By a reverse flow of protons, the electrochemically stored energy is used for ATP synthesis (Mitchell 1966). The potential gradient can also be dissipated by the basal ion efflux, which depends on the electrical permeability of the membranes. The rise and decay of the transmembrane electrical difference can be FRAX597 followed by the electrochromic absorbance changes (ΔA515) of the pigments embedded in the membrane, which correlates with the
transmembrane electric field (Junge 1977; Witt 1979). We have obtained ΔA515 decay times comparable with those observed for barely under similar conditions (Garab et al. 1983). The initial amplitude of ΔA515 is lower for dgd1 than JSH-23 concentration for WT, but this can be attributed to the decreased content of PSI reaction centers in the mutant (Ivanov et al. 2006). These data are also in line with the data of Härtel et al. (1997) showing that dgd1 Selleckchem NCT-501 is capable of maintaining a low lumenal pH, needed for the xanthophyll cycle operation. Effects of DGDG on the thermal stability of thylakoid membranes The temperature dependencies of the various CD bands reveal that whereas LHCII (characterized by (−)650 nm Chl b excitonic band) preserved its stability, the Ψ-type (CD(685–730) and CD(685–671)) and the excitonic Chl a CD bands
(CD(448–459) and CD(448–438)) are significantly less stable in the mutant (Fig. 1; Table 1). The latter two Chl a CD signals most probably originate from the core complexes of PSII and/or PSI which bind only Chl a (Chitnis 2001; Smith et al. 2002; Ben-Shem et al. 2003), and thus, their thermal behavior indicates a lower stability of these complexes in the mutant than in the WT. This was further confirmed by green gel electrophoresis, which clearly demonstrates that the thermal degradation of LHCII follows the same pattern in WT and dgd1, but PSI degrades faster in dgd1 than in WT (Fig. 2). This fact strongly next suggests that the lower thermal stability of Chl a excitonic CD bands (see above) is at least partially due to the faster degradation/disassembly
of PSI in dgd1 than in WT. Faster degradation of the photosynthetic complexes in dgd1 is also confirmed by the temperature dependence of the Chl a average fluorescence lifetime above 45°C (Fig. 4). This dependence is rather similar to the one observed for the CD bands at around 450 nm (Fig. 1b; Table 1) and, hence, it can be suggested that PSI degradation significantly contributes to it. These data are complementary to the observation of Guo et al. (2005) who revealed that PSI in dgd1 thylakoids is more susceptible to chaotropic agents and demonstrated the presence of PSI lacking LHCI and subunit PsaD, which could be detached from the core complex with mild detergents.