Crystallization of pseudopolymorphs of some gamboge pigments. Pyrirdine, dimethylformamide and dimethylsulfoxide solvates of morellic acid, gambogic acid and guttiferic acid

Morellic acid, gambogic acid and guttiferic acid are related naturally-occu rring xanthone Figments that yield X-ray quality crystals only from solvent s like pyridine, dimethylformamide (dmf) and dimethyl sulfoxide (dmso). The structures of four of these crystals have been determined and are found to contain solvents of crystallization. The solvents hydrogen bond to the car boxyl groups with O-H...O/N motifs previously seen in other carboxylic acid s. Distinctive, however, is the presence of an extended though somewhat dif fuse array of C-H...O hydrogen bonds that aggregates the entire solute-solv ent assemblage in a multi-point manner. Pyridine and dmf are able to mimic each other with respect to their hydrogen bond donating and accepting chara cteristics and in this respect play equivalent roles in their solvates with morellic acid and gambogic acid. Dmso is seen to self-associate in its gut tiferic acid solvate. It is possible that these solvents with multiple hydr ogen bonding donor and acceptor capability can act as hydrogen bond nucleat ors, providing just enough rigidity to the solutes to ensure crystallizatio n.