Could well be a cation-pi interaction. Have been reading up on Arg/Trp interactions and sitting above the ring is a classic place for a cation. See The Cation-π Interaction DENNIS A. DOUGHERTY ACCOUNTS OF CHEMICAL RESEARCH ’ 885–893 ’ 2013 ’ Vol. 46, No. 4 for up to date or I found Ma, J. C., and Dougherty, D. A. (1997) The cation-pi interaction. Chemical Reviews 97, 1303-132 better from a structural biology point of view. I would go with a metal ion or ammonium but as to which one I have no idea. Best wishes Nick -- Prof Nicholas H. Keep Executive Dean of School of Science Professor of Biomolecular Science Crystallography, Institute for Structural and Molecular Biology, Department of Biological Sciences Birkbeck, University of London, Malet Street, Bloomsbury LONDON WC1E 7HX email [email protected] Telephone 020-7631-6852 (Room G54a Office) 020-7631-6800 (Department Office) Fax 020-7631-6803 If you want to access me in person you have to come to the crystallography entrance and ring me or the department office from the internal phone by the door