Molecular motions of a glycopeptide from human serum transferrin studied by 13C nuclear magnetic resonance.

The molecular motions of a 21-amino-acid glycopeptide (Gp21) containing multiple glycoforms of an N-linked diantennary oligosaccharide were studied by two-dimensional 1H-detected 13C relaxation measurements at natural abundance. Gp21 was derived from human serum transferrin, its amino acid sequence is QQQHLFGSNVTDCSGNFCLFR, and its N-glycan structure is [Formula: see text] The measured longitudinal and transverse relaxation rate constants and the nuclear Overhauser enhancements for the methine carbons of Gp21 were analyzed by using the model-free approach to obtain information about the internal motions in the molecule. The calculated order parameters S2 of the alpha carbons in both NH2- and COOH-terminal segments of the peptide are smaller than those in the interior segment of the Gp21 peptide moiety, implying that the internal motions in the terminal segments are less restricted than in the interior segment. The average S2 value is 0.72-0.91 for the glycosyl residues in the pentasaccharide core of Gp21, 0.58-0.59 for the interior GlcNAc-5,5' residues in the two branches, and 0.35-0.51 for the terminal GlcNAc-5, Gal-6,6', and NeuAcN,N' residues in the two branches, indicating that the internal motions in the glycan core are more restricted than in the two branches.