¹⁵N NMR spectroscopy. 23—Shift effects of protecting groups in oligopeptides of glycine and alanine

Abstract

The natural abundance 15N NMR spectra of glycylglycine and alanylalanine derivatives with various N-or 0-protecting groups were measured in dimethyl sulphoxide, pyridhe and formic acid. The nitrogens directly attached to the protecting group have chemical shifts relative to NOo-in the range -220 to -358 ppm. The influence of the amino end group on the next peptide nitrogen (Gly-Gly bond) q o u n t s to, at most, 1.7 ppm. The influence of the 0-protecting group is also weak (A6 ~2 ppm), but strongly dependent on the solvent.

15N NMR spectroscopy. 30—structure/shift relationships of oligopeptides and copolypeptides, including gramicidin S

“…22 The NREs of N-protected tripeptides and their esters have the same sign, but the value can differ up to 0.8 ppm. However, the pair Z-GlyAla-* Ala-OHIZ-Gly-Ala-* Ala-OMe is an ex- ception, because the signs of the NREs are opposite in pyridine (Table 2).…”

“…N-protected dipeptides and their esters behave similarly, and a potential explanation of this spectroscopic behaviour has been presented in a previous paper. 22 The NREs of N-protected tripeptides and their esters have the same sign, but the value can differ up to 0.8 ppm. However, the pair Z-Gly-Ala-* Ala-OHIZ-Gly-Ala-* Ala-OMe is an ex- ception, because the signs of the NREs are opposite in pyridine ( Table 2).…”

Solvent Effects on Nitrogen Chemical Shifts

“…Solvent effects were investigated for a series of N and C terminally protected glycine and alanine containg peptides, including dipeptides 311-315 shown in Fig. 39 (Table 32) [180]. In formic acid, strong hydrogen bonding and protonation of the amide carbonyl oxygens promote delocalization of the unshared electron pair of the nitrogen atoms and shift the amide-iminol tautomer equilibria, yielding a 3.5-5.5 ppm deshielding of the δ 15N of Ala 2 and Gly 2 residues as compared to that observed for dimethyl sulfoxide-d 6 and pyridine-d 5 solutions.…”

15N NMR spectroscopy 31—preliminary study on the insulin‐a chain (bunte salt form)

“…2(b) and (c) again allow a differentiation between nitrogens on the basis of mobility. Eight signals (7,8,13,14,18,20,21,22) have longer Tl values compared to the other signals and, hence, correspond to nitrogens with greater segmental mobility.…”