Microscopic protonation constants in tobramycin. An NMR and pH study with the aid of partially N-acetylated derivatives.

Three tetra-N-acetyl derivatives and one tri-N-acetyl derivative of tobramycin (1) have been prepared by partial N-acetylation. Comparison of the pKa values, determined by NMR chemical shift titrations and pH titration of the derivatives, with those of unprotected 1 suggests that protonation equilibria at any particular amino group in 1 are not likely to be influenced by those at other sites. pH-Dependent conformational changes in 1 were assessed on the basis of 1H and 13C chemical shift changes in the derivatives.