemical-shift difference between the diastereotopic b-CH2 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-Iva14/15, the values range from 0.38 to 0.63 ppm, whereas, for D-Iva1, 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 0.19 ppm). Again, an opposite behavior is seen for the left-handed tetrapeptide. Overall, our method enables the nondestructive assignment of the configuration of each Iva residue in peptides of known helical screw sense.