1. Research Group for Carbohydrates of the Hungarian Academy of Sciences, H-4010 Debrecen, P.O. Box 55, Hungary, Department of Biochemistry, University of Debrecen, H-4010 Debrecen, P.O. Box 55, Hungary, and Department of Organic Chemistry, University of Debrecen, H-4010 Debrecen, P.O.Box 20, Hungary