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NMR of carboxylic‐ 17 O in amino acids
Author(s) -
Steinschneider A.,
Valentine B.,
Burgar M. I.,
Fiat D.
Publication year - 1985
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260230211
Subject(s) - chemistry , aqueous solution , chemical shift , deprotonation , relaxation (psychology) , zwitterion , titration , proton nmr , analytical chemistry (journal) , crystallography , stereochemistry , medicinal chemistry , molecule , organic chemistry , ion , social psychology , psychology
17 O NMR studies were conducted on the α‐COOH groups of the amino acids Val, Leu, Ile, His and Pro and on the γ‐COOH group of Glu. Limit chemical shifts, downfield from external water, were 249‐257 ppm in aqueous solution at low pH, and 234–243 ppm in HSO 3 F. The relative nuclear screening order for α‐COOH 17 O between pH ‐1 and 13 and in HSO 3 F was usually Pro>Leu>His>Val>Ile, reflecting on interactions with the side chain. Concomitant with ionization in aqueous solution, the α‐ and γ‐COOH resonances were displaced to lower field, by 13–18 aid 24 ppm, respectively, presumably owing to overriding effects of the bond order‐charge density term in the paramagnetic contribution toward nuclear shielding. Titration with alkali above neutrality resulted in 2–4 ppm upfield shifts related to deprotonation of the α‐NH 3 + group. T 2 relaxation times were inversely related to the molecular weight. Line widths of amino acid 17 O‐COOH resonances near neutrality were between 280 and 525 Hz, corresponding to T 2 relaxation times between 0.6 and 1.1 ms. Titration from acidic to neutral pH increased T 2 by up to 1.7‐fold. T 1 measured for 17 O‐Gly and γ‐ 17 O‐Glu changed in parallel with T 2 . This was attributed to decreasing rotational correlation times, τ c , in line with expected changes in the nuclear quadrupole coupling constant. τ c values calculated from 17 O relaxation in solutions near neutrality (zwitterion predominant) were in good agreement with τ η , expected from solution properties and with the assumption of isotropic rotation. T 2 decreased when the pH was increased above neutrality, partly owing to increasing solution viscosity.