1Bioorganic Research Group, Department of Life Sciences, Polytechnic of Central London, 115 New Cavendish Street, London W.1, England
2Medical Research Council, Biophysics Unit, King's College, 26–29 Drury Lane, London W.C.2, England
3Institut fu¨r Biochemie, Universita¨t Frankfurt (Main) Germany
4Department of Biochemistry, University College, Gower Street, London W.C.1, England