Search

Labelling phospholipid membranes with luminophores without altering the biophysical characteristics of the system is particularly challenging due to the small size of the phospholipid molecules and the sensitivity of membrane properties to the presence of fused heterocyclic molecules. Here the design and synthesis of a luminescent lipid mimetic Ir(III) N-heterocyclic carbene complex of the form [Ir(ppy)2(C^N)] (where ppy = 2-(phenyl)-pyridine and C^N is a N-heterocyclic carbene ligand) conjugated to stearic acid is described. This complex was synthesised by the reaction of an acetate functionalised Ir(III) precursor complex with tert-butyl N-(2-aminoethyl)carbamate (mono-BOC protected ethylene diamine) and after deprotection of the amine group this complex was coupled to stearic acid using the peptide coupling reagent 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). The photophysical properties of the synthesised complexes were evaluated and they showed blue-green luminescence in the range of 514–520 nm. Fluorescence microscopy studies showed that the lipid mimetic complex successfully incorporated into liposomes composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), while dynamic light scattering (DLS) and differential scanning calorimetry (DSC) studies showed that the complex had negligible influence on the biophysical properties of the liposomes.

Six ruthenium(II) complexes of the general form cis-alpha-[Ru(N4-tetradentate)(N2-bidentate)]Cl2 have been synthesized from the two related tetradentate ligands 1,6-di(2'-pyridyl)-2,5-dimethyl-2,5-diazahexane (picenMe2) and 1,6-di(2'-pyridyl)-2,5-dibenzyl-2,5-diazahexane (picenBz2) and the bidentate ligands 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen) and dipyrido[3,2-f:2'3'-h]quinoxaline (dpq). Synthetic intermediate species of the general form cis-alpha-[Ru(II)(N4-tetradentate)(DMSO)Cl][PF6] were isolated. The N4-tetradentate ligand picenMe2 formed only the cis-alpha stereoisomer, while picenBz2 formed both the cis-alpha and cis-beta stereoisomers. These latter stereoisomers were resolved by fractional crystallisation. Dimer self-association constants, K(D), were estimated from the concentration dependence of the 1H NMR shifts for some of these complexes in aqueous solutions at 25 degrees C. The values of K(D) ranged from 0.6 to 7.9 M(-1) and a relationship was observed between the aromatic surface area of the bidentate component and the degree to which self-association occurred, whereby a greater level of self-association correlates with a larger surface area for the bidentate ligand. Some of these complexes demonstrate an ability to bind to DNA that is consistent with intercalation of the bidentate molecular component between the base pairs of the DNA molecule. Using calf-thymus DNA, the equilibrium binding constants, K(B), were determined for some of the complexes using intrinsic methods and these ranged from 3.32 to 5.11 M(-1), the intercalating abilities of the different bidentate ligands being in the order dp qphenbipy. This relationship between aromatic surface area of the bidentate ligand and the degree of DNA binding activity is the same as that observed in the self-association study.

Seven dinuclear gold(I) complexes of bidentate N-heterocyclic carbene ligands have been evaluated for their ability to induce mitochondrial membrane permeabilisation (MMP) in isolated rat liver mitochondria. Six of the compounds, at concentrations of 10 μM, induced Ca2+-sensitive MMP as evidenced by mitochondrial swelling. In the absence of low concentrations of exogenous Ca2+, the compounds were either inactive or their activity was significantly decreased. The mitochondrial swelling was completely blocked by the addition of cyclosporin A, a well established inhibitor of the mitochondrial permeability transition pore (MPT) that is believed to be responsible for MMP. The rates and levels of uptake of these compounds into mitochondria were estimated by measuring mitochondrial Au levels using inductively coupled plasma optical emission spectroscopy. Significant differences were found in the levels at which the different compounds accumulated in the mitochondria, but these differences did not correlate with the rate at which they induced mitochondrial swelling. These results suggest that the mechanism by which MMP is induced by these lipophilic cationic Au(I)–carbene complexes is not purely a function of the level of compound accumulation. Instead, a more specific mechanism, possibly involving disruption of the function of a particular enzyme, or interaction with a MPT component, appears to be more likely.