Comparison of 19 aminoquinolines supports the hypothesis that chloroquine and related antimalarials act by complexing ferriprotoporphyrin IX (Fe(III)PPIX), inhibiting its conversion to beta-hematin (hemozoin) and hence its detoxification. The study suggests that a basic amino side chain is also essential for antiplasmodial activity. 2- And 4-aminoquinolines are unique in their strong affinity for Fe(III)PPIX, and attachment of side chains to the amino group has relatively little influence on the strength of complex formation. Association with Fe(III)PPIX is necessary, but not sufficient, for inhibiting beta-hematin formation. Presence of a 7-chloro group in the 4-aminoquinoline ring is a requirement for beta-hematin inhibitory activity, and this is also unaffected by side chains attached to the amino group. In turn, beta-hematin inhibitory activity is necessary, but not sufficient, for antiplasmodial activity as the presence of an aminoalkyl group attached to the 4-amino-7-chloroquinoline template is essential for strong activity. We thus propose that the 4-aminoquinoline nucleus of chloroquine and related antimalarials is responsible for complexing Fe(III)PPIX, the 7-chloro group is required for inhibition of beta-hematin formation, and the basic amino side chain is required for drug accumulation in the food vacuole of the parasite.

Chemical analysis has shown that Plasmodium falciparum trophozoites contain 61+/-2% of the iron within parasitized erythrocytes, of which 92+/-6% is located within the food vacuole. Of this, 88+/-9% is in the form of haemozoin. (57)Fe-Mössbauer spectroscopy shows that haemozoin is the only detectable iron species in trophozoites. Electron spectroscopic imaging confirms this conclusion.

The crystal structures of three porphyrin diacid species, [H 4 OEP](ClO 4 ) 2 , [H 4 TPP](ClO 4 ) 2 , and [H 4 TMP]-(ClO 4 ) 2 , have been determined from low-temperature X-ray diffraction data to delineate how the peripheral substituents of the porphyrin affect the overall molecular flexibility. [H 4 OEP](ClO 4 ) 2 (|C b | ) 0.46 Å), [H 4 TMP](ClO 4 ) 2 (|C b | ) 0.67 Å, molecule 1), and [H 4 TPP](ClO 4 ) 2 (|C b | ) 0.93 Å) show increasingly saddled core conformations with effective D 2d symmetry. The mean porphyrin-aryl group dihedral angles in [H 4 TPP](ClO 4 ) 2 and [H 4 TMP](ClO 4 ) 2 (molecule 1) are 27(2)°and 63(13)°, respectively. The steric bulk of the mesityl substituents in [H 4 TMP] 2+ limits the range of observed porphyrin-aryl group dihedral angles to >50°and, consequently, the magnitude of the core distortion. [H 4 TMP] 2+ is therefore less flexible than [H 4 TPP] 2+ . Molecular mechanics calculations, using a modified version of MM2 (87) and a newly developed force field for porphyrin diacids, correctly predict that the flexibility of mesotetraaryl porphyrin diacids decreases as the steric bulk of the peripheral substituentsrelated minima with D 2d -saddled and C 2h -stepped core conformations. The in Vacuo strain energy barrier to inversion of the lowest-energy D 2d -saddled conformation increases from 0.45 kcal/mol in [H 4 porphine] 2+ to 1.9 kcal/mol in [H 4 T-2,6-Cl 2 PP] 2+ . The calculations indicate that the relative stability and magnitude of distortion of the D 2d isomer increases as the steric bulk of the peripheral substituents increases; [H 4 OEP] 2+ is therefore calculated to be less distorted than [H 4 TPP] 2+ , in agreement with the X-ray structures of these species.