[Molecular mechanism of self-assembly of aggregated bacteriochlorophyll c].

The intermolecular interaction of bacteriochlorophyll c and its pheophytin was studied in nonpolar solvents and solid films with the aid of absorption and infra-red (in the region of 1800--1600 and 3800--3000 cm-1) spectra. The influence of water removing and its addition on these spectra has been investigated. Besides the effect of pyridine treatment and pigment concentration were examined. The self-assemblage of all types of bacteriochlorophyll c aggregated forms absorbing in the range 680--745 nm is due to the formation of intermolecular bonds in which keto groups of cyclopentanone rings take part. Keto groups form coordinate bonds with the central magnesium atom (keto-C = O...Mg). Hydroxyl groups interact coordinately with magnesium and simultaneously form hydrogen bonds with pyrrol nitrogen. In contrast to chlorophyll a and bacteriochlorophyll a, water molecules in the case of bacteriochlorophyll c do not participate in the intermolecular bond formation in the course of long-wave aggregated forms production. The thermostability of bacteriochlorophyll c aggregates and their rather high stability to desaggregating agents is related to the mentioned peculiarities of their structure. Bacteriopheophytin c in any state (solution or solid film) is not capable to form intermolecular bonds by its carbonyl groups and long-wave aggregates. The specific features of the assemblage of bacteriochlorophyll c aggregates modelling antenna of the green photosynthetic bacteria are discussed.