Alkoxide coordination of iron(iii) protoporphyrin IX by antimalarial quinoline methanols: a key interaction observed in the solid-state and solutionElectronic supplementary information (ESI) available: SCD data tables and additional figures, as well as MS, EXAFS and UV-visible spectroscopic data tables and spectra are presented. The SCD structural data for MQ-Fe(iii)PPIX and QD-Fe(iii)PPIX determined in the current work have been deposited with the Cambridge Crystallographic Data Centre. CCDC 10

The quinoline methanol antimalarial drug mefloquine is a structural analogue of the Cinchona alkaloids, quinine and quinidine. We have elucidated the single crystal X-ray diffraction structure of the complexes formed between racemic erythro mefloquine and ferriprotoporphyrin IX (Fe( iii )PPIX) and show that alkoxide coordination is a key interaction in the solid-state. Mass spectrometry confirms the existence of coordination complexes of quinine, quinidine and mefloquine to Fe( iii )PPIX in acetonitrile. The length of the iron( iii )-O bond in the quinine and quinidine complexes as determined by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy unequivocally confirms that coordination of the quinoline methanol compounds to Fe( iii )PPIX occurs in non-aqueous aprotic solution via their benzylic alkoxide functional group. UV-visible spectrophotometric titrations of the low-spin bis-pyridyl-Fe( iii )PPIX complex with each of the quinoline methanol compounds results in the displacement of a single pyridine molecule and subsequent formation of a six-coordinate pyridine-Fe( iii )PPIX-drug complex. We propose that formation of the drug-Fe( iii )PPIX coordination complexes is favoured in a non-aqueous environment, such as that found in lipid bodies or membranes in the malaria parasite, and that their existence may contribute to the mechanism of haemozoin inhibition or other toxicity effects that lead ultimately to parasite death. In either case, coordination is a key interaction to be considered in the design of novel antimalarial drug candidates. Quinoline methanol antimalarial drugs coordinate to ferriprotoporphyrin IX via their alkoxide oxygen atom in both the solid-state and non-aqueous solution.