pH-Dependent Nuclear Magnetic Resonance Spectra of ^(15)N-Enriched Glycine. Line Shape and Relaxation Studies

Abstract

The influence of pH on the line shapes, chemical shifts, and relaxation times of the ^(15)N resonances in aqueous solutions of 95%-enriched ^(15)N glycine is discussed. Below pH 6.4, the observed ^(15)N and ^1H spectra of glycine clearly reflect chemical-exchange modulation of the ^(15)N-^1H scalar interaction. The observed increase in the scalar relaxation, as well as the concomitant decrease in the nuclear Overhauser enhancement of the ^(15)N resonance as the pH increases, can also be explained on the basis of chemical exchange. A comparative study of ^(15)N enriched glycine and ethyl glycinate shows that the ^(15)N chemical-shift changes found for glycine in the acid region reflect the electronic changes at the carbonyl function. Furthermore, the observed differences in the line shapes for the two compounds correlate well with the differences in pK_a's for the ^+NH_3 group. The changes in line shape for both the decoupled and undecoupled ^(15)N spectra of glycine and its ethyl ester above pH 6.4 suggest that ^(15)N chemical-shift averaging occurs as the result of exchange between the protonated and deprotonated forms.

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