Efficiency of T-T energy transfer with participation of chlorophyll-like molecules

Abstract

EFFICIENCY OF T-T ENERGY TRANSFER WITH PARTICIPATION OF CHLOROPHYLL-LIKE MOLECULES E. I. Sagun, A. P. Losev, and N. D. Kochubeeva UDC 535.35 Recently, it has become clear that the spectral characteristics, and hence also the func- tional role of chlorophyll in vivo is to a large extent dependent on both the structural organiza- tion and the character of the molecular environment of the pigment under native conditions. In this sense, the ability of the chlorophyll molecule to add different numbers of ligands to form a 5- or 6-coordinated state of the Mg atom acquires special importance. By using the NMR and IR spectrscopy methods [I, 2] it was shown that the accurate position of the long-wave absorption band of chlorophyll depends on both the nature of the solvent and the number and type of ligands bound to the Mg atom. The authors of [3] believes that for most of chlorophyll-like molecules, the bathochromic shift of the absorption bands in a given sol- vent may be due to increase in the coordination number of the Mg atom in the presence of nucleophilic additives. They proposed an empirical method with which it was possible to find the state of the central metal atom in the molecule of Bacteriochlorophyll (BCHI). This pro- cedure was confirmed in [4] by the data on resonance Raman scattering spectroscopy. Since the coordination of the ligand molecules to chlorophyll caused noticeable changes in the duration of the excited singlet state [5, 6], and in the absorption and fluorescence spectra [7-9], we should have expected that the properties of the triplet state would also depend on the condi- tions at which the pigment molecule is present. According to our data, the monomolecular rate constant of the nonradiative deactivation of the triplet state of the chlorophyll molecule q noticeably differs for solvents in which the coordination number of Mg is 5 (q = 4.5.102 sec -I) and 6 (q = 5.9"102 sec -I) [i0, ii]. Similar results were obtained in the study of phosphor- escence of mono- and disolvates of chlorophyll [12-14]. In [15], phosphorescence was discov- ered from two closely located triplet levels of Zn-pheophytins "a" and "c". It was shown in [16-18] that the electronic structure and also the parameters of the triplet state splitting in a zero field, at temperatures of 2 and 4.2~ change, depending on the coordination state of the Mg atom, and hence on the number of ligands attached to the chlorophyll ring. In [19] results were also obtained which confirm the view of the authors of this paper that the chlorophyll molecules can exist in two forms, to which the total diversity of the monomeric pigment in solutions can be attributed. In nonpolar and weakly polar solvents, chlorophyll exists in a ligandless form (ChI'L 0) with a coordination number of the Mg atom equal to 4, while in strong electron-donor solvents, due to increase in the coordination number of Mg to 5, chlorophyll exists in the form of a monoligand compound (ChI.LI). In an investigation of the T-T energy transfer [20], we have already found that the efficiency of transfer from the chlorophyll molecule to B-carotene in pyridine at 20~ is anomalously low in the whole series of solvents studied. Since energy transfer over the triplet levels occurs during con- tact of the interacting shells [21, 22], it was assumed that the pyridine molecules bound to chlorophyll and oriented axially to the plane of the ~-electronic macrocycle [23], steri- cally hinder the approach of the chromophoric groups of the triplet energy donor and acceptor. This observation showed that the T-T energy transfer can serve as a basis for a new method of studying the state of solvation of chlorophyll-like molecules in solutions. We therefore systematically studied the influence of the addition of ligands to chlorophyll molecules on the efficiency of migration of the triplet energy in liquid solutions. The donors of the triplet energy (D) were pheophytin "a" and its Mg (chlorophyll "a") and Zn-complexes, while p-carotene was the acceptor (A). Chlorofphyll "a" was extracted from nettle (Urtica) and was purified by chromatography on sugar [24]. Pheophytin "a' was obtained by treating spectrally pure chlorophyll "a" with 10% hydrochloric acid in an ether-acetone solution, washing several times with water, and drying the ether solution over anhydrous Na2SO ~. Zn-pheophytin t'a" was obtained by heating pheophytin "a" with Zn(CHsCOO) ~ in propanol, followed by chromatographic purification on sugar. Isopropanol and pyridine, purified by methods described in [25], were used as solvents. The viscosities of the solvents at the working temperatures were determined Translated from Zhurnal Prikladnoi Spektroskopii, Volo 42, No. 5, pp. 779-785, May, 1985. Original article submitted January 13, 1984o