1. Department of Food Science, Cornell University, Ithaca, New York 14853, Department of Biochemistry, Bristol University, Bristol BS8 1TD, United Kingdom, H. H. Wills Physics Laboratory, Bristol University, Bristol BS8 1TL, United Kingdom, and Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau Drive, Mail Stop 6102, Gaithersburg, Maryland 20899