Energy Transfer and Charge Separation in Photosystem I: P700 Oxidation Upon Selective Excitation of the Long-Wavelength Antenna Chlorophylls of Synechococcus elongatus

Abstract

Photosystem I of the cyanobacterium Synechococcus elongatus contains two spectral pools of chlorophylls called C-708 and C-719 that absorb at longer wavelengths than the primary electron donor P700. We investigated the relative quantum yields of photochemical charge separation and fluorescence as a function of excitation wavelength and temperature in trimeric and monomeric photosystem I complexes of this cyanobacterium. The monomeric complexes are characterized by a reduced content of the C-719 spectral form. At room temperature, an analysis of the wavelength dependence of P700 oxidation indicated that all absorbed light, even of wavelengths of up to 750 nm, has the same probability of resulting in a stable P700 photooxidation. Upon cooling from 295 K to 5 K, the nonselectively excited steady-state emission increased by 11- and 16-fold in the trimeric and monomeric complexes, respectively, whereas the quantum yield of P700 oxidation decreased 2.2- and 1.7-fold. Fluorescence excitation spectra at 5 K indicate that the fluorescence quantum yield further increases upon scanning of the excitation wavelength from 690 nm to 710 nm, whereas the quantum yield of P700 oxidation decreases significantly upon excitation at wavelengths longer than 700 nm. Based on these findings, we conclude that at 5 K the excited state is not equilibrated over the antenna before charge separation occurs, and that approximately 50% of the excitations reach P700 before they become irreversibly trapped on one of the long-wavelength antenna pigments. Possible spatial organizations of the long-wavelength antenna pigments in the three-dimensional structure of photosystem I are discussed.

Defining the Far-red Limit of Photosystem I

“…All data points were taken using the same power of the excitation laser flashes (20 mJ) at different wavelengths. This result extends the action spectrum for PS I almost 80 nm further out into the red if compared with previous reports (53). It is noteworthy that P 700 was oxidized in almost 70% of the centers when using excitation light at 780 nm where 532 99 100 730 87 39 780 65 2 820 20 0…”

“…This shows that charge separation can be driven with light 140 nm above the absorption peak at 700 nm for P 700 and more than 100 nm above the absorption band of the so-called red Chls (see below) (31). Our results also extend the action spectrum of PS I 80 nm further out to the red than reported previously (53).…”

“…In this study, we investigated the far-red limit and efficiency for oxidation of P 700 , the primary donor in PS I. PS I-induced photochemistry was earlier reported at 760 nm in Synechococcus elongatus (53). In contrast to this, we show that, in thylakoid membranes from spinach where both PS I and PS II are present together with their respective large membrane-bound antenna systems, P 700 ϩ can be formed in almost all PS I centers when using excitation light up to 780 nm.…”

Species-specific Differences of the Spectroscopic Properties of P700

“…For example, although the flash-induced (P700 ϩ⅐ -P700) absorbance difference spectra of T. elongatus and Spinacia oleracea (spinach) are rather similar (15,16), those recorded for Anabaena variabilis (15) and C. reinhardtii (17, 18) look different.…”

Carotenoids assist in assembly and functions of photosynthetic complexes in cyanobacteria

“…They were modelled as β-carotene (Jordan et al 2001) which has been previously shown as the dominant carotenoid in PSI (Palsson et al 1998). Seventeen carotenoid molecules showed the all-trans configuration, 2 are 9-cis, and 3 contain one of 9, 9'-cis 9, 13'-cis and 13-cis bonds.…”