Theory of disk accretion onto supermassive black holes

Accretion onto supermassive black holes produces both the dramatic phenomena associated with active galactic nuclei and the underwhelming displays seen in the Galactic Center and most other nearby galaxies. I review selected aspects of the current theoretical understanding of black hole accretion, emphasizing the role of magnetohydrodynamic turbulence and gravitational instabilities in driving the actual accretion and the importance of the efficacy of cooling in determining the structure and observational appearance of the accretion flow. Ongoing investigations into the dynamics of the plunging region, the origin of variability in the accretion process, and the evolution of warped, twisted, or eccentric disks are summarized.Comment: Mostly introductory review, to appear in "Supermassive black holes in the distant Universe", ed. A.J. Barger, Kluwer Academic Publishers, in pres

Intercalation of small molecules into DNA in chromatin is primarily controlled by superhelical constraint

The restricted access of regulatory factors to their binding sites on DNA wrapped around the nucleosomes is generally interpreted in terms of molecular shielding exerted by nucleosomal structure and internucleosomal interactions. Binding of proteins to DNA often includes intercalation of hydrophobic amino acids into the DNA. To assess the role of constrained superhelicity in limiting these interactions, we studied the binding of small molecule intercalators to chromatin in close to native conditions by laser scanning cytometry. We demonstrate that the nucleosome-constrained superhelical configuration of DNA is the main barrier to intercalation. As a result, intercalating compounds are virtually excluded from the nucleosome-occupied regions of the chromatin. Binding of intercalators to extranucleosomal regions is limited to a smaller degree, in line with the existence of net supercoiling in the regions comprising linker and nucleosome free DNA. Its relaxation by inducing as few as a single nick per ~50 kb increases intercalation in the entire chromatin loop, demonstrating the possibility for long-distance effects of regulatory potential

Luminescent nitridoosmium(VI) complexes with aryl- and alkylacetylide ligands: Spectroscopic properties and crystal structures

A series of air- and moisture-stable nitridoosmium(VI) complexes with arylacetylide ligands, [ nBu 4N][OsN(C≡CC 6H 4R-p) 4 ] (R = H (1), Et (2), OEt (3), Ph (4)), and the alkyl analogue [ nBu 4N] [OsN(C≡C tBu) 4] (5) were synthesized. The structures of 1, 2, and 4 were determined by X-ray crystal analyses. The Os≡N (1.65(1) Å in 1, 1.620(8) Å in 2, and 1.619(6) Å in 4) and Os-C (mean 2.03 Å) distances are similar to that observed for [OsN(CH 2SiMe 3) 4] -, whereas the C≡C distances (mean 1.20 Å) correspond to terminal arylacetylide groups. The UV-vis absorption spectra of 1-4 display intense absorptions at 248-324 nm (ε ≈ 5 × 10 4 dm 3 mol -1 cm -1) that are attributed to overlapping of the [p π*(N 3-) → d π*(d xz, d yz)] and [π → π*(arylacetylides)] transitions. The moderately intense absorptions at λ max 403-418 nm (ε ≈ 10 3 dm 3 mol -1 cm -1) are assigned to admixture of [(d xy) 2 → (d xy) 1(d π*) 1] with intraligand charge-transfer transition of the arylacetylides. The broad structureless solid-state luminescence of 1-5 at 298 (λ max 600-632 nm) and 77 K (λ max 599-635 nm) are assigned to the triplet 3[(d xy) 1(d π*) 1] excited state. The 77 K glassy solutions of 1-5 also exhibit 3[(d xy) 1(d π*) 1] emissions at λ max 597-628 nm. Like [OsNX 4] - (X = Cl, Br), complexes 1-5 are nonemissive in dichloromethane at room temperature. Notably, 1 emits weakly at λ max 643 nm in benzene at room temperature, whereas 5 exhibits a more intense and long-lived luminescence at λ max 640 nm in diethyl ether. Correspondingly, a well-defined absorption at λ max 428 nm in the excitation spectrum of 5 (λ em 640 nm, concentration 8 × 10 -4 mol dm -3) is assigned to the 1[(d xy) 2] → 1[(d xy) 1(d π*) 1] transition.link_to_subscribed_fulltex