Photophosphorylation, sensitized by chlorophills a and b, pheophytin and beta-carotene in a modelled system]

In aqueous solutions in air atmosphere chlorophylls a and b, pheophytin and beta-carotene adsorbed on aluminium oxide powder are capable of sensibilizing electron transfer from phosphate ions coupled with the formation of high energy bonds of adenosine phosphates. The highest activity of chlorophylls a and b and pheophytin is observed within the pH range of 7.5-7.8; that of beta-carotene--at pH 7.3-7.5.     

Activity of methane-oxidizing bacteria in the adsorbed state]

Adsorption of pure cultures of methane oxidizing bacteria, Methylosinus trichosporium 20 and Methylococcus ucrainicus 21, on glass and coal was studied; the former strain was sorbed on both sorbents, the latter strain was sorbed on coal but not on glass. The rate of methane oxidation by the cells of adsorbed microorganisms was higher than in the case of free cells, and increased with a decrease in dimensions of the sorbent particles.     

Triplet state quenching by photoinduced radicals in the sensitized ethanol photolysis

UV irradiation of a rigid solution of 2-amino-pyrimidine in ethanol at 77 degrees K results in a sensitized production of ethyl radicals. The radicals are formed via a biphotonic process. The increase of the radical yield with irradiation time is directly correlated to a decrease of the number of triplet state molecules as detected by ESR. A kinetic model of the biphotonic process is presented and verified by experimental results. It supports the hypothesis of a catalytically enhanced radiationless decay process of the triplet states due to an interaction between the radicals and the triplet state molecules.     

Tentoxin Suppresses Stomatal Opening by Inhibiting Photophosphorylation

Tentoxin and, to a lesser extent, dihydrotentoxin (both at 10mmol m^–3) reduce stomatal opening in epidermal stripsof Commelina communis in the light but not in darkness. Thiseffect was significantly greater in normal air than in CO₂-freeair. Fusicoccin overcame the tentoxin effect. However, tentoxindid not inhibit stomatal opening in the light in epidermal stripsof Paphiopedilum harrisianum, a species which lacks guard cellchloroplasts. It is concluded that tentoxin exerts its actionon stomata not by an ionophorous effect in the plasmalemma ofguard cells but by the inhibition of photophosphorylation intheir chloroplasts. The effects of DCMU and tentoxin on guardcells are discussed in terms of their effects on chloroplastsand the extent to which energy is supplied from this organelleduring stomatal opening in the light. The results indicate thatneither photophosphorylation nor non-cyclic electron transportin guard cell chloroplasts are essential for stomatal opening. Key words: Commelina, epidermal strips, Paphiopedilum, photophosphorylation, stomata, tentoxin     

Photophosphorylation and the chemiosmotic perspective

Photophosphorylation was discovered in chloroplasts by D. Arnon and coworkers, and in bacterial ‘chromatophores’ (intercytoplasmic membranes) by A. Frenkel. Initial low rates were amplified by adding electron-carrying compounds such as FMN, later shown to support the ‘pseudocyclic’ electron flow. ATP synthesis, and coupling to electron flow, was detected accompanying linear electron flow from H₂O to either NADP⁺ or ferricyanide. Another pattern of electron flow supporting photophosphorylation was that of a cycle around Photosystem I (PS I). Isolation and analysis of the ATP synthase showed, as with mitochondrial and bacterial analogues, an intrinsic membrane complex (CF₀) and an extrinsic complex (CF₁). CF₁ is a latent ATPase, activated additively by the high-energy state of the thylakoids, and by reduction of a disulfide bond on the gamma subunit. Once reduced, ATP synthesis occurs at lower energy levels. The search for an ‘intermediate’ linking electron flow and ATP synthesis led to the discovery of post-illumination ATP synthesis by thylakoids, where turnover occurs in the dark. Once interpreted by P.Mitchell's chemiosmotic hypothesis, this led to the discovery of light-driven proton uptake into the thylakoid lumen, with accompanying Cl⁻ intake and Mg²⁺ and K⁺ output. Chemiosmosis was confirmed in several ways, including ATP synthesis in the dark due to an acid-to-base transition of thylakoids, and photophosphorylation accomplished in artificial lipid vesicles containing both the proton-pumping bacterial rhodopsin and a mitochondrial ATPase complex. The now generally accepted chemiosmotic interpretation is able to clarify some other aspects of photosynthesis as well. This revised version was published online in June 2006 with corrections to the Cover Date.     

Fluorescence quenching of chlorophyll and its analogs by oxidants]

The quenching of the fluorescence of chlorophyll, Zn- and Cd-pheophytinates, Mg-, Zn-, Cd-, Al-, Ga-, In-, Hf-tetraphenylporphine in different solvents with chlorinanyl, p-benzoquinone and m-dinitrobenzole has been studied. It is concluded from the dependence of quenching on medium viscosity that static and kinamatic mechanisms are in the basis of quenching. The first one dominates within the high values of viscosity, the latter within the law ones. In case of metals on one group the quenching constants are decreased with the growth of the ordinal number of the central atom. This fact supports the great role of the dynamic factor during the quenching of fluorescence of tetrapirrole pigments.     

Stimulation of Photophosphorylation and Cytochrome c Photooxidation by Pteridines

A number of pteridines were examined for activity in promoting photophosphorylation in broken spinach chloroplasts and in stimulating cytochrome c photooxidation in sonicated chloroplasts. Correlation was found between activities for the 2 reactions. Photophosphorylation promoted by pteridines was inhibited by DCMU and by anaerobic conditions. It is concluded that pteridines may stimulate photophosphorylation by linking photosystem 1 with molecular oxygen and thereby allowing noncyclic electron flow.

Aromatic pteridines in both the 2,4-dihidroxy- and 2-amino-4-hydroxy-series were active; substitution at the 6 (or 7) position was a necessary but not sufficient condition for activity in both reactions.

Reducing agents increased photophosphorylation activity of aromatic pteridines and an oxidant increased activity of a tetrahydropteridine. It is postulated that pteridines are most active in their semiquinone or unstable dihydro forms.