Synthesis and conformational analysis of N‐glycopeptides. II. CD, molecular dynamics, and nmr spectroscopic studies on linear N‐glycopeptides

The comprehensive structural analysis reported herein of eight N‐glycopeptides, in three different solvents, is based on quantitative CD experiments, homonuclear nuclear Overhauser effect measurements, and molecular dynamics (MD) calculations. Although several orientations of the two amide planes attached to the carbohydrate pyranose ring are possible, according to NOE, CD data, and MD simulations, of all of the glycopeptide models, regardless of the type of the carrier peptide, only one dominant conformer population was found. This conformer is characterized by a nearly trans orientation of the CH and NH hydrogens of both acetamido groups. This finding is in perfect agreement with x‐ray crystallographic data on the solid state conformation of the 1‐N‐acetyl‐ and 1‐N‐(β‐aspartoyl)‐2‐acetamido‐2‐deoxy‐β‐D‐glucopyranosylamine. The precise identification of this dominant conformer of N‐glycopeptides in solution was the major question addressed herein by the structural analyses. A “CD additivity” experiment was carried out using an equimolar solution of Boc‐Pro‐Asp‐NHCH3 and l‐N‐acetyl‐3,4,6‐tri‐O‐acetyl‐2‐acetamido‐2‐deoxy‐β‐D‐gluco‐pyranosylamine at ambient temperature in acetonitrile. The CD spectrum obtained from the equimolar solution of the above two molecules (the “spectroscopic sum”) was identical with the CD curve obtained from the algebraic summation of the individually recorded CD spectra of the peptide and the carbohydrate moiety (“mathematical sum”).