作者单位:Institute for Organic Chemistry and Chemical Biology, Center for Biomolecular Magnetic Resonance, Johann Wolfgang Goethe-University, Max von Laue-Str. 7, D-60438 Frankfurt am Main, Germany[1]Laboratory of Nucleic Acid Chemistry, ?cole Polytechnique Fédérale de Lausanne, EPFL-BCH, 1015 Lausanne, France[2]