1. Department of Biochemistry, August Krogh Institute, University of Copenhagen, Universitetsparken 13, DK-2100, Copenhagen Ø, Denmark, Centre de Recherches sur les Macromolécules Végétales, CNRS at Joseph Fourier University, B. P. 53 F-38041, Grenoble Cedex 9, France, and Department of Chemistry, Carlsberg Laboratory, Gamle Carlsberg Vej 10, DK-2500 Valby, Denmark