Hydrogen-bonding patterns in trimethoprim trifluoroacetate

In the title compound, trimethoprim trifluoroacetate [or 2,4-diamino-5-(3,4,5-trimethoxybenzyl)pyrimidin-1-ium trifluoroacetate], C14H19N4O3+.C2F3O2-, the trimethoprim molecule is protonated at N-1. The carboxylate group of the trifluoroacetate anion binds with the protonated pyrimidine ring of trimethoprim (TMP) by two nearly parallel N-H...O hydrogen bonds. This is reminiscent of the carboxylate-trimethoprim interaction observed in dihydrofolate reductase (DHFR)-trimethoprim complexes. The pyrimidine moieties of the trimethoprim cations are centrosymmetrically paired through a pair of N-H...N hydrogen bonds involving the 4-amino group and atom N3. The 2-amino group of one TMP motif and the 4-amino group of another motif (both of these motifs are members of a base pair) are bridged by one of the methoxy O atom of a third TMP motif, leading to a five-membered (excluding H atoms) hydrogen-bonded chelate. One of the H atoms of the 2-amino group is also involved in a bifurcated hydrogen bond involving two methoxy O atoms of a trimethoprim motif, leading to a five-membered (including the H atom) hydrogen-bonded chelate. The pyrimidine ring makes a dihedral angle of 83.69 (10)degrees with the phenyl ring in the trimethoprim cation. In the trifluoroacetate moiety, the average F-C bond distance is 1.261 Angstrom and F-C-C and F- C-F bond angles are 114.7 and 103.7degrees respectively.