Thermodynamics and NMR studies on Duck, Heron and Human HBV encapsidation signals

Hepatitis B virus (HBV) replication is initiated by binding of its reverse transcriptase (P) to the apical stem-loop (AL) and primer loop (PL) of epsilon, a highly conserved RNA element at the 5′-end of the RNA pregenome. Mutation studies on duck/heron and human in vitro systems have shown similarities but also differences between their P–epsilon interaction. Here, NMR and UV thermodynamic data on AL (and PL) from these three species are presented. The stabilities of the duck and heron ALs were found to be similar, and much lower than that of human. NMR data show that this low stability stems from an 11-nt internal bulge destabilizing the stem of heron AL. In duck, although structured at low temperature, this region also forms a weak point as its imino resonances broaden to disappearance between 30 and 35°C well below the overall AL melting temperature. Surprisingly, the duck- and heron ALs were both found to be capped by a stable well-structured UGUU tetraloop. All avian ALs are expected to adhere to this because of their conserved sequence. Duck PL is stable and structured and, in view of sequence similarities, the same is expected for heron - and human PL

Comparison of the H-imino NMR spectra (5°C, HO) of (top) and (bottom) apical stem-loop

<p><b>Copyright information:</b></p><p>Taken from "Thermodynamics and NMR studies on and HBV encapsidation signals"</p><p></p><p>Nucleic Acids Research 2007;35(8):2800-2811.</p><p>Published online 11 Apr 2007</p><p>PMCID:PMC1885660.</p><p>© 2007 The Author(s)</p> Assigned peaks are labeled by residue numbers and unassigned peaks by asterisks. Residue numbers in the upper part of are superscripted with an asterisk for clarity

Photochemical isomerization of N-heterocyclic carbene ruthenium hydride complexes:In situ photolysis, parahydrogen, and computational studies

Low-temperature UV irradiation of the N-heterocyclic carbene complex Ru(IEt2Me2)(PPh3)2(CO)H2 (IEt2Me2 = 1,3-bis(ethyl)-4,5-dimethylimidazol-2-ylidene) leads to a remarkable photoisomerization reaction. By combining in situ photolysis and parahydrogen experiments to characterize the ultimate photoproducts and DFT calculations to interrogate the structures of the key 16-electron intermediates, the importance of both PPh3 and H2 loss pathways has been established.</p