[Gallium(III) protoporphyrin IX]2: a soluble diamagnetic model for malaria pigment.

Gallium(III) protoporphyrin IX forms a dimeric propionate-bridged dimer, 2, that is a soluble diamagnetic analogue of hematin anhydride. The single-crystal structure of 2 corresponds to a nondisordered inversion-symmetric dimer similar to malaria pigment but, unlike it, has a six-coordinate metal and an intraporphyrin rather than an interporphyrin hydrogen bond. NMR NOE correlations demonstrate the presence of the propionate linkage in solutions with pyridine. Taken together, this is the first single-crystal X-ray diffraction study of a propionate-linked dimer as found in malaria pigment and the first evidence for its presence in solution.