1. Federal Institute of Materials Research and Testing, Project Group I.3903, Richard Willstaetter Str. 11, D-12489, Berlin, Germany, Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK, The Royal Veterinary College, Royal College Street, London, NW1 0TU, UK, Department of Clinical Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge, CB3 0ES, UK, and Centre for Equine Studies, Animal Health Trust, Lanwades Park, Kentford, Newmarket, Suffolk, CB8 7UU, UK