1. Contribution from the Department of Biochemistry, Eötvös Loránd University, 1/C Pázmány P. sétány, H-1117 Budapest, Hungary, Protein Chemistry Research Group, Hungarian Academy of Sciences, 8 Dóm tér, H-6720 Szeged, Hungary, Department of Chemistry, Tartu University, 2 Jakobi Street, EE−51014 Tartu, Estonia, and Department of Medical Chemistry, University of Szeged, 8 Dóm tér, H-6720 Szeged, Hungary