Intermediary Electron Acceptor Research Articles

Arrangement and interaction of pigment molecules in reaction centers of Rhodopseudomonas viridis Photodichroism and circular dichroism of reaction centers at 100 K

The linear dichroism of light-induced absorbance changes (photodichroism, p) accompanying photooxidation of the primary electron donor ( P), a bacteriochlorophyll dimer (BChl) 2-960, or photoreduction of the intermediary electron acceptor, bacteriopheophytin (BPh), as well as circular dichroism (CD) of the reaction centers of Rhodopseudomonas uiridis at 100 K in their various redox states: P · BPh, P ⨥ · BPh ⨪ and P · BPh ⨪ , have been measured. Photooxidation of P is accompanied by bleaching of the absorption and CD bands at 980 nm and a considerable change of the frequency, a decrease of the dipole and rotational strengths of the transition at 850 nm with the preservation of dipole and rotational strengths of the BPh transition at 790 nm. On the other hand, the photoreduction of BPh is accompanied by bleaching the absorption and CD bands of BPh at 790 nm and a considerable change of the frequency, a decrease of dipole and rotational strengths of the transition at 830 nm with the preservation of the dipole and rotational strengths of the transitions at 850 and 980 nm. Consideration of these data in terms of the static quantum treatment of reaction centers as molecular aggregates shows: (i) a direct interaction between BPh and (BChl) 2-960 is not observed; (ii) BPh mainly interacts with BChl monomer absorbing at 830 nm (BChl-830); (iii) (BChl) 2-960 mainly interacts with BChl monomer absorbing at 850 nm (BChl-850); and (iv) there is also an interaction between BChl-830 and BChl-850 since the dipole strengths of these transitions differ from each other but their sum corresponds to the sum of the dipole strengths of two BChl monomers. The p values of the transitions have been used for the estimations of the angles between the dipole-vectors of the transitions in reaction centers. Estimations of the distances and interaction energy between the transitions of the pigment molecules in the reaction center are given.

Zum wirkungsmechanismus der ascorbinsäure I. Isolierung einer ascorbinsäureabhängigen DPNH-oxydase aus nebennierenmikrosomen

1. 1. An enzyme system has been isolated from suprarenal microsomes that oxidizes DPNH in the presence of ascorbic acid and O 2. 2. 2. The spectra and absorption maxima point to a flavin constituent and cytochrome b 5 as components. 3. 3. The enzyme system contains SH groups, which are concerned in the enzyme reaction described. The reaction is entirely blocked by p-chloromercuri-benzoate. The enzyme is also blocked by Mn ++, but not by KCN, NaN 3, monoiodoacetic acid, or antimycin A. 4. 4. Neither dehydroascorbic acid, nor glutathione, nor adrenalin, can displace ascorbic acid. D-Isoascorbic acid is active but distinctly weaker than ascorbic acid itself. Under the experimental conditions described, TPNH is not oxidized in the presence of ascorbic acid. 5. 5. Fe +++ in complex form is reduced by DPNH in the presence of the enzyme. 6. 6. The speed of the DPNH-oxidation by ascorbic acid is dependent on the O 2 content of the reaction medium. 7. 7. On the basis of the experiments it appears that “monodehydroascorbic acid” acts as an intermediary electron acceptor. The system ascorbic acid ⤥ monodehydroascorbic acid forms the electron carrier between DPNH and O 2. Intermediary hydroxyl radicals probably arise in this way.