1. Contribution from the School of Physical Sciences, University of Kent, Canterbury CT2 7NR, United Kingdom, Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester, United Kingdom, EPSRC Multi-frequency EPR Service, Dept of Chemistry, The University of Manchester, Manchester M13 9PL, United Kingdom, and Research School of Biosciences, University of Kent, Canterbury CT2 7NJ, United Kingdom