Premium
Multinuclear two‐dimensional NMR: Assignments of natural abundance polypeptide 13 C, 1 H and 15 N chemical shifts and demonstration of isomer interconversion
Author(s) -
Gray George A.
Publication year - 1983
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/omr.1270210207
Subject(s) - homonuclear molecule , heteronuclear molecule , chemical shift , chemistry , proton , cis–trans isomerism , spectral line , diastereomer , stereochemistry , nmr spectra database , crystallography , nuclear magnetic resonance , two dimensional nuclear magnetic resonance spectroscopy , proton nmr , nuclear magnetic resonance spectroscopy , molecule , physics , organic chemistry , quantum mechanics , astronomy
The polypeptide carbobenzoxy‐glycyl‐ L ‐prolyl‐ L ‐leucyl‐ L ‐alanyl‐ L ‐proline (0.2 M in DMSO‐ d 6 ) was investigated using 13 C, 1 H and 15 N NMR in natural abundance at 4.7 tesla. The existence of cis – trans ‐Gly‐Pro and ‐Ala‐Pro bonds permits up to four isomers, and all four were observed (in a 60:30:7:3 ratio). 13 C shifts of the proline β‐CH 2 resonances are consistent only with the 60% form being trans – trans . The 30% form is either trans – cis or cis – trans (order as above) and was tentatively assigned as cis ‐ trans on the basis of relaxation behavior. Refocused INEPT studies aided the 13 C assignments. The 15 N data were obtained using both NOE and INEPT excitation, with signals evident for the three major isomers. The spectra were analysed by starting from the 13 C data, which were assigned based on known regularities in peptide spectra. A 13 C 1 H heteronuclear two‐dimensional chemical shift correlation experiment allowed direct assignment of proton shifts for major and minor isomers. The NH proton shifts were assigned by running a homonuclear two‐dimensional chemical shift correlation experiment and noting the correlation with the previously assigned α‐CH protons. The 15 N resonances were then assigned from a 15 N 1 H heteronuclear two‐dimensional chemical shift correlation experiment, relating the 15 N signals directly to the NH proton resonances. Isomer interconversion between the two major isomers was demonstrated by performing a magnetization transfer homonuclear 2D experiment. Off‐diagonal intensity was noted relating the major and minor isomer alanine NH proton, as well as for the major and minor isomer leucine NH protons.