[Aggregation of 4-vinyl-protochlerophyll and protochlorophyll in solutions].

Abstract

The addition of water to dioxane solution of protochlorophyll (PChl) and 4-vinyl-protochlorophyll (4VPChl) results in a number of transformations related with pigment molecule associations into polymer formations and with their conversion. By variation of pigment concentration and composition of binar mixture of solvents a set of aggregated forms with different electronic spectra can be obtained. At the first stage the monomeric pigment converts into polymer with long-wavelenght absorption maximum of 633 nm (P633). During 1-3 hours P633 converts into other aggregated forms: either P639 or P645 depending on the proportion of dioxane and water. The pigment aggregates are characterized by fluorescence (quantum yield less than or equal to 1%) with maxima close to the main maxima of absorption spectra. Alongside with the main aggregated forms of PChl and 4VPChl the presence of a small number of longer-wavelength aggregates in solutions is established by fluorescence spectra. The investigation of dependence of fluorescence spectra on excitation wavelength, as well as of fluorescence excitation spectra of pigments in binary mixture shows that the long-wavelength aggregates can form part of complex associate and be effective trapping centres of electronic excitation of shorter wavelength aggregated forms. The phenomenon of 4VPChl association in concentrated solutions of castor oil (c greater than-10(-2) m/l) is discovered, which is especially marked after a long storage of the solutions at lower temperature (t=-2 degrees C). Under these conditions the two aggregated forms P640 and P650 alongside with monomer are present. The aggregate P650 is characterized by fluorescence with maximum of 655 nm and spectrally similar to the natural form of photoactive protochlorophyll. The experimental data show that the protochlorophyll pigments form a set of aggregated forms and that the medium conditions favour the predominance of some of the forms and their interconversion. The results obtained evidence that the main reason of the long-wavelength shift of electronic spectra for PChl natural forms is the intermolecular interaction related with the association of pigment monomeric molecules.