b -Carboline (norharman) Robert J. Thatcher* and Richard E. Douthwaite Department of Chemistry, University of York, Heslington, York YO10 5DD, England Correspondence e-mail: rjt509@york.ac.uk Received 16 May 2011 Accepted 6 June 2011 Online 23 June 2011 The structure of -carboline, also called norharman (systematic name: 9H-pyrido[3,4-b]indole), C 11 H 8 N 2 , has been determined at 110 K. Norharman is prevalent in the environ- ment and the human body and is of wide biological interest. The structure exhibits intermolecular N—HN hydrogen bonding, which results in a one-dimensional herringbone motif. The three rings of the norharman molecule collectively result in a C-shaped curvature of 3.19 (13)  parallel to the long axis. The diffraction data show shorter pyridyl C—C bonds than those reported at the STO-3G level of theory. Comment Norharman, (I), is the prototypical -carboline alkaloid that is the basic structural unit for a wide range of important natu- rally occurring compounds. Norharman is found in numerous plants and animals, including humans (Fekkes et al. , 1992). It is also prevalent in the environment, for example, as a consti- tuent of cigarette smoke, and can be absorbed from numerous foodstuffs and other environmental sources (Herraiz, 2004). The biological function of norharman appears to be varied and its toxicity and therapeutic uses have been investigated. For example, norharman has been implicated as both a neurotoxin relevant to Parkinson’s disease (Kuhn et al., 1996) and a mediator in the mutagenesis of DNA in the presence of other small molecules (Mori et al., 1996). Norharman has also been suggested as a potential neuroprotective agent (Haghdoost- Yazdi et al., 2010). The molecular-based interactions between norharman and biological materials such as DNA and proteins are likely to arise primarily from hydrogen bonding and quadrupolar effects, due to the -derived electrons. However, no adducts or cocrystals of norharman have been published to date. Despite considerable investigation into the properties and biological activity of norharman and the calculation of its optimized structure at the STO-3G level of theory (Konschin et al., 1987), a high-quality crystal structure has not yet been reported. Crystallographic studies undertaken by Ray (1957) and later by Roychowdhury & Roychowdhury (1981), deter- mined unit-cell parameters and presented gross structural features. Given the ongoing research into the biological function of norharman and the many related -carboline derivatives, a single-crystal X-ray structure of norharman would be of use in theoretical modelling and related structural work. A fragment search of the Cambridge Structural Data- base (CSD, Version 5.32; Allen, 2002) using the unsaturated norharman skeleton returned 48 hits for structures where the three-dimensional coordinates have been reported. The bond lengths and distances were subjected to principal-component and cluster analyses (Barr et al., 2005; Fletcher et al., 1996) and correlation coefficients were calculated, but no meaningful trends were observed. Investigation of the bonding in these molecules is restricted, in part due to the limited number of structures reported. Therefore, the structure of norharman will be of use as a fundamental reference as more -carboline structures are obtained. Single crystals of norharman were grown as part of ongoing efforts to explore the metal–ligand bonding and electron distribution in complexes incorporating the norharman motif. The needle-like morphology resulted in poor diffraction, but a data set of sufficient quality to locate the H atoms was obtained. The unit-cell parameters and space group are consistent with those reported previously (Ray, 1957). Upon refinement, the H atoms were observed in the difference map, although all but amino atom H2 (Fig. 1) were refined using a riding model. The molecular structure of norharman (Fig. 1) exhibits curvature parallel to the longest axis, with angles between the least-squares planes of the benzene/pyrrole and pyrrole/ pyridyl rings of 0.68 (14) and 2.60 (14)  , respectively, defining a C- rather than S-shaped geometry. The X-ray structures of other -carbolines in the CSD generally exhibit curvatures of 0–5  . The bond lengths of the benzene fragment are identical, within experimental error, to those observed for aniline, where the latter exhibits asymmetry about the Ph—N axis due organic compounds Acta Cryst. (2011). C67, o241–o243 doi:10.1107/S0108270111021706 # 2011 International Union of Crystallography o241 Acta Crystallographica Section C Crystal Structure Communications ISSN 0108-2701 Figure 1 The molecular structure of norharman, (I). Displacement ellipsoids are drawn at the 50% probability level.