Elucidation of monomerization effect of PVP on chlorin e6 aggregates by spectroscopic, chemometric, thermodynamic and molecular simulation studies.

Molecular aggregation in aqueous media is one of the factors which largely affects the efficacy of photosensitizers in photodynamic therapy. Chlorin e6 (Ce6) in aggregated form is known to exhibit markedly reduced therapeutic effects. In the present study, aggregate to monomer conversion of Ce6 was investigated as a function of pH and polyvinylpyrrolidone (PVP) concentration by simple absorption and fluorescence spectroscopic techniques. Aggregation of Ce6 was observed in the pH range of 2 to 6 as indicated by changes in UV-vis absorption spectra, fluorescence emission spectra and relative quantum yield. Novel chemometric approach was considered for determining the relative monomerization efficiency of different grades of PVP. The chemometric analysis and binding constant study both strongly revealed that the Ce6-PVP complex was more efficiently formed with PVP of the lowest molecular weight (K17). Thermodynamic parameters, such as the heat of entropy and enthalpy, showed that complex formation was largely attributed to hydrophobic interaction between Ce6 and PVP. This was found to be consistent with the results obtained from molecular simulation study.