Over the past decade our physical understanding of gamma-ray bursts (GRBs) has progressed rapidly thanks to the discovery and observation of their long-lived afterglow emission. Longduration (T > ∼ 2 s) GRBs are associated with the explosive deaths of massive stars ("collapsars" 1 ), which produce accompanying supernovae, 2-4 while the short-duration (T < ∼ 2 s) GRBs arise from a different origin, which has been argued to be the merger of two compact objects, [5][6][7] either neutron stars or black holes. Here we present observations of GRB 060614, a 100-s long burst discovered by the Swift satellite, 8 which require the invocation of a new explosive process: 2 Gal-Yam et al.either a massive "collapsar" that powers a GRB without any associated supernova, or a new type of engine, as long-lived as the collapsar but without any such massive stellar host. We also discuss the properties of this burst's redshift z = 0.125 host galaxy, which distinguish it from other long-duration GRBs and suggest that an entirely new type of GRB progenitor may be required.On 14 June 2006, 12:43 UT, the burst alert telescope (BAT) on board the Swift satellite detected the γ-Ray Burst (GRB) 060614.8 The BAT detected γ-rays from this event for 120s, and the burst showed strong variability during much of that period, as confirmed by parallel observations by the Konus-Wind satellite. 9 Note that while some evolution in the temporal and spectral properties of this GRB were observed, the emission remained highly variable and relatively hard for tens of seconds, unlike the situation observed for a few short bursts with long, soft "tails". 10,7 This indicates sustained activity of an engine, rather than the early onset of the afterglow. The γ-ray properties of this event are similar to those of other bursts from the long-duration subgroup of GRBs. Swift autonomously slewed to the GRB position and began taking data with the X-ray telescope and UVoptical telescope. 11 We began observing this event ≈ 26 minutes later using the 40 inch telescope at Siding Springs Observatory. The evolution of the optical radiation from this event as traced by our data, augmented by Swift observations and additional data from the literature is shown in Fig. 1. As the optical source decayed, we noticed that it was apparently superposed on a faint dwarf host galaxy. On June 19, 2006 UT We obtained a spectrum of the host using the GMOS-S spectrograph mounted on the Gemini-south 8m telescope at Cerro Pachon, Chile. From this spectrum we derived the redshift of the host galaxy, and by association of the GRB, and found it to be z = 0.125, a low value for long GRBs. We confirmed this redshift with a higher quality spectrum obtained using the same instrument on July 15, 2006 UT (Fig. 2). Previous long GRBs at such low redshifts showed clear signatures of the underlying supernova (SN) explosions at comparable age post-burst. 3,12 However, such signatures were lacking in the case of this long GRB. well-detected in our first-epoch WFPC2 observations, and is apparently gone du...

Cation-π interactions are common in biological systems, and many structural studies have revealed the aromatic box as a common motif. With the aim of understanding the nature of the aromatic box, several computational methods were evaluated for their ability to reproduce experimental cation-π binding energies. We find the DFT method M06 with the 6-31G(d,p) basis set performs best of several methods tested. The binding of benzene to a number of different cations (sodium, potassium, ammonium, tetramethylammonium, and guanidinium) was studied. In addition, the binding of the organic cations NH4+ and NMe4+ to ab initio generated aromatic boxes as well as examples of aromatic boxes from protein crystal structures were investigated. These data, along with a study of the distance dependence of the cation-π interaction, indicate that multiple aromatic residues can meaningfully contribute to cation binding, even with displacements of more than an angstrom from the optimal cation-π interaction. Progressive fluorination of benzene and indole was studied as well, and binding energies obtained were used to reaffirm the validity of the “fluorination strategy” to study cation-π interactions in vivo.

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