1. Department of Protein Chemistry, Institute of Molecular Biology, University of Copenhagen, Øster Farimagsgade 2A, DK-1353 Copenhagen K, Denmark
2. Department of Chemistry and Biochemistry, University of Texas, Austin, TX 78712, U.S.A.
3. Carlsberg Research Laboratory, Gamle Carlsberg Vej 10, DK-2500 Copenhagen V, Denmark