Quinone Headgroup Research Articles

The molecular arrangement of a vitamin K 1 monolayer at the air-water interface

The quinone head group orientation is determined during compression of a vitamin K 1 monolayer on water. Resonant second harmonic generation indicates that the long axis of the quinone group remains in the surface plane upon film compression. In previous work, the short axis of the head group was shown to orient at 24° relative to the surface normal upon film compression.

Macroscopic and microscopic surface characterization of a vitamin K 1 monolayer on water

Film pressure and surface potential measurements are used to elucidate chain orientational changes in vitamin K 1 and phytol monolayers during compression and expansion cycles. Second harmonic generation studies on vitamin K 1 show a major reorientation in the quinone head group during compression. In the expanded film, phytol chains are freely rotating and head groups are randomly oriented. Upon compression, the short axis of the quinone head groups orients with a tilt angle of 24° to the surface normal. Molecular interactions may cause the surface potential to change as chains become more nearly vertically oriented.

Structural and optical properties of langmuir-blodgett films of the electron acceptor 2-octadecylthio-1,4-benzoquinone

The electron acceptor 2-octadecylthio-1,4-benzoquinone forms stable monolayers at air/water interfaces. Transfer to hydrophobic substrates yields Y-type Langmuir-Blodgett films. By studies of multilayers using X-ray diffraction and spectroscopy with polarized light a structure model is obtained which shows that a simultaneously close packing of the head group and alkyl chains is possible. The oxygen-carbon bonds in the quinone head group are oriented along the substrate normal according to the structure model.

Proton and electron transfer in the acceptor quinone complex of Rhodobacter sphaeroides reaction centers: characterization of site-directed mutants of the two ionizable residues, GluL212 and AspL213, in the QB binding site

Proton and electron transfer events in reaction centers (RCs) from Rhodobacter sphaeroides were investigated by site-directed mutagenesis of glutamic acid at position 212 and aspartic acid at 213 in the secondary quinone (QB) binding domain of the L subunit. These residues were mutated singly to the corresponding amides (mutants L212EQ and L213DN) and together to give the double mutant (L212EQ/L213DN). In the double mutant RCs, the rate of electron transfer from the primary (QA) to the secondary (QB) acceptor quinones is fast (tau approximately 300 microseconds) and is pH independent from pH 5 to 11. The rate of recombination between the oxidized primary donor, P+, and QB- is also pH independent and much slower (tau approximately 10 s) than in the wild type (Wt), indicating a significant stabilization of the QB- semiquinone. In the double mutant, and in L213DN mutant RCs at low pH, the P+QB- decay is suggested to occur significantly via a direct recombination rather than by repopulating the P+QA- state, as in the Wt. Comparison of the behavior of Wt and the three mutant RC types leads to the following conclusions: the pK of AspL213 in the Wt is approximately 4 for the QAQB state (pKQB) and approximately 5 for the QAQB-state (pKQB-); for GluL212, pKQB approximately 9.5 and pKQB- approximately 11. In L213DN mutant RCs, pKQB of GluL212 is less than or equal to 7, indicating that the high pK values of GluL212 in the Wt are due largely to electrostatic interaction with the ionized AspL213 which contributes a shift of at least 2.5 pH units. Transfer of the second electron and all associated proton uptake to form QBH2 is drastically inhibited in double mutant and L213DN mutant RCs. At pH greater than or equal to 8, the rates are at least 10(4)-fold slower than in Wt RCs. In L212EQ mutant RCs the second electron transfer and proton uptake are biphasic. The fast phase of the electron transfer is similar to that of the Wt, but the extent of rapid transfer is pH dependent, revealing the pH dependence of the equilibrium QA(-)QB- in equilibrium with QAQBH-. The estimated limits on the pK values--pKQA-QB-less than or equal to 7.3, pKQAQB2- greater than or equal to 10.4--are similar to those derived earlier for Wt RCs [Kleinfeld et al. (1985) Biochim. Biophys. Acta 809, 291-310] and may pertain to the quinone head group, per se.(ABSTRACT TRUNCATED AT 400 WORDS)

Synthetic quinones influencing herbicide binding and Photosystem II electron transport. The effects of triazine-resistance on quinone binding properties in thylakoid membranes

We have analyzed the binding of synthetic quinones and herbicides which inhibit electron transport at the acceptor side of Photosystem II (PS II) of the photosynthetic electron-transport chain in thylakoid membranes. These data show that quinones and PS II-directed herbicides compete for binding to a common binding environment within a PS II region which functions as the Q − PQ oxidoreductase. We observed that (1) synthetic quinones cause a parallel inhibition of electron transport and [ 14C]herbicide displacement, and (2) herbicide binding is affected both by the fully oxidized and fully reduced form of a quinone. Quinone function and inhibitor binding were also investigated in thylakoids isolated from triazine-resistant weed biotypes. We conclude the following. (1) The affinity of the secondary accepting quinone, B, is decreased in resistant thylakoids. (2) The observation that the equilibrium concentration of reduced Q after transferring one electron to the acceptor side of PS II is increased in resistant as compared to susceptible chloroplasts may be explained both by a decrease in the affinity of PQ for the herbicide / quinone binding environment, and by a decrease of the midpont redox potential of the B B − couple. (3) The binding environment regulating quinone and herbicide affinity may be divided roughly into two domains; we suggest that the domain regulating quinone head-group binding is little changed in resistant membranes, whereas the domain-regulating quinone side-group binding (and atrazine) is altered. This results in increased inhibitory activity of tetrachloro- p-benzoquinone and phenolic herbicides, which are hypothesized to utilize the quinone head-group domain. The two domains appear to be spatially overlapping because efficient atrazine displacement by tetrachloro- p-benzoquinone is observed.

Interactions of herbicides and azidoquinones at a Photosystem II binding site in the thylakoid membrane

6-Azido-5-decyl-2,3-dimethoxy- p-benzoquinone (6-azido-Q 0C 10) was found to replace the native plastoquinone at B (the second stable electron acceptor to Photosystem II (PS II)). The 6-azido-Q 10C 10 would accept electrons from the primary electron-accepting quinone, Q, thus allowing electron transport through PS II to the plastoquinone pool in thylakoids. The synthetic azidoquinone also competes with the PS II herbicides ioxynil and atrazine for binding. This observation strongly favors the hypothesis that PS II herbicides block electron transport by replacing the native quinone which acts as the second electron carrier on the reducing side of PS II (termed B). Covalent linkage of 6-azido-Q 0C 10 to its binding environment by ultraviolet irradiation greatly reduces herbicide-binding affinity but does not lead to a loss in herbicide-binding sites. We take this as evidence that covalent attachment of 6-azido-Q 0C 10 allows some freedom of quinone head-group movement such that the herbicides can enter the binding site. This indicates that the protein determinants which regulate quinone and herbicide binding are very closely related, but not identical. A compound somewhat related to 6-azido-Q 0C 10 is 2-azido-3-methoxy-5-geranyl-6-methyl- p-benzoquinone (2-azido-Q 2). This compound was found to be an ineffective competitor with respect to herbicide binding. Thus, interactions with protein-binding determinants are highly dependent on the molecular structure of quinones. The 2-azido-Q 2 was an inhibitor of electron flow in the intersystem portion of the chain.