Methods for crystallizing a molecule of interest, such as a polypeptide, in complex with nucleic acid, including contacting the molecule of interest with selenium-derivatized nucleic acid and crystallizing the molecule of interest/selenium-derivatized nucleic acid complex are provided. Methods for determining the X-ray crystal structure of molecule of interest/selenium-derivatized nucleic acid complexes are also provided. Typically, the method of X-ray crystal structural determination includes selenium single-wavelength anomalous phasing of the selenium-derivatized nucleic acid. In some embodiments the phases for the X-ray crystal structure of the molecule of interest are not provided from another crystal. Also disclosed are methods of affecting a biological process by administering a functional nucleic acid to a cell or a subject and/or by bringing into contact a nuclease and a functional nucleic acid, where the functional nucleic acid is selenium-derivatized nucleic acid.
