Role of functional groups and surfactant charge in regulating chlorophyll aggregation in micellar solutions

A series of modified chlorophylls, namely, pyrochlorophyll a, Zn-pheophytin a, Zn-pheophorbide a, chlorophyllide a, [3-acetyll]-chlorophyll a, and bacteriochlorophyll a, have been investigated in micellar solutions. The study is aimed at establishing the role played by the different functional groups of the chlorophyll in the molecular organization of chlorophylls in a microheterogeneous environment. The surfactants (AOT, CTAB. and Triton X-100) have been chosen mainly on the basis of the different charges carried by their polar heads, to study the effect of a point charge on the spectral characteristics of the pigments. Besides optical techniques used to investigate the spectral properties of the pigments, the state of the micellar system was studied by resonance light scattering (RLS) and NMR (self-diffusion and relaxation time measurements). The results of the UV-vis measurements evidence the role played by the functional group, or the chlorophylls in the formation of the different species in solution. In view of the few cases reported in the literature on blue-shifted chlorophyll species, special attention has been devoted, to the behavior of the modified chlorophylls in CTAB where a species absorbing around 6421 nm is always formed. CID, RLS, and NMR data identify this species as a pigment-surfactant aggregate, in which the positive charge of the surfactant interacts with the C-13(2) ketoester group of a monomeric chlorophyll.