organic papers o1178 Raj et al. C 6 H 10 N 3 O 2 + BF 4 doi:10.1107/S1600536806006192 Acta Cryst. (2006). E62, o1178–o1180 Acta Crystallographica Section E Structure Reports Online ISSN 1600-5368 L-Histidinium tetrafluoroborate S. Gokul Raj, a * G. Ramesh Kumar, a Thenneti Raghavalu, a R. Mohan a and R. Jayavel b a Department of Physics, Presidency College, Chennai 600 005, India, and b Crystal Growth Centre, Anna University, Chennai 600 025, India Correspondence e-mail: gokulrajs@yahoo.com Key indicators Single-crystal X-ray study T = 293 K Mean (C–C) = 0.003 A ˚ R factor = 0.025 wR factor = 0.067 Data-to-parameter ratio = 5.1 For details of how these key indicators were automatically derived from the article, see http://journals.iucr.org/e. Received 30 January 2006 Accepted 20 February 2006 # 2006 International Union of Crystallography All rights reserved The title compound, C 6 H 10 N 3 O 2 + BF 4 , crystallizes with one histidinium cation and one tetrafluoroborate anion in the asymmetric unit. The protonated cation and the deprotonated anion are linked through a number of intermolecular N— HO and N—HF hydrogen bonds, forming a three- dimensional network. Comment In our earlier studies, we have reported the crystal structure of l-histidinium trifluoroacetate (Gokul Raj et al., 2006). The present work reports the crystal structure of l-histidinium tetrafluoroborate, (I). Other l-histidine complexes, such as l-histidine hydrochloride monohydrate (Fuess et al. , 1977), l-histidine sulfate (Kumar et al., 2005), l-histidine dinitrate (Asath Bahadur et al., 1992), l-histidine l-tartarate (March- ewka et al., 2003), l-histidine dihydrogen arsenate orthoar- senic acid (Ratajczak et al., 2000) and l-histidine tetrafluorosuccinate (Akkurt et al., 2004), have been reported previously. The asymmetric unit contains an l-histidine molecule protonated at a ring N atom and a tetrafluoroborate anion. The conformation of the histidine side chain can be described by two torsion angles, 1 [C 3 —C 4 —C 5 —N 3 ] and 21 [N 2 —C 3 — C 4 —C 5 ] or 22 [C 2 —C 3 —C 4 —C 5 ] [IUPAC–IUB Commission on Biochemical Nomenclature, 1970]. The sum of 22 and 21 is 180 . 1 , which gives the orientation of the side chain with respect to main chain, can take values of 60 , 60 or 180 , corresponding to gauche I, gauche II and open conformations. The title compound has 1 = 61.3 (3) and hence it is in the gauche II form, which gives a closed conformation (Pratap et al., 2000). The torsion angles 21 and 22 are 130.8 (3) and 49.9 (3) , respectively. This deviation from the preferred value of 90 is due to interactions of the imidazole ring and is also observed in other compounds, for example, l-histidine tetra- fluorosuccinate [102.7 (2) and 79.2 (3) ], l-histidinium trifluoroacetate [54.1 (5) and 126.2 (4) ] and l-histidine sulfate [156.3 (6) and 28.0 (12) ]. The imidazole group of l- histidine is in a trans conformation [C3—C4—C5—C6,