[en] The structure of a very common RNA hairpin, 5'GGAC(UUCG)GUCC, has been determined in solution by NMR spectroscopy. The loop sequence, UUCG, occurs exceptionally often in ribosomal and other RNAs, and may serve as a nucleation site for RNA folding and as a protein recognition site. Reverse transcriptase cannot read through this loop, although it normally transcribes RNA secondary structure motifs. A hairpin with that loop displays unusually high thermodynamic stability; its stability decreases when conserved nucleotides are mutated. The three-dimensional structure for the hairpin was derived from interproton distances and scalar coupling constants determined by NMR using distance geometry, followed by restrained energy minimization. The structure was well-defined despite the conservative use of interproton distances, by constraining the backbone conformation by means of scalar coupling measurements. A mismatch G·U base pair, with syn-guanosine, closes the stem. This hairpin has a loop of only two nucleotides; both adopt C_2'-endo sugar pucker. A sharp turn in the phosphodiester backbone is stabilized by a specific cytosine-phosphate contact, probably a hydrogen bond, and by stacking of the cytosine nucleotide on the G·U base pair. The structural features of the loop can explain the unusual thermodynamic stability of this hairpin and its sensitivity to mutations of loop nucleotides

[en] The determination of equilibrium constants, seen, e.g., in the usual Scatchard plots, may be highly inaccurate because generally the values of the bound-ligand response are known with large uncertainty. A common cause of this inaccuracy stems for the use of samples that contain small amounts of free ligand, hardly avoided, or from equilibrium dialysis which is generally affected by the Donnan effect, dye losses, etc. From an analysis of this problem a simple procedure is derived here, so that, in spite of the above drawbacks, the equilibrium parameters can be obtained with better precision

[en] Signs and magnitudes of scalar coupling constants-spin-spin splittings-comprise a very large amount of data that can be used to establish the conformations of RNA molecules. Proton-proton and proton-phosphorus splittings have been used the most, but the availability of ¹³C-and ¹⁵N-labeled molecules allow many more coupling constants to be used for determining conformation. We will systematically consider the torsion angles that characterize a nucleotide unit and the coupling constants that depend on the values of these torsion angles. Karplus-type equations have been established relating many three-bond coupling constants to torsion angles. However, one- and two-bond coupling constants can also depend on conformation. Serianni and coworkers measured carbon-proton coupling constants in ribonucleosides and have calculated their values as a function of conformation. The signs of two-bond coupling can be very useful because it is easier to measure a sign than an accurate magnitude

[en] Infrared spectra of supernova 1987A, taken 18, 110/2, 192, 255, 284 and 349 days after the explosion are presented. The atmospheric windows from 1.05 to 4.1 μm were covered on each occasion, usually at a spectral resolution > 1000. Data to 4.75 μm are also shown for one occasion. All data are of spectrophotometric quality and allow the extraction of line intensities typically to 10 per cent accuracy except where blending, continuum placement or telluric absorption gives problems. We give plausible identifications for the majority of the emission and absorption features. SN 1987A is the first supernova in which molecular emission has been seen: we find CO and CO⁺. There is also evidence of emission from CS. (author)

[en] The presence and decay of radioactive cobalt in the ejecta of supernova 1987A are inferred from the [Co II] 1.547-μm line which was observed between days 255 and 576 after the explosion. The cobalt/iron abundance ratio and its temporal behaviour are obtained by examination of the relative strengths of near-infrared spectral lines. This has the advantages of being relatively insensitive to uncertainties in temperature, A-values and optical depths. The presence of both ⁵⁶Co and ⁵⁷Co is revealed. This work provides the earliest evidence of ⁵⁷Co in the ejecta of SN 1987A. The abundance ratios are found to be consistent with the predictions of explosive nucleo-synthesis calculations, and constrain the ultimate ⁵⁷Fe/⁵⁶Fe abundance ratio to between one and two times solar. (author)

[en] Photoexcited iron porphyrins can be of some interest in both fine and industrial chemistry in view of the preparation of new efficient biomimetic catalysts, working with high selectivity under mild temperature and pressure

[en] The high-resolution NMR structure of the acyl-carrier protein from the pathogen B. burgdorferi determined to a r.m.s. deviation of 0.4 Å over the protein backbone is reported. The NMR structure was determined using multidimensional NMR spectroscopy and consists of four α-helices and two 3₁₀-helices. Structural comparison reveals that this protein is highly similar to the acyl-carrier protein from A. aeolicus. Nearly complete resonance assignment and the high-resolution NMR structure of the acyl-carrier protein from Borrelia burgdorferi, a target of the Seattle Structural Genomics Center for Infectious Disease (SSGCID) structure-determination pipeline, are reported. This protein was chosen as a potential target for drug-discovery efforts because of its involvement in fatty-acid biosynthesis, an essential metabolic pathway, in bacteria. It was possible to assign >98% of backbone resonances and >92% of side-chain resonances using multidimensional NMR spectroscopy. The NMR structure was determined to a backbone r.m.s.d. of 0.4 Å and contained four α-helices and two 3₁₀-helices. A structure-homology search revealed that this protein is highly similar to the acyl-carrier protein from Aquifex aeolicus