Influence of Substituents on the Energy and Nature of the Lowest Excited States of Heteroleptic Phosphorescent Ir(III) Complexes: A Joint Theoretical and Experimental Study

Abstract: A series of Ir(III)-based heteroleptic complexes with phenylpyridine (ppy) and 2-(5-phenyl-4H-[1,2,4]triazol-3-yl)-pyridine (ptpy) derivatives as coordinating ligands has been characterized by a number of experimental and theoretical techniques. Density functional theory (DFT) calculations were able to reproduce and rationalize the experimental redox and excited-states properties of the Ir complexes under study. The introduction of fluorine and trifluoromethyl substituents is found not only to modulate the emission energy but also often to change the ordering of the lowest excited triplet states and hence their localization. The lowest triplet states are best characterized as local excitations of one of the chromophoric ligands (ppy or ptpy). The admixture of metal-to-ligand charge-transfer (MLCT) and ligand-to-ligand charge-transfer (LLCT) character is small and strongly depends on the nature of the excited state; their role is, however, primordial in defining the radiative decay rate of the complexes. The extent of charge-transfer contributions depends on the energy gaps between the relevant molecular orbitals, which can be modified by the substitution pattern

Structural determinants of TAR RNA-DNA annealing in the absence and presence of HIV-1 nucleocapsid protein

Annealing of the TAR RNA hairpin to the cTAR DNA hairpin is required for the minus-strand transfer step of HIV-1 reverse transcription. HIV-1 nucleocapsid protein (NC) plays a crucial role by facilitating annealing of the complementary hairpins. To gain insight into the mechanism of NC-mediated TAR RNA–DNA annealing, we used structural probes (nucleases and potassium permanganate), gel retardation assays, fluorescence anisotropy and cTAR mutants under conditions allowing strand transfer. In the absence of NC, cTAR DNA-TAR RNA annealing depends on nucleation through the apical loops. We show that the annealing intermediate of the kissing pathway is a loop–loop kissing complex involving six base-pairs and that the apical stems are not destabilized by this loop–loop interaction. Our data support a dynamic structure of the cTAR hairpin in the absence of NC, involving equilibrium between both the closed conformation and the partially open ‘Y’ conformation. This study is the first to show that the apical and internal loops of cTAR are weak and strong binding sites for NC, respectively. NC slightly destabilizes the lower stem that is adjacent to the internal loop and shifts the equilibrium toward the ‘Y’ conformation exhibiting at least 12 unpaired nucleotides in its lower part