[1] Allegheny Hlth Network Res Inst, Lab Bimol Med, Pittsburgh, PA 15212 USA
[2] Oregon State Univ, Dept Biochem & Biophys, Corvallis, OR 97331 USA
[3] Carnegie Mellon Univ, Ctr Nucle Acids Sci & Technol, Pittsburgh, PA 15213 USA
[4] Carnegie Mellon Univ, Dept Chem, Pittsburgh, PA 15213 USA