Nuclear magnetic resonance studies of the microscopic protonation of L-cysteine

The protonation scheme of L-cysteine has been investigated by nuclear magnetic resonance. Chemical shift data as a function of pH are given for L-cysteine, L-cysteine methyl ester and S-methyl-L-cysteine. Use of derivatives of L-cysteine permits determination of the effect of protonation of the amino, sulfhydryl and carboxylic sites on the chemical shift of the -CH and -CH₂ protons. On the basis of these results, a set of simultaneous equations was written whose solution yielded the fraction of protonation of each site upon the addition of n equivalents of acid to the initially completely deprotonated molecule.