NMR analysis and site-specific protonation constants of streptomycin.

Streptomycin, the classical aminoglycoside antibiotic, generally considered the most basic drug compound was characterized in terms of protonation macro- and microconstants. ¹H NMR-pH and ¹H-¹³C HSQC-pH titrations were carried out on streptomycin and streptidine, a symmetrical constituent compound of reduced complexity to monitor the proton-binding processes of the basic sites. Accurate, undistorted, electrodeless pH measurement was ensured by a new set of in tube indicators. The microscopic protonation constants of the two guanidino groups of streptomycin were calculated by evaluating the various NMR-pH data and transferring the pair-interactivity parameter from streptidine to streptomycin. Inherent guanidino basicities fall in the range of 13.03-13.39 log k units, which drop to 12.48-12.85 upon protonation of the other site. pH-dependent distribution of the major microspecies and charge-related biological consequences are provided.