Aggregation of chlorophylls a and b in polymer films and monolayers

Abstract

Abstract Polarized absorption and fluorescence spectra as well as lifetime of fluorescence (τ) of the mixtures of chlorophyll a (Chl a) and chlorophyll b (Chl b) in anisotropic polyvinyl alcohol (PVA) films were measured. Two mixtures with various concentrations ratios of Chl a to Chl b were investigated. For comparison, the fluorescence excitation and emission spectra of Chl b + phytol mixture in Langmuir—Blodgett films were also measured. At high Chl a and low Chl b concentrations (MI) the following three forms of Chl a aggregates occurred: ‘dry’ Chl a (aggregated with polymer) with a fluorescence maximum at 676 nm (F676), hydrated dimer (Chl a 2H2O)2 or (Chl a H2O)2 (F700–705 nm), and oligomers of hydrated dimers (F750–770 nm). Chl b in the same matrix also occurs in at least three forms: ‘dry’ (F653 nm), ‘wet’ - hydrated aggregates (F662 nm) and aggregated forms with the fluorescence at 710–720 nm and/or 730–740 nm regions. The orientation of various chlorophyll forms in the PVA matrix is different. The efficiency of excitation energy transfer from Chl b to various aggregates of Chl a is different. The yield of energy transfer from Chl b to Chl a oligomers is high. Fluorescence decay of various aggregates is different and the experimental decay can be analysed always on two or three exponential components. Results are discussed on the basis of previous data concerning the monolayers of Chl a with phytol and literature data concerning aggregation of chlorophylls in various model systems. In the spectra of chlorophyll in several model systems, as follows from the literature, it is possible to distinguish the contributions from the three or even more aggregated forms of pigment similar to those reported by us. Some similarities between aggregated forms or chlorophylls and pigment forms observed in organisms are noted. In polymer films, aggregated forms of pigments are stable and oriented; therefore, their properties and the condition of their formation can be established.