Light-dark transient changes

Fig. 5. Indirect redox titration of FeS-X (A) Light-induced EPR changes of P700 in TSF-I particles as a function of redox potential at pH 10 and at 15 K (B) Plot of the extent of dark decay of the EPR signals in (A) at pHs 8, 9 and 10 (C) Plot of the initial amplitude of the light-induced EPR changes in (A) at pHs 8, 9 and 10. Figure source Ke, Dolan, Sugahara, Hawkridge, Demeter and Shaw (1977) Electrochemical and kinetic evidence for a transient electron acceptor In the photochemical charge separation in photosystem I, in Photosynthetic Organelles (special issue of Plants Celt Physiol) pp. 195, 196.

Typical experiments measure transient concentration changes at a fixed position and concentration profiles. Transient concentration changes are measured during photosynthesis with the fast light-dark shift (Revsbech 1983) or for the in situ determination of diffusion coefficients (Cronenberg 1994a, b). For these types of experiments, we refer to the literature the interpretation of profiles will be discussed below. 

Upon addition of [TMPD+Asc] to re-reduce P700 in the dark, an absorbance-change transient with a biphasic decay, typical ofnon-cyclic electron flow could be elicited by a light flash, as shown by the lower transient in Fig. 3 (D). The ratio ofthe amplitude ofthe slow-decay portion ofthe absorbance change due to reduction ofP700, to that ofthe fast-decay portion, due to oxidation ofP430 , is 4 1. 

The light-minus-dark, or reduced-minus-oxidized, difference spectra for FeS-X and P430 constructed from flash-induced absorbance-change transients and shown in Fig. 14 (A) both show minima in the 420-430 nm region the differential molar absorptivities were estimated to be 13,000 500 and 15,000 500 M cm for P430 and FeS-X, respectively. More importantly, the difference spectrum of FeS-X differs from that ofP430 by a narrow shoulder in the 410-418 nm region and a diminished absorbance above 450 nm. 

Fig. 3. (A) Difference spectrum constructed from absorbance change transients in PS-1 particles poised at —625 mV [in the presence of dithionite] in a pH 10 buffer. Each transient was induced by a 3-s iliumination (B) AA obtained directiy with a P700-enriched, PS-i particie in a conventionai, commerciai spectrophotometer in the light-minus-dark mode (C) AA between 350 and 750 nm measured at 200 K with a Triton-fractionated, PS-1 particle from pea chloroplasts the spectrum shown was obtained by first illuminating at 200 K for 45 m and then at 215 K for three 30-m incremental periods. Figure source (A) Swarthoff, Gast, Amesz and Buisman (1982) Photoaccumulation of reduced primary electron acceptors of photosystem I of photosynthesis. FEBS Lett 146 131 (B) Ikegami and Ke (1984) A 160-kilodalton photosystem-l reaction-center complex. Low temperature absorption and EPR spectroscopy of the early electron acceptors. Biochim Biophys Acta 764 75 (C) Mansfield and Evans (1985) Optical difference spectrum of the electron acceptor Ao in photosystem I. FEBS Lett 190 239.

When a dark-adapted leaf or a living plant cell is exposed to light, the intensity of the chlorophyll fluorescence shows characteristie ehanges. These changes were found by Kautsky and Hirseh in 1931 [259] and have been termed fluoreseenee induction, fluorescence transients, or Kautsky effeet [192, 193, 345]. The general behaviour of the fluoreseenee intensity is shown in Fig. 5.30. 

Complementary to the method of microwave-recovery, the method of micro-wave-induced delayed phosphorescence (MIDP) is sometimes used for studying population kinetics. MIDP is particularly suited for two-level systems in which only one of the levels is radiative, whereas the other, dark level, is long-living. The microwave recovery is mostly applied when both levels are radiative. In the MIDP experiment the exciting light is chopped (or pulsed). In the dark time, after optical excitation, a resonant microwave pulse is applied at the delay time f(j. At t(j> the population still present in the non-radiative level is in part transferred to the radiative level. Thus a phosphorescence intensity change is induced, the amplitude of which is proportional to the population present in the non-radiative level, at time Measurement of the amplitude decay with gives the decay transient for the non-radiative level. The (fast) decay of the delayed phosphorescence transient at times t>t is typical of the decay of the radiative level. 

The transient fluorescence rise after switching off the actinic light may be due to ATP-induced back electron transport to Q or by changes in the rates of direct and reverse electron transport between the donors of Photosystem 2. The appearance of this transient in Cl-CCP treated leaves indicates the second possibility (Pig. 2b). The amplitude of transient rise in the dark enhances with the increase of Cl-CCP concentration. Illiunination of Cl-CCP treated leaves by light 1 (after switching off the actinic light) sharply accelerates the dark relaxation of variable... 

From 5-11 hrs illumination (Figs. 1 f-i), the lag phase was replaced by a transient, rapid and almost complete oxidation, followed by a partial rereduction and a subsequent slow reoxidation, also on the order of hours. At about 1 hr before dark, the kinetics gradually became similar to those of the early morning. It is noteworthy that no changes in light intensity occur until the chamber dark-transition. 

FIGURE 25-33 Dopamine release during cocaine selfadministration, Rats were trained to press a lever owhead) to receive a small intravenous Injection of cocaine. The lower trace shows changes in dopamine. Peak 1 indicates an increase in dopamine before the rat pressed the lever. The two peaks (2) indicate transients that occurred after the lever press. Underneath the trace, the dark blue bar marks the time the audiovisual cues associated with the lever press were on. The light blue bar indicales when the pump was activated to deliver cocaine. The cyclic voltammogram (black) of behaviorally evoked dopamine matches the electrically evoked voltammogram (blue). (Adapted with permission from P. E. M. Phillips, Nature, 2003, A22. 614.)... 

Fig. 2 - Relative change in the level of depolarization reached by the peak of the transient component, V /Vr (V, level of depolarization reached by the test response, Vq, level of depolarization reached by the control one), as a function of the dark interval. Adapting lights respectively of 3 sec (A), 6 sec (B), 10 sec (C), 20 sec (D).

As pointed out by Bader et al. (1976),. changes in duration of the transient phase of the receptor potential of the retinula cells of the drone are more marked and last longer than changes in the level of depolarization in the experiments presented here, however, it is possible to observe that, while a decrease in the response amplitude is possibly present only for very short times after the end of the adapting lights, in the first few seconds of dark an increase in amplitude can be observed. This fact could be interpreted as a transient increase in sensitivity, but for this hypothesis to be confirmed more detailed experimental work is required. 

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