Surface-Bound Ruthenium Diimine Organometallic Complexes: Excited-State Properties

Ruthenium complexes of the general formula [Ru­(CO)­(H)­(L₂)­(L′₂)]­[PF₆] (L₂ = <i>trans-</i>2PPh₃, L′ = η²-4,4′-dicarboxy­bipyridine (<b>1</b>); L₂ =<i>trans</i>-2Ph₂PCH₂CH₂COOH, L′₂ = bipyridine (<b>2</b>); L₂ = Ph₂PCHCHPPh₂, L′ = η²-5-amino-1,10-phenanthroline (<b>3</b>); L₂ = <i>trans-</i>2PPh₃, L′₂ = η²-4-carboxaldehyde-4′-methylbipyridine (<b>4</b>)) have been shown to have longer emission lifetimes and higher quantum yields in solution compared with more symmetrical molecules such as [Ru­(bpy)₃]­[Cl]₂. Compound <b>4</b> is obtained as a mixture with the corresponding acetal, <b>4′</b>. These less symmetrical complexes have been covalently immobilized on the surface of silica polyamine composites, and their photophysical properties have been studied. The surface-bound complexes have been characterized by solid-state CPMAS ¹³C, ³¹P, and ²⁹Si NMR, UV–vis, and FT-IR spectroscopies. Excited-state lifetime studies revealed that, in general, the lifetimes of the immobilized complexes are 1.4 to 8 times longer than in solution and are dependent on particle size (300–500 μm versus 10–20 nm average diameter silica gels), polymer structure (linear poly­(allylamine) versus branched poly­(ethylenimine)), and the type of surface tether. One exception to this trend is the previously reported complex [Ru­(bpy)₂(5-amino-1,10-phenanthroline)]­[PF₆]₂ (<b>5</b>), where only a slight increase in lifetime is observed. Only minor changes in emission wavelength are observed for all the complexes. This opens up the possibility for enhanced heterogeneous electron transfer in photocatalytic reactions

Bacterial Pleckstrin Homology Domains: A Prokaryotic Origin for the PH Domain

Pleckstrin homology (PH) domains have been identified only in eukaryotic proteins to date. We have determined crystal structures for three members of an uncharacterized protein family (Pfam PF08000), which provide compelling evidence for the existence of PH-like domains in bacteria (PHb). The first two structures contain a single PHb domain that forms a dome-shaped, oligomeric ring with C5 symmetry. The third structure has an additional helical hairpin attached at the C-terminus and forms a similar but much larger ring with C12 symmetry. Thus, both molecular assemblies exhibit rare, higher-order, cyclic symmetry but preserve a similar arrangement of their PHb domains, which gives rise to a conserved hydrophilic surface at the intersection of the β-strands of adjacent protomers that likely mediates protein–protein interactions. As a result of these structures, additional families of PHb domains were identified, suggesting that PH domains are much more widespread than originally anticipated. Thus, rather than being a eukaryotic innovation, the PH domain superfamily appears to have existed before prokaryotes and eukaryotes diverged