Robert L. Lichter

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

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_as 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.

15N magnetic resonance spectroscopy—XI. Nitrogen-proton spin-spin coupling as source of proton resonance line broadening in amino acids

The geminal ^(15)N—H coupling constant in some amino acids was found to increase with pH and this increase can account for the reported broadening of the α proton resonances of the corresponding ^(14)N amino acids with increasing pH.

One- and two-bond 13C–15N coupling constants in saturated amides. Absence of three-bond coupling on rotational averaging

One-bond carbon-nitrogen coupling constants in amides vary over a narrow range, while two-bond couplings show a much more pronounced structural dependence; no three-bond values are observed.

13 C And 15N nuclear magnetic resonance parameters of ethyl diazoacetate

15 N Chemical shifts of both singly labelled and the doubly labelled isotopic modifications of ethyl diazoacetate accord with qualitative expectations; the 13C–15N and 15N–15N coupling constants compare with calculated values which demonstrate the dominance of the Fermi contact term and the importance of the nitrogen lone pair.