1. Contribution from the Department of Biophysics, P. J. Šafárik University, Jesenná 5, 041 54 Košice, Slovakia, Instituto de Estructura de la Materia, CSIC, Serrano, 121, 28006 Madrid, Spain, and LPBC (CNRS URA 2056), Université P. et M. Curie, Case 138, 4 Place Jussieu, 75231 Paris Cedex 05, France