PubMed · 2084472
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H M Shapiro. 1990. Cell membrane potential analysis.. https://doi.org/10.1016/s0091-679x(08)60509-9
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Effects on the PS II acceptor side caused by exposure to strong white light (180 W/m2) of PS II membrane fragments (spinach) at pH 6.5 and 0 degrees C were analyzed by measuring low temperature EPR signals and flash-induced transient changes of the fluorescence quantum yield. The following results were obtained: (a) the extent of the light induced g = 1.9 EPR signal as a measure of photochemical Fe2+QA- formation declines with progressing photoinhibition. The half-life of this effect is independent of the absence or presence of an exogenous electron acceptor during the photoinhibitory treatment; (b) in samples photoinhibited in the absence of an electron acceptor and subsequently incubated with K3[Fe(CN)6] in the dark, the extent of the g = 8 EPR signal (reflecting the oxidized Fe3+ form of the endogenous non-heme iron center) and of the flash-induced change of the fluorescence yield (as a measure of fast electron transfer from QA- to Fe3+ after the first flash; [see (1992) Photosynth. Res. 31, 113-126] exhibits the same dependence on photoinhibition time as the g = 1.9 EPR signal; (c) in samples photoinhibited in the presence of an exogenous electron acceptor, the signals reflecting Fe(3+)-formation and fast electron transfer from QA- to Fe3+ decline faster than the g = 1.9 EPR signal. These results provide for the first time direct evidence that the endogenous non-heme iron center located between QA and QB is susceptible to modifications by light stress. The implications of this finding will be discussed.
Potentiometric titrations, at ionic strengths (I) ranging from 0.0057 to 0.336, suggest that Ca2+, Cu2+, Li+ and Na+ bind to heparin in a manner that depends on cation identity. These interactions were less affected by the value of I than those of heparin with Mg2+, which binds weakly below I0.050 and with K+, which binds weakly at I0.0057. Of the interactions studied, that of heparin with Cu2+ was the least readily reversible.