13C and 15N solution and solid-state nuclear magnetic resonance study of the intermolecular interactions in the 1:1 trimethoprim: sulphamethoxazole complex

The combined use of natural-abundance solid-state 13C and 15N n.m.r. shows that the structure of the trimethoprim:sulphamethoxazole complex (TMP:SMZ) is best described by transfer of the proton from N(7) of SMZ to N(1) of TMP. Stabilisation of the complex is achieved by the resulting ionic interaction and by the formation of two intermolecular hydrogen bonds. When the complex is dissolved in dimethyl sulphoxide it is largely dissociated into neutral free base TMP and SMZ.