1971-1976 - Profiting from a New Beginning in Geneva.

A few projects of organic photochemistry, which have been investigated in Geneva in 1971-76, are reviewed summarily. They include excited state-selective processes from the n,π* and π,π* triplets and selectively occurring π → π* induced reactions of α,ß-unsaturated cyclic ketones, oxadi-π-methane rearrangements and 1,3-acetyl shifts of ß,γ-unsaturated methyl ketones, and di-π-methane rearrangements of benzoylnaphthobarrelene compounds. Furthermore, intramolecular triplet energy transfer dependence on rigid donor-acceptor syn-anti topologies between indanone and naphthalene moieties most likely reflects competition between radiationless energy dissipation by through-space aromatic π interaction (amounting to intramolecular exciplex action) and through-σ-bond exchange transfer. Finally, electronically integrating actinometry and time-resolved IR spectroscopy of excited states and reaction intermediates are mentioned.

New photoproducts from irradiation of NADH with near-UV light.

Adenosine 5'-diphosphoribose (ADPR) and a second compound, which may be nicotinamide, are the newly discovered photoproducts resulting from irradiation of beta-nicotinamide adenine dinucleotide (beta-NADH) in the wavelength range of 300-400 nm under oxygen-poor conditions. Both products emerge there even exclusively, whereas, at higher oxygen concentrations, the oxidized form of nicotinamide adenine dinucleotide (NAD+) is additionally formed, although still as a minor product. The development of ADPR and NAD+ is clearly oxygen-dependent, while, for the formation of the second photoproduct, small quantities of oxygen appear to be sufficient.

Two independent, light-sensing two-component systems in a filamentous cyanobacterium.

Two ORFs, cphA and cphB, encoding proteins CphA and CphB with strong similarities to plant phytochromes and to the cyanobacterial phytochrome Cph1 of Synechocystis sp. PCC 6803 have been identified in the filamentous cyanobacterium Calothrix sp. PCC7601. While CphA carries a cysteine within a highly conserved amino-acid sequence motif, to which the chromophore phytochromobilin is covalently bound in plant phytochromes, in CphB this position is changed into a leucine. Both ORFs are followed by rcpA and rcpB genes encoding response regulator proteins similar to those known from the bacterial two-component signal transduction. In Calothrix, all four genes are expressed under white light irradiation conditions, albeit in low amounts. For heterologous expression and convenient purification, the cloned genes were furnished with His-tag encoding sequences at their 3' end and expressed in Escherichia coli. The two recombinant apoproteins CphA and CphB bound the chromophore phycocyanobilin (PCB) in a covalent and a noncovalent manner, respectively, and underwent photochromic absorption changes reminiscent of the P(r) and P(fr) forms (red and far-red absorbing forms, respectively) of the plant phytochromes and Cph1. A red shift in the absorption maxima of the CphB/PCB complex (lambda(max) = 685 and 735 nm for P(r) and P(fr), respectively) is indicative for a noncovalent incorporation of the chromophore (lambda(max) of P(r), P(fr) of CphA: 663, 700 nm). A CphB mutant generated at the chromophore-binding position (Leu246-->Cys) bound the chromophore covalently and showed absorption spectra very similar to its paralog CphA, indicating the noncovalent binding to be the only cause for the unexpected absorption properties of CphB. The kinetics of the light-induced P(fr) formation of the CphA-PCB chromoprotein, though similar to that of its ortholog from Synechocystis, showed differences in the kinetics of the P(fr) formation. The kinetics were not influenced by ATP (probing for autophosphorylation) or by the response regulator. In contrast, the light-induced kinetics of the CphB-PCB complex was markedly different, clearly due to the noncovalently bound chromophore.

Aggregation of synthetic metallochlorins in hexane. A model of chlorosomal bacteriochlorophyll self-assemblies in green bacteria.

Metal 3(1)-hydroxy-13(1)-oxo-chlorins were systematically prepared and their visible and circular dichroism spectra were measured in a solution. All the synthetic complexes were monomeric in tetrahydrofuran. The Ni/Cu/Pd/Ag(II) complexes were still monomeric after dilution with 99-fold hexane. In contrast, the Co(II) complex, as well as the Mg/Zn/Cd(II) complexes, self-aggregated in 1% (v/v) tetrahydrofuran-hexane to form oligomers. In the less polar organic solvent, the Mn(III) complex fully dimerized and the Fe(III) complex partially dimerized. Infrared spectra of the synthetic metal chlorins in solid thin films revealed that the Ni/Cu/Pd/Ag(II) and ClFe(III) chlorins were 4- and 5-coordinated monomers, respectively, the AcOMn(III) chlorin formed a 6-coordinated dimer by mutual coordination of 3(1)-OetaetaetaMn, and the Co(II) chlorin as well as the Mg/Zn/Cd(II) chlorins self-aggregated by 13-C=OetaetaetaO-Hetaetaetametal to form large oligomers.