1. Chemistry & Biochemistry Laboratory, Department of Agriculture, University of Reading, P.O. Box 236, Earley Gate, Reading RG6 6AT, and School of Pharmacy and Department of Food Biosciences, University of Reading, P.O. Box 226, Whiteknights, Reading RG6 6AP, and Polymer Laboratories Ltd., Essex Road, Church Stretton, Shropshire SY6 6AX, United Kingdom