title: Configurational assignment of D- and L-isovalines in intact, natural, and synthetic peptides by 2D-NMR spectroscopy.
abstract: 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<0.90 ppm) than those of the L-Iva residues (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 <or=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.
authors.affiliation: Institute of Biomolecular Chemistry, CNR, Padova Unit, Department of Chemical Sciences, University of Padova, I-35131 Padova, Italy.
authors.forename: Marta
authors.initials: M
authors.lastname: De Zotti
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Schievano
authors.forename: Stefano
authors.initials: S
authors.lastname: Mammi
authors.forename: Bernard
authors.initials: B
authors.lastname: Kaptein
authors.forename: Quirinus B
authors.initials: QB
authors.lastname: Broxterman
authors.forename: Sheo B
authors.initials: SB
authors.lastname: Singh
authors.forename: Hans
authors.initials: H
authors.lastname: Brückner
authors.forename: Claudio
authors.initials: C
authors.lastname: Toniolo
mesh_terms: D000595:Amino Acid Sequence; D002942:Circular Dichroism; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000737:chemistry*; D017433:Protein Structure, Secondary; D013237:Stereoisomerism; D014633:Valine / Q000737:chemistry*
chemical_list: D009842:Oligopeptides; C456979:integramide A; C456980:integramide B; C032737:isovaline; D014633:Valine