Carbon-13 Chemical Shielding Tensors in L-Threonine*

The carbon- 13 chemical shielding tensors of the amino acid L-threonine have been determined by solid state proton-enhanced NMR of a single crystal at 3.52 T. For the first time a carbinol shielding tensor is determined quantitatively and unambiguously: the most shielded axis lies 4 ” from the C-O bond. A coordinate system which approx-imates the alpha carbon tensor orientation is proposed, and it is suggested that this “local symmetry ” may be general to many amino acid alpha carbons. The carboxyl and methyl tensor orientations closely approximate previously established “local symmetry, ” although the methyl (Cl) C,. symmetry is slightly disrupted, apparently by the neighboring (07) oxygen. The influence of the carbon-nitrogen dipolar interaction on the observed spectra is discussed, and used to make unique assignments for the carboxyl, alpha, and beta carbon tensors