Abstract
Abstract Tentative assignment for each carbon resonance in the natural abundance high resolution carbon-13 nuclear magnetic resonance Fourier transform spectrum at 25.2 MHz of the biologically active cyclic octapeptides, synthetic oxytocin (43 carbons) and lysine-vasopressin (46 carbons) and the cyclic dodecapeptide bacitracin (66 carbons) has been made using a number of different techniques. The 13C assignments of the constituent amino acids were used to investigate the conformation of these cyclic peptides in aqueous solution. The chemical shift nonequivalence found for the end chain and β-substituted methyl carbons of the 3 isoleucyl residues in bacitracin may result from different chemical environments imposed by the conformation of this peptide. Furthermore, the small differences (less than 2 ppm) in chemical shifts of the cyclic peptide 13C resonances relative to the amino acids (correcting for internal peptide bond, N- and C-terminal, and pH effects) and linear peptides may also reflect the extent that 13C shifts are affected by different conformations.
Highlights
Since detailed information on the assignments of 13C chemical shifts in proteins is limited, it is important that model compounds be investigated by 13C NMR to delineate the chemical shift characteristics of t’hese systems. 13C studies have been conducted on most of the amino acids [9], a variety of linear peptides ranging from dipeptides to the amino-terminal 1 to 13, 1 to 15, and 1 to 20 residues of RNase A2 and the cyclic peptide gramicidin S-d [10,11,12,13]
The r3C spectra of the three cyclic peptides are given in Figs. 1 to 3
The extent of resolution is apparent in the spectra for BACITRACIN -=C (25.2 MHz) pprn UPFIELD FROM CS 2
Summary
Tentative assignment for each carbon resonance in the natural abundance high resolution carbon-13 nuclear magnetic resonance Fourier transform spectrum at 25.2 MHz of the biologically active cyclic octapeptides, synthetic oxytocin (43 carbons) and lysine-vasopressin (46 carbons) and the cyclic dodecapeptide bacitracin (66 carbons) has been made using a number of different techniques. 13C studies have been conducted on most of the amino acids [9], a variety of linear peptides ranging from dipeptides to the amino-terminal 1 to 13, 1 to 15, and 1 to 20 residues of RNase A2 and the cyclic peptide gramicidin S-d [10,11,12,13] In this investigation the results are reported of a 1% study of oxytocin, lysine-vasopressin, and bacitracin. Hon-ever, detailed spectral analysis has been complicated by the small chemical shift range for protons and by the extensive overlap of peaks caused by the broadness of resonances of native proteins [2] In this respect, the much larger spectral dispersion encountered with carbon-13 magnetic resonance makes this nucleus a potentially useful probe to investigate conformat,ions and interact’ions in macromolecular systems.
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