Threonine chemical shifts

C-N.m.r. Chemical-shift Data for Various a- and /i-D-Clycosyl-L-threonine Derivatives... 

On the other hand, resonance assignments for CP of threonine and serine, and C and Cy of hydroxy proline, were difficult to make, because of their proximity to carbohydrate carbon resonances. In most cases then, the resonances were assigned on the basis of the effects of pH on the chemical shifts of those resonances. It was shown that the chemical shifts for the carbohydrate carbon resonances were virtually unaffected (AS 0.4 p.p.m.) when going from the cationic state (pH 2) to the anionic state (pH 11) of the amino acid residues. The chemical shifts of C and CP of the amino acid residues, however, shifted considerably (up to 3.1 and 6.6 p.p.m. for C" and CP, respectively see Table VI). 

C-N.m.r. Chemical-shift Data for the Anomeric-Carbon Atoms and C, and Coupling Constants ( /ch) Measured for Selected Gland /S-D-Glycosyl-L-threonine Model Compounds19- 1,82-84... 

The presence of two different probes, namely, Gal C-l and Thr C7, giving rise to signals situated in two different, uncrowded regions of the spectrum, with chemical-shift differences (AdC-l up to 8.35 p.p.m., and Ad Thr C7, up to 6.7 p.p.m.), allows the facile detection of any racemiza-tion occurring at either, or both, of the asymmetric carbon atoms of the threonine. It would be difficult to distinguish a-D-Gal — L-Thr (27) from its P analog (28), or / -D-Gal — D-alloThr (58) from / -D-Gal — L-alloThr (56), on the basis of the anomeric-carbon chemical-shifts only. However, they can be differentiated on the basis of Thr Cr chemical-shift data. It should be noted that neither optical rotation nor -n.m.r. spectroscopy could have elucidated this point.85... 

Such electronic effects are readily deduced from chemical shifts. A recent example from our laboratory concerns the antimicrobial peptide drosocin.61 This 19-amino-acid peptide is secreted by Drosophila species in response to septic injury and is a potent antibacterial. The sequence of the peptide (shown below using the one-letter amino acid code) contains a glycosylated threonine at a midchain position that appears to be important in enhancing antimicrobial activity. 

Schraml et al.(137) found that the Si resonances of a series of silyl ester, alkoxy-silyl, and amino-silyl derivatives appear in different regions of the spectrum. On the basis of this information they suggested that Si NMR can be used for the structure elucidation of silylated hydroxy- or aminoacids. The silicon chemical shifts for DL-serine [55] and DL-threonine [56 ] are shown in ppm. The variation in the shielding for the alkoxy-silyl group demonstrates the previously noted sensitivity of Si NMR to the nature of R in Mc3SiOR. (140)... 

PS may be assigned to a glycine (Gly) or threonine (Thr), because PS is close to both the expected chemical shift data sets ... 

The second problem that arises from the use of pre-calculated CS hypersurfaces is the neglect of the molecular environment in the calculation of small model systems. For chemical shift tensors of zwitterionic amino acids this effect was studied by de Dios et Quite large effects were observed for the carbonyl carbons when the crystal lattice was included in the ab initio calculations in the form of a point charge distribution. The an tensor component of L-tyrosine changed by 16 ppm and the 072 tensor component by 37 ppm. The changes for L-threonine were only slightly smaller. 

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