Kinetic characterization of T-jump thermoluminescence in isolated chloroplasts

Estimation of the activation energy for millisecond delayed fluorescence from uncoupled chloroplasts

“…The weighted mean for these values of Eac over the whole temperature range studied is 0.64 eV. This value agrees well with the value of 0.68 -+ 0.08 eV recently published for 0.5 to 1 sec delayed fluorescence [18] but does not agree with previous estimates of the activation energy barrier for one msec delayed fluorescence [15,16]. From the earlier work it was reported that the barrier for the msec emission process was much smaller, a conclusion which can probably be partly accounted for both by the presence of/2xp and its temperature sensitivity.…”

“…The corrected value of Eac compares reasonably well with that reported by Malkin and Hardt [18] but unfortunately their elegant analysis was not applied to studies with uncoupled systems and failed to make allowances for time dependent changes in prompt fluorescence yield, factors which are probably signifi-cant for the part of the decay that they investigated (0.5-1 sec). Accepting that msec delayed fluorescence results from a back reaction in the photosystem two reaction centre and realising that more than one oxidation state of Z is probably involved [21,22] then a value for Eac of 0.61 eV indicates that about 1.19 eV can be extracted from the 1.8 eV available from the singlet excited chlorophyll for driving primary reactions in photosynthesis, a value which is consistant with theoretical arguments [ 13].…”

“…1 that if a suspension of preilluminated chloroplasts is subjected to a sudden increase of temperature then the intensity of delayed fluorescence should increase. Such a stimulation by a T-jump was first observed by Mar and Govindjee [20] and has since been studied in some depth for 10 sec [17] and 0.5 to 1 sec [18] delayed fluorescence. As shown in fig.…”

“…1 are not all independent, but it does nevertheless show the involvement of the high-energy state in reducing the activation energy Eac: a complication which clearly perturbs the measurement of Eac and which has been neglected in earlier temperature studies of msec delayed fluorescence [15,16]. The object of the work presented in this paper was to measure Eac in the absence of the high-energy state by the T-jump method [17,18] and discuss the results in terms of the concepts presented above.…”

“…It has been assumed also that the formation and non-radiative degradation of the precursors Z ÷ and Q-are temperature insensitive and that their concentrations do not change during the T-jump. There is no way to check or correct for these effects because of the nature of the experiments and the methods used, but Malkin and Hardt [18] have emphasised the necessity for such corrections which they were able to make for T-jump measurements at longer dark times.…”