1. Department of Biophysics, Molecular Biology and Genetics, University of Calcutta, 92 A.P.C. Road, Calcutta 700 009, India, Biophysics Division, Saha Institute of Nuclear Physics, 37, Belgachia Road, Calcutta 700 037, India, and Biochemistry Department, Bose Institute, Centenary Building, Calcutta 700 054, India