Ulrich Siggel

Evidence for electronic and ionic interaction between electron transport chains in chloroplasts

Abstract Electron transport, phosphorylation and the absorption changes of plastoquinone and chlorophyll-a1 have been measured as a function of 3(3,4-dichlorophenyl)-1,1-dimethylurea(DCMU) concentration. The differential effect of DCMU observed is not consistent with the concept of isolated chains of electron carriers. Two types of cooperation are realized. 1. 1. Electron exchange is possible between at least 10 chains within a common plastoquinone strand. This follows from the different sensitivity of the electron transfer reactions against DCMU with regard to short and long time excitation under uncoupled conditions. 2. 2. All the electron transport chains of one thylakoid seem to interact by means of the internal protons which are pumped into the common inner phase by the chains and react on them by means of pH-sensitive electron transfer reactions. This is the interpretation of the different DCMU dependence of basal, coupled and uncoupled electron transport.

Electron transfer in oligothiophene-bridged bisporphyrins.

Oligothiophene-bridged zinc-tin bisporphyrinates were synthesized. Their absorption spectra have been analyzed in terms of exciton interaction and porphyrin-bridge coupling by through-bond interaction and the steady-state fluorescence spectra in terms of differential Stokes shifts for the electron-donating zinc and the electron-accepting tin porphyrinates. Strong quenching of the fluorescence intensity and acceleration of the fluorescence decay as compared to porphyrinate monomers (ZnTPP, SnTPP) were observed. Both phenomena were traced back to light-induced electron transfer by the occurrence of ion pair absorption bands in picosecond transient absorption spectra. Similar absorption spectra of both chromophores caused always simultaneous excitation and, consequently, two concurrent photoreactions. Combined evaluation of the time-dependent absorption and fluorescence data allowed the estimation of rates for the electron transfer reactions. The found dependence on the separation distance was much smaller than for donor-acceptor systems with saturated spacers. A damping factor of 0.05 was calculated for the charge separation proceeding from the excited state of the zinc porphyrin. The polarity of the solvent had a profound influence on the transfer rates. The charge recombination was 300 times faster in polar tetrahydrofuran than in nonpolar toluene.

Lateral, Chiral Tin(IV) Porphyrin Assemblies

The μ-Oxo stacks of tin(IV) porphyrins rearrange to staircase-type and lateral aggregates upon replacement of the oxygen ligands by chloride ions. The lateral aggregation of tin(IV) 2,18-dipropionate porphyrins in hydrochloric acid at pH 0–0.5 is favoured by 8,13-ethyl groups instead of the natural 8,13-vinyl groups of protoporphyrin IX and is impeded by hydrogen atoms at these positions. Replacement of axial chloride counterions to the tin(IV) central ions by cyanate counterions at pH 4.5 leads to similar aggregates if the cyanate ions are connected by hydrogen bonding to acetic or lactic acid. In this case, aggregation is not necessarily impeded by hydrogen atoms at positions 8 and 13. D- and L-lactic acid enforce chiral assemblies of the tin(IV) deuteroporphyrin 1a with mirror image CD (circular dichroism) spectra (θ ≈ 8 × 105deg cm2dmol−1), whereas the gluconoyl hydrazide-substituted tin(IV) deuteroporphyrin 1d does not form aggregates at all.

Micellar fibres of tin(IV) porphyrins with axial hydrogen chloride ligands as facial head groups

Amphiphilic tin(IV) porphyrinate dichlorides form micellar fibres with an average diameter of 5 nm and a length of several µm at pH 0; electronic and circular dichroism spectra indicate a lateral arrangement of facially protonated monomers.

Different Types of Cooperation between Electron Transport Chains in Chloroplasts

In the past in many laboratories a number of electron carriers have been identified and investigated which mediate the transfer of electrons from water to NADP+. A simplified scheme of one electron transport chain is shown in Fig. 1. The pathways of electrons have been regarded as isolated chains. At least there has been no discussion of a possible interaction until recently.