Action Spectra of Photophosphorylation–i. Atp Formation Coupled to Non‐cyclic Electron Flow

Abstract

Abstract— Photophosphorylation action spectra were measured with the utmost precision to identify the different pigment forms involved in the light‐harvesting process for formation ofadenosine–5′‐triphosphate (ATP). With ferricyanide‐ions as electron acceptor freshly prepared thylakoid membranes yield a typical photosystem 2 (PS II) action spectrum peaking at 672 nm and with pronounced shoulders at 650 and 680 nm. Because ferricyanide‐ions can be reduced by both photosystems we suggest that due to the poor accessibility of these hydrophilic ions to the PS II acceptor site, the PS II electron flow is rate‐limiting. When the thylakoid membranes are aged, the maximum of the action spectrum shifts to 675 nm and bands in the spectral region690–725 nm are observed, which are generally attributed to photosystem 1 (PS I). Obviously, as a result of the aging, the rate‐limiting effect of PS II electron flow decreases and activity of both photosystems is observed. With nicotinamide adenine dinucleotide phosphate (NADP) and ferredoxin as electron acceptors also a typical PS II action spectrum is obtained peaking at 676 nm and with a sharp drop in activity at 695 nm. However, the spectrum shows a pronounced peak at706–707 nm, which is attributed to a pigment involved in the light‐harvesting for either of the two photosystems. Curve‐analysis of the action spectra suggests that four forms of Chi a exist at room temperature in the spectral region660–685 nm. However, a definite conclusion about the occurrence of different pigment forms will be drawn on the basis of combined results from this and the following publication. This is done to yield a conclusion that is statistically more reliable. ATP–adenosine‐5′‐triphosphate; BSA– bovine serum albumin; Chl–chlorophyll; DBMIB–2,5‐dibromo‐3‐methyl‐6‐isopropylbenzoquinone; DCMU–3‐(3,4)‐dichlorophenyl‐1,1‐dimethyl urea; NADP –nicotinamide adenine dinucleotide phosphate; Pi–orthophosphate‐ion; PMS–N‐methyl‐phenazonium sulphate or phenazinemethosulphate;PS–1–photosystem 1;PS–2–photosystem 2.

Distribution of the chlorophyll spectral forms in the chlorophyll-protein complexes of photosystem II antenna

“…Another level of antenna heterogeneity concerns the spectroscopic characteristics of the chi antenna pigments. Thus, at least five distinct chi a spectral forms have been detected in PS II antenna by derivative spectroscopy and Gaussian band decomposition of low-temperature absorption spectra (French et al, 1972;Brown & Schoch, 1981;Van Dorssen et al, 1987b) and by Gaussian analysis of room temperature spectra (Van Ginkel & Kleinen-Hammans, 1981;Zucchelli et al, 1990). The absorption maxima (Qy transition) of these spectral forms are approximately at 660, 670, 678, 684, and 695 nm.…”