1. A. F. A. Marsden, J. Cortés, N. J. Dunster, P. F. Leadlay, Cambridge Centre for Molecular Recognition and Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge, CB2 1QW, UK.
2. B. Wilkinson and J. Staunton, Cambridge Centre for Molecular Recognition and University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK.