1. Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares, CIQUS, and Departamento de Química Orgánica; Universidade de Santiago de Compostela; calle Jenaro de la Fuente s/n 15782 Santiago de Compostela Spain
2. Department of Biochemistry; University of Zurich; 8057 Zurich Switzerland
3. Current address: Department of Biochemistry and Biophysics; University of North Carolina, School of Medicine; Chapel Hill NC 27599 USA
4. Institute of Cell and Molecular Biosciences, Medical School; University of Newcastle upon Tyne; Catherine Cookson Building, Framlington Place Newcastle upon Tyne NE2 4HH UK