Kõu Timpmann

Flexible color adaptation to available ecological niches is vital for the photosynthetic organisms to thrive. Hence, most purple bacteria living in the shade of green plants and algae apply bacteriochlorophyll a pigments to harvest near infra-red light around 850–875 nm. Exceptions are some Ca²⁺-containing species fit to utilize much redder quanta. The physical basis of such anomalous absorbance shift equivalent to ~5.5 kT at ambient temperature remains unsettled so far. Here, by applying several sophisticated spectroscopic techniques, we show that the Ca²⁺ ions bound to the structure of LH1 core light-harvesting pigment–protein complex significantly increase the couplings between the bacteriochlorophyll pigments. We thus establish the Ca-facilitated enhancement of exciton couplings as the main mechanism of the record spectral red-shift. The changes in specific interactions such as pigment–protein hydrogen bonding, although present, turned out to be secondary in this regard. Apart from solving the two-decade-old conundrum, these results complement the list of physical principles applicable for efficient spectral tuning of photo-sensitive molecular nano-systems, native or synthetic.

Spectrally-resolved pico- and nanosecond fluorescence kinetics of photosystem II reaction center (RC) preparations and RC together with CP47 proximal antenna have been studied at room temperature and at 77 K. The following conclusions have been made. (1) At room temperature the average trapping time proportional to the charge separation time and the number of cooperating pigments is observed. This time is equal to 13 +/- 3 ps in RCs and 25 +/- 10 ps in RC + CP47 complexes. (2) All other decays at room temperature are of a recombination origin and reflect complex relaxation of the metastable radical pair state. (3) At low temperatures an energetically directed excitation transfer, qualitatively very similar to the one observed in the core antenna of some purple bacteria, is observed. This energy transfer is relatively slow with an apparent transfer time 10 - 20 ps at 77 K.