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Dear David, have a look at the Ammonium Transporter structure, AmtB, PDB entries 1XQE, 1XQF by Zheng et al. or 2NMR and so on by Javelle et al. (search for AmtB on www.pdb.org). We've seen such an unusual shared hydrogen bond between two histidines in the middle of the ammonia channel. Best regards, Dirk. david lawson (JIC) wrote: > Dear All, > > We have solved a structure that has an intersubunit His-His H-bond > (ND1---NE2). The protein most likely undergoes conformational changes > that involve the making and breaking of H-bonds at this interface. In > all the protein structures I have previously studied I don't recall ever > seeing such a bond - His-His ring stacking yes, but never H-bonding. I > was wondering if anyone else had seen this type of interaction and > whether it had any functional significance. The crystals were grown at > pH 5.6. > > Many thanks > > Dave Lawson > > ------------------------------- > > Dr. David M. Lawson > Biological Chemistry Dept., > John Innes Centre, > Norwich, > NR4 7UH, UK. > Tel: +44-(0)1603-450725 > Fax: +44-(0)1603-450018 > Email: [log in to unmask] > Web: http://www.jic.bbsrc.ac.uk/staff/david-lawson/index.htm > -- **************************************** Dirk Kostrewa Paul Scherrer Institut Biomolecular Research, OFLC/110 CH-5232 Villigen PSI, Switzerland Phone: +41-56-310-4722 Fax: +41-56-310-5288 E-mail: [log in to unmask] http://sb.web.psi.ch ****************************************