Aggregated enhanced Raman scattering in Fe(III)PPIX solutions: the effects of concentration and chloroquine on excitonic interactions.

Resonance Raman spectra of hematin and hemin solutions are reported for 413 and 514 nm excitation wavelengths. Enhancement of A1g modes (1569 and 1370 cm(-1)) and B1g modes (1124 and 755 cm(-1)) as a function of increased concentration are observed when irradiating with 514 nm laser excitation but not 413 nm. This can be rationalized by considering an excitonic coupling mechanism. As the concentration of hematin increases there is an increased probability of supramolecular interactions between iron(III) protoporphyrin IX (Fe(III)PPIX) units occurring. The Fe(III)PPIX concentration reaches a saturation point in solution and excitonic coupling reaches a maximum causing the enhancement profile to plateau when applying 514 nm excitation. In contrast, when using 413 nm excitation there were no changes in band intensity with increased concentration showing that excitonic coupling through supramolecular interactions for aggregated solutions is wavelength dependent. Electronic absorption spectra show that as the concentration of Fe(III)PPIX increases in solution the Soret band is slightly blue shifted and the Q-band significantly broadens supporting the excitonic hypothesis. Understanding the mechanism that accounts for the Raman photophysical behavior of hemes at high concentrations provided an indirect method to monitor antimalarial drug interactions. A second aim was to investigate chloroquine binding to Fe(III)PPIX-OH/H2O monomers, pi-pi dimers and micro-oxo dimers formed in highly concentrated solutions approaching those of the digestive vacuole of the P. falciparum malaria parasite using excitonic Raman enhancement. It was hypothesized that the Raman excitonic enhancement mechanism could be impeded in heme aggregated solutions by the addition of chloroquine. This would result in a reduction in heme bands associated with the A1g modes including nu4. Resonance Raman spectra recorded using 514 nm excitation show that chloroquine (CQ) acts as a molecular spacer and binds noncovalently through dispersion interactions giving rise to pi-pi interactions, between micro-oxo dimer units of Fe(III)PPIX as evinced by the decrease in intensity of nu4 in the Raman spectrum as a function of increasing CQ mole ratio. In comparison, electronic spectra show that CQ can bind to the unligated face of Fe(III)PPIX-OH/H2O monomers, potentially reducing the formation of pi-pi dimers. This study has important implications in determining the effectiveness of potential antimalarial compounds that are thought to exert their effectiveness by binding through supramolecular interactions to the unligated faces of Fe(III)PPIX-OH/H2O monomers and micro-oxo dimers.