Electrochromic shift

The most efficient factor in stabilizing the electronic state is the dipole-dipole interaction. This creates a local electric field (reactive field) around the excited dye interacting with its dipole [14]. If the charges are present in its vicinity, they create an electric field that interacts with the dye dipole and induces electrochromic shifts of absorption and fluorescence spectra. The direction of these shifts depends on the relative orientation of the electric field vector and the dye dipole. These effects of electrochromism are overviewed in [15]. 

Finally we should mention the electrochromic shifts which are the changes in the energy of electronic transitions when an external electric field is applied to the sample. These effects are quite small but have proved useful for the measurements of the dipole moments and polarizabilities of excited molecules. 

The electrochromic shift of the carotenoids is usually calibrated with K-diffusion potential in the presence of valinomycin. One problem is that the shifts observed in respiring chromatophores (where the proton electrochemical potential is predominantly in the form of a membrane potential) are much larger than those induced by the calibrating diffusion potential, so that an extensive extrapolation is required. Thus, the carotenoids in illuminated chromatophores may indicate a membrane potential in excess of 300 mV, whereas the distribution of CNS, an electrically permeant anion, in the same system only indicates 140 mV [32]. The extent of this discrepancy, and the uncertainty as to whether the carotenoids see the bulk-phase potential, or only the local electrical field within the membrane, limits the confidence with which carotenoids may be used for quantitative as opposed to qualitative potential measurements. 

Evidence for the existence of a Q cycle mechanism also in the intersystem chain of higher plant chloroplasts has been obtained this evidence rests, however, on more indirect experimental approaches, such as a slow phase of the electrochromic shift of carotenoids and an H /e stoicheiometry of proton translocation higher than one, as would be postulated by a linear loop including plastoquinone as a transmembrane proton translocator. 

Martin etal also used 150-/5 laser pulses but at 850-nm to excite predominantly the Qy band of the primary electron donor of Rb. sphaeroides reaction centers. The kinetics were monitored at wavelengths specific to the various pigment molecules, e.g., 1240 nm for production of P870, 545 nm for BO photoreduction, and an 805-nm absorbance decrease for BChl reduction as well as for the electrochromic shift of the BChl-absorption band in the 800 nm region (see Fig. 8). 

A decade after the work of Dekker et al., the absorbance change shown in Fig. 1 (A) was reexamined by Lavergne and by Leeuwen, Heimann and van Gorkom and the results are shown in Figs. 1 (B) and (C), respectively. To facilitate their comparison, all data are presented on the same As scale. For simplicity and clarity, the region above 400 nm where electrochromic shifts due to chlorophyll molecules occur was omitted. Note that the spectra vary somewhat from the different sources. Nevertheless, all the difference spectra assigned to the Si->S2 transition show some common features, e.g., a peak at -310-320 nm and, to some degree, a shoulder at -350 nm, as well as a peak or shoulder at 385 nm. 

In Fig. 4, we have included a portion of the spectral region above 400 nm, where the electrochromic shifts of chlorophyll spectra, possibly ofP680 itself, induced by the presence of positive charges on the various S-states are very prominent. While this electrochromic shift is absent in the S2->S3 transition, those appearing in the spectra for the 8,- 83 and Sq- S, transitions are interestingly almost mirror images of each other. 

Additional modulations of the optical spectra by nearby amino acid residues were considered by including the residues surrounding BCHL 6 within 5.5 A. The Qy transition is calculated at 740 nm vs 808 nm without the residues (model b). Most of this effect is attributable to an electrochromic shift induced by the positively charged arginine (ARG 90) that lies 5A from the keto group on ring V. Uncharged aromatic residues cause only small perturbations in the optical spectra. 

Figure 1. Diurnal pattern of the kinetics of reoxidation of the chloroplast ATP-ase in the dark. The y-axis represents the extent of the slow decay phase of the flash-induced electrochromic shift which is used as a measure of the fraction of ATP-ase in the oxidized form. See Text.

The results in this paper can be interpreted with a modification of the model previously proposed (2) as adapted for the intact plants by ourselves (3). In this modified scheme, Calvin cycle enzymes and intermediates alter the rate of reoxidation of the redox pool in equilibrium with the ATP-ase. This study suggests that measurements of the electrochromic shift in intact plants with the type of instrumentation used here may become an important tool in the study of the energetics and biochemistry of the intact plant. 

As a control for quinone reduction, VIS/NIR difference spectra were recorded. Figure 2 shows the electrochromic shift of the BPheo absorption band upon quinone reduction. A differential feature at 750 nm and 768 nm was taken as evidence for the reduction of [21]. The signal observed on application of an oxidizing potential (fig. 2, dashed line)... 

Figure 2 Difference spectra of Rb. sphaeroides R26 RC. Electrochromic shift of BPheo absorption arising from electrochemical reduction and reoxidation of Q. ...

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