Joseph E. Sarneski

Carbon-13 NMR studies of amino acids: Chemical shifts, protonation shifts, microscopic protonation behavior

The 13C chemical and protonation shifts of a number of the naturally occurring α-amino acids and some related compounds are tabulated. A computer program is described which fits chemical shift vs pH data to predict the macroscopic pKa values. This program operates on the data in a global manner utilizing simultaneously all of the 13C resonance data at the various pH conditions in order to achieve a self-consistency in the calculated pKa values. The protonation shifts and pKa values thus obtained are used to determine the microscopic protonation behavior of lysine and hydroxylysine; it was found to be necessary to use model compounds to ascertain the influence of amine protonation on the chemical shifts of remote carbons in the latter systems. Only in this way could these microscopic protonation equilibria be properly characterized.

The Use of Protonation Induced Changes in Carbon-13 NMR Chemical Shifts to Investigate the Solution Microscopic Structure of Partially Protonated Polybasic Molecules

Abstract Changes in the NMR chemical shift of carbon-13 nuclei upon protonation of a nearby basic center are shown to be a useful method of probing the microscopic site of protonation of molecules containing multiple basic centers, such as diethylenetriaminepenta-acetic acid (DTPA). This approach is also shown to support the predominance of the pyridine amine protonated tautomer in the 4-pyridone ↔ 4-hydroxypyridine equilibrium.