1. Division Molecular Biophysics (B020), Deutsches Krebsforschungszentrum (DKFZ), Im Neuenheimer Feld 580, D-69120 Heidelberg, Germany, Molecular Biophysics Unit, Indian Institute of Science, Bangalore 560012, India, Department of Physics, Banaras Hindu University, Varanasi-221005, India, and Department of Chemistry, Indian Institute of Technology, Guwahati-781039, Assam, India