The reduced aza-peptide fragment (C-alpha-CH2-NH-N(alpha)R-CO-NH-C-alpha) is obtained by reduction of the semicarbazone moiety (C-alpha-CH=N-N(alpha)R-CO-NH-C-alpha) in an imino aza-peptide. It differs from the aminomethylene link (C-alpha-CH2-NH-C(alpha)HR-CO-NH-C-alpha) found in the reduced peptides by the absence of protonation in water in the 2-12 pH range. The reduced aza-analogue of the Pro-Ala dipeptide has been studied in solution by H-1-NMR and IR spectroscopy, and in the solid state by X-ray diffraction. Its structure is quite similar to that of the neutral reduced dipeptide analogue in solution. (C) 1997 Elsevier Science Ltd.
The reduced aza-peptide fragment (C-alpha-CH2-NH-N(alpha)R-CO-NH-C-alpha) is obtained by reduction of the semicarbazone moiety (C-alpha-CH=N-N(alpha)R-CO-NH-C-alpha) in an imino aza-peptide. It differs from the aminomethylene link (C-alpha-CH2-NH-C(alpha)HR-CO-NH-C-alpha) found in the reduced peptides by the absence of protonation in water in the 2-12 pH range. The reduced aza-analogue of the Pro-Ala dipeptide has been studied in solution by H-1-NMR and IR spectroscopy, and in the solid state by X-ray diffraction. Its structure is quite similar to that of the neutral reduced dipeptide analogue in solution. (C) 1997 Elsevier Science Ltd.
The homologous RCO-Pro-Xaa-NHR' model pseudodipeptides containing the reduced peptide ((CCH2NHCalpha)-C-alpha), reduced azapeptide ((CCH2NHNalpha)-C-alpha), methyleneoxy ((CCH2OCalpha)-C-alpha), and iminoazapeptide ((CCH)-C-alpha=NNalpha) surrogate of the middle amide group have been prepared. Their structural analysis has been carried out in solution by H-1 NMR and IR spectroscopy and in the solid state by X-ray diffraction. The last three fragments, not protonated in the pH range 2-12, and the reduced fragment in its neutral amine form induce quite similar molecular structures characterized by a hydrogen bond between NHR' and the N/O atom replacing the amide NH group and closing a five-membered cycle. The neutral amine or protonated ammonium state of the reduced amide fragment, with a pK(a) value of about 7, depends on the environment. Protonation induces a conformational transition due to the strong proton donating properties of the ammonium group which interacts with the RCO carbonyl.
The semicarbazone peptidomimetic group in imino aza peptides
The semicarbazone moiety (C-CH=N-NR-CO-NH-C), either obtained by coupling a peptide aldehyde with a semicarbazide, or by action of an alkylisocyanate on a peptide hydrazone, is a dipeptide isostere. The structure of four imino aza dipeptides, analogues of the Pro-Gly, Pro-Ala and Pro-Phe dipeptides, has been studied in solution by H-1-NMR and IR spectroscopy, and in the solid state by X-ray diffraction.