Action Spectra of Photophosphorylation–ii. Atp Formation Catalyzed by Phenazinemethosulphate Suggesting the Involvement of Long Wavelength Pigment Forms in the Light‐harvesting Process for Ps Ii and Ps I

Abstract

Abstract— Action spectra of phenazinemethosulfate (PMS)‐catalyzed photophosphorylation, measured under aerobic conditions without added reductants, show the involvement of both photosystem 2 (PS II) and photosystem 1 (PS I). Addition of low concentrations 3‐(3,4)dichlorophenly‐l, 1‐dimethylurea (DCMU) changes the shape of the curves and causes formation of typical PS II action spectra. This can be understood by the partial inhibition of PS II electron flow by DCMU, which therefore becomes rate‐limiting. The presence of DCMU causes a strong decrease of photochemical activity in these spectra at 638, 660 and 690 nm, which we attribute to decreasing activity of PS I pigments. Curve analysis of the action spectra suggests the presence of different Chi forms at room temperature peaking at: 641.6; 650.3; 661.2; 669.8; 677.1; 684.0 and 690.6 nm. Comparison of our data with published data from low temperature absorption spectra indicates that lowering the temperature causes a decrease in halfwidth of the absorption bands but no significant change in peak position. Our data support the hypothesis of French and co‐workers that there are four universal forms of Chi a also at 25â°C. Careful investigation of data in literature does not give evidence that the pigment peaking at 641 nm is a second form of Chi b, as is sometimes assumed. Because our action spectra show two small but distinct maxima at703–705 and718–720 nm when PS II is rate‐limiting, we also measured action spectra of PS I (reduced PMS + DCMU) and PS II (ferricyanide‐ions + 2,5‐dibromo‐3‐methyl‐6‐isopropylbenzoquinone) separately. They show, in agreement with published data, that PS I is operating more efficiently in the spectral region690–725 nm than PS II. The Chi form peaking at 690 nm is absent in the PS II action spectrum, whereas this curve has two small but distinct maxima around703–705 and718–720 nm, respectively, which are also present in the PS I action spectrum. We, therefore, suggest that light absorbed by these pigments can be transferred to either of the two photosystems, and not to PS I exclusively as is sometimes assumed. ATP,adenosine–5′‐triphosphate; BSA, bovine serum albumin; Chi, 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, JV‐methylphenazonium sulphate or phenazinemethosulphate; PS I, photosystem 1; PS II, photosystem 2.

Action Spectroscopy on Dense Samples of Photosynthetic Reaction Centers of Rhodobacter sphaeroides WT Based on Nanosecond Laser-Flash 13C Photo-CIDNP MAS NMR

“…Photosynthesis of plants and bacteria has been explored by action spectroscopy since the days of Warburg. In general, physiological parameters, such as adenosine triphosphate production, are observed to measure the action [5]. Alternatively, action spectroscopy in photosynthesis has been measured by optical, photothermal and photoacoustic methods [6–8].…”