We have previously shown that horse cytochrome c (cyt c) forms oligomers by domain swapping its C-terminal α-helix when interacting with ethanol. Although folding of cyt c has been studied extensively, formation of domain-swapped oligomers of cyt c during folding has never been reported. We found that domain-swapped oligomeric cyt c is produced during refolding from its guanidinium ion-induced unfolded state at high protein concentrations and low temperatures. The obtained dimer exhibited a domain-swapped structure exchanging the C-terminal α-helical region between molecules. The extent of dimer formation decreased significantly for the folding of C-terminal cyt c mutants with reduced hydrophobicity achieved by replacement of hydrophobic residues with Gly in the C-terminal region, whereas a large amount of heterodimers was generated for the folding of a mixture of N- and C-terminal mutants. These results show that cyt c oligomers are formed through intermolecular hydrophobic interaction between the N- and C-terminal α-helices during folding. A slow phase (4-5 s) was observed in addition to a 400-500 ms phase during folding of a high concentration of cyt c in the presence of 1.17 M guanidine hydrochloride. The fast phase is attributed to the intramolecular ligand exchange process, and we attribute the slow phase to the ligand exchange process in oligomers. These results show that it is important to consider formation of domain-swapped oligomeric proteins when folding at high protein concentrations.

Coordination complexes can be used to photocage biologically active ligands, providing control over the location, time, and dose of a delivered drug. Dual action agents can be created if both the ligand released and the ligand-deficient metal center effect biological processes. Ruthenium (II) complexes coordinated to pyridyl ligands generally are only capable of releasing one ligand in H2O, wasting equivalents of drug molecules, and producing a Ru(II) center that is not cytotoxic. In contrast, Ru(II) polypyridyl complexes containing diazine ligands eject both monodentate ligands, with the quantum yield (ϕps) of the second phase varying as a function of ligand pKa and the pH of the medium. This effect is general, as it is effective with different Ru(II) structures, and demonstrates that diazine-based drugs are the preferred choice for the development of light-activated dual action Ru(II) agents.

The tecton I [4b,5,7,7a-tetrahydro-4b,7a-epiminomethanoimino-6H-imidazo-[4,5-f]-[1,10]-phenanthroline-6,13-dione] with inherent hydrogen bond acceptor (O atoms) and donor (N-H atoms) groups reacts with cis-Ru(bpy) 2 Cl 2 (bpy ) 2,2′-bipyridine) in a 1:1 molar ratio to form a monomeric supramolecular synthon [Ru(bpy) 2 (I)]Cl 2 (1), which generates a 3-D metal-organic framework (MOF) by a self-assembly process. The crystal structure reveals an intricate hydrogen-bonding network in which two symmetry-related molecules are connected through N-H‚‚‚O interactions along with the chlorine anions (C-H‚‚‚Cl, C-H‚‚‚O, N-H‚‚‚Cl, and N-H‚‚‚O) resulting in the formation of channels. Water clusters buttressed by chlorine anions and fortified by N-H‚‚‚O and C-H‚‚‚O interactions are encapsulated within these channels. The reversibile encapsulation of lattice water molecules and the corresponding effect on the overall structure is probed by IR and in situ high-temperature X-ray powder diffraction studies.