1. Department of Biology & Biochemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
2. Division
of Chemistry and Structural Biology, Australian Research Council Centre
of Excellence in Advanced Molecular Imaging, Institute for Molecular
Bioscience, University of Queensland, Brisbane, Queensland 4072, Australia
3. School
of Biochemistry, University of Bristol, University Walk, Bristol BS8 1TD, U.K.