Carbon-13 Nuclear Magnetic Resonance Titration Shifts in Amino Acids

Nmr titration curves for the individual carbon atoms in a series of representative amino acids have been determined. A computer model involving multiple ionizations has been used to obtain ionization constants and titration shifts, even when ionizations overlap. CNDO/2 molecular orbital calculations suggest a rationalization of the observed changes in ¹³C shielding on deprotonation. For carbon atoms near the site of ionization a decrease in excitation energy dominates the chemical shift expression, resulting in deshielding despite an increase in electron density; for more distant carbon atoms changes in electron density dominate, yielding shifts in either direction.