The triplet state of chlorophyll-a in whole algal cells

The triplet state of chlorophyll-a (Chl-a) can be observed at 4K in intact algal cells using optically detected magnetic resonance (ODMR).In this Thesis experiments are described, to determine, to which kind of physically distinguishable Chl-a molecules, involved in the process of photosynthesis in algae, the observed triplet state belongs. From the wavelength of the fluorescence (~715-725 nm in intact algal cells), where the Chl-a triplet is detected, it appears, that the triplet is located within photosystem I.In the photosystem I containing chlorophyll-protein complex (CP-I), isolated from the blue-green alga Anacystis nidulans , the triplet state of Chl-a is also observed. This triplet has a number of properties in common with the triplet observed in intact cells. low temperature fluorescence spectra and triplet zerofield splitting parameters of the triplet detected in this spectral region, are identical for the alga and its complex.For CP-I it is demonstrated by light minus dark absorption difference spectroscopy and ESR, that the reaction centers are in oxidized state under ODMR conditions. From this observation it is concluded, that the triplet is located in the antennapigment system in the intact algal cells, as well as in the chlorophyll-protein complexIn the ODMR spectrum of intact cello the triplet signals of Chl-a appear with opposite sign compared to those of Chl-a in vitro , i.e. in intact cells the triplet is detected as an increase of fluorescence, while in vitro it is detected as a decrease.Several models are conceivable to explain this sign reversal. The mechanism of energy transfer between parts of the pigmentsystem is considered to be the most probable candidate for the cause of the observed sign reversal of both ODMR signals. As yet, it cannot be completely excluded, however, that the sign reversal is caused by altered kinetics as a result of hydrogen-bonding of amino acids of the surrounding protein to ring V keto- and/or estergroups of Chl-a.The occurrence of the radical pair mechanism is unlikely, because the observed triplets are located in the antennapigment and not in reaction centers.The theory of radiationless transitions is applied to Chl-a, using calculations of Petke and coworkers, of the energies and configurational composition of the ground and excited states of Chl-a. This offers an explanation, why only two of the expected three triplet signals are observed in the ODMR experiment. Furthermore, predicted effects of ligation and hydrogen-bonding to Chl-a molecules on kinetics of Chl-a in vivo are in good agreement with the results from experiments on complexes and aggregates of Chl-a in solution.Finally, the presence of paramagnetic metal ions, especially Cu ²⁺, in the vicinity of the triplet state of Chl-a in vivo causes a line-broadening of ODMR spectra as well as a slow-down of the rate of photochemical breakdown of Chl-a due to irradiation with blue light at low temperature. These observations indicate that the triplet state of Chl-a in algae is involved in this photochemical breakdown.