Spectroscopic studies of the chlorophyll d containing photosystem I from the cyanobacterium, Acaryochloris marina

Abstract

Absorbance difference spectroscopy and redox titrations have been applied to investigate the properties of photosystem I from the chlorophyll d containing cyanobacterium Acaryochloris marina. At room temperature, the (P740(+)-P740) and (F(A/B)(-)-F(A/B)) absorbance difference spectra were recorded in the range between 300 and 1000 nm while at cryogenic temperatures, (P740(+)A(1)(-)-P740A(1)) and ((3)P740-P740) absorbance difference spectra have been measured. Spectroscopic and kinetic evidence is presented that the cofactors involved in the electron transfer from the reduced secondary electron acceptor, phylloquinone (A(1)(-)), to the terminal electron acceptor and their structural arrangement are virtually identical to those of chlorophyll a containing photosystem I. The oxidation potential of the primary electron donor P740 of photosystem I has been reinvestigated. We find a midpoint potential of 450+/-10 mV in photosystem I-enriched membrane fractions as well as in thylakoids which is very similar to that found for P700 in chlorophyll a dominated organisms. In addition, the extinction difference coefficient for the oxidation of the primary donor has been determined and a value of 45,000+/-4000 M(-1) cm(-1) at 740 nm was obtained. Based on this value the ratio of P740 to chlorophyll is calculated to be 1 : to approximately 200 chlorophyll d in thylakoid membranes. The consequences of our findings for the energetics in photosystem I of A. marina are discussed as well as the pigment stoichiometry and spectral characteristics of P740.

Redox potentials of primary electron acceptor quinone molecule (Q A ) − and conserved energetics of photosystem II in cyanobacteria with chlorophyll a and chlorophyll d

“…However, experimental evidence for this interpretation was completely indirect. Moreover, it has been shown that the potential of P740 in A. marina is almost the same as that in Synechocystis (+435 mV) (6,13). The cytochrome b 6 f complexes are situated between the PS II and PS I complexes, although we have no data on the potential of these complexes in A. marina.…”

“…In PS I, the redox potential of P740 (corresponding to P700 for other oxygenic photosynthetic organisms) is the same as that of P700 (6,13), suggesting that no specific mechanism exists to compensate for the lower energy from the longer wavelength absorption of Chl d. Thus, one can conclude that the redox potentials of the special pair in oxygenic photosynthetic organisms are determined by the oxidizing side of the PS and not by the reducing side. These considerations suggest that a determining factor for the potential of the special pair with a specific pigment(s) might be related to an oxidation reaction that includes ligands and/or hydrogen bonds from the amino acid(s) of proteins to the pigments.…”

Phosphorescence study of chlorophyll d photophysics. Determination of the energy and lifetime of the photo-excited triplet state. Evidence of singlet oxygen photosensitization

“…Tomo et al 2007;Schenderlein et al 2008). This reflects in a loss of between 75 and 95 meV (3-4 k b T) in energy available for the photochemical reactions, which is intriguing from the evolutionary point of view (Hu et al 1998;Tomo et al 2007;Santabarbara et al 2007;Schlodder et al 2007;Schenderlein et al 2008).…”

“…Though, chemically, Chl d is distinct from Chl a only for a formyl substitution at C 3 position of ring I of the tetra-pyrrol ring (Holt and Morley 1959), the energy of the lowest singlet excited state is redshifted by more than 30 nm in pigment solutions (Nieuwenburg et al 2003) and by 30-40 nm in the photosynthetic apparatus of A. marina (e.g. Tomo et al 2007;Schenderlein et al 2008). This reflects in a loss of between 75 and 95 meV (3-4 k b T) in energy available for the photochemical reactions, which is intriguing from the evolutionary point of view (Hu et al 1998;Tomo et al 2007;Santabarbara et al 2007;Schlodder et al 2007;Schenderlein et al 2008).…”

“…Although the chemical nature of this chlorophyll has been known for over five decades (Manning and Strain 1943;Holt and Morley 1959), its involvement in biologically relevant processes has not been recognized until the discovery of the novel class of photosynthetic phototrophic organisms of which A. marina is the best characterized (e.g., Miyashita et al 1996;Kühl et al 2005). It has been demonstrated that in this organism, Chl d not only plays a major role in light harvesting, but it is part of the photochemical reaction centers (RC) of both Photosystem I and Photosystem II (PS I and PS II) (e.g., Hu et al 1998;Tomo et al 2007;Santabarbara et al 2007;Schenderlein et al 2008). Though, chemically, Chl d is distinct from Chl a only for a formyl substitution at C 3 position of ring I of the tetra-pyrrol ring (Holt and Morley 1959), the energy of the lowest singlet excited state is redshifted by more than 30 nm in pigment solutions (Nieuwenburg et al 2003) and by 30-40 nm in the photosynthetic apparatus of A. marina (e.g.…”