NMR of carboxylic-17O in amino acids

✍ Scribed by A. Steinschneider; B. Valentine; M. I. Burgar; D. Fiat

• Publisher: John Wiley and Sons
• Year: 1985
• Tongue: English
• Weight: 701 KB
• Volume: 23
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260230211

✦ Synopsis

N M R studies were conducted on the a-COOH groups of the amino acids Val, Leu, Ile, His and Pro and on the y-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 HS0,F. The relative nuclear screening order for a-COOH 170 between pH -1 and 13 and in HSO& was usually Pro> Leu> His> Val>Ile, reflecting on interactions with the side chain. Concomitant with ionization in aqueous solution, the a-and y-COOH resonances were displaced to lower field, by 13-18 and 24ppm, respectively, presumably owing to ovemding effects of the bond order-charge density term in the paramagnetic contribution toward nuclear shielding. Titration with alkali above neutrality resulted in 24ppm upfield shifts related to deprotonation of the a-NH3* group. T2 relaxation times were inversely related to the molecular weight. Line widths of amino acid "0-COOH resonances near neutrality were between 280 and 525 Hz, corresponding to T2 relaxation times between 0 . 6 and 1.1ms. Titration from acidic to neutral pH increased T2 by up to 1.7-fold. Tl measured for "0-Gly and y-l'O-Glu changed in parallel with T2. This was attributed to decreasing rotational correlation times, T ~, in line with expected changes in the nuclear quadrupole coupling constant. T, values calculated from 170 relaxation in solutions near neutrality (zwitterion predominant) were in good agreement with expected from solution properties and with the assumption of isotropic rotation. T2 decreased when the pH was increased above neutrality, partly owing to increasing solution Viscosity.