EVIDENCE FOR THE CONFORMATIONAL RIGIDITY OF TRIPLEX D(C+T)8-D(AG)8.D(CT)8 ON SILVER ELECTRODE REVEALED BY FOURIER TRANSFORM RAMAN SCATTERING STUDIES

Abstract Fourier transform surface enhanced Raman scattering spectroscopy (FT-SERS) has been first applied to characterize the triple stranded helix d(C + T) 8 −d(AG) 8 ·d(CT) 8 (pH 4.5) (triplex) and its corresponding double-stranded helix d(AG) 8 ·d(CT) 8 (pH 7.0) (duplex) at an ex situ roughened silver electrode polarized at between 0.0 and −1.0 V vs. Ag AgCl . The triplex adsorbed on the silver electrode yields five intense and one weak SERS bands located at 241, 839, 1189, 1293, 1643, and 1530 cm −1 , respectively. These bands are not seen in the FT-SERS spectrum of the duplex, which showed a pattern similar to that of an oligo-DNA duplex observed by Koglin and Sequaries (Top. Curr. Chem., 134 (1989) 1). In the case of the triplex, the occurrence of the 839 cm −1 band with concomitant disappearance of the adsorbed adenine ring-breathing band at 734 cm −1 of the duplex indicates that the helical structure of the triplex near the surface of the silver electrode is well preserved and exhibits conformational rigidity. We observed a desorption process of the triplex from the silver electrode when the electrode approached −0.9 V (potential of zero charge of the triplex) vs. Ag AgCl . This indicates that the highly negatively charged triplex shows a more favorable absorption on a highly positively charged silver surface. The structure of the triplex has also been studied by Fourier transform infrared (FTIR) and Raman spectroscopy. The vibrational spectra obtained clearly revealed that the conformation of the sugar moiety of the purine strand and one pyrimidine strand in the triplex is C 2′ -endo/anti, whereas that of the other pyrimidine strand is C 3′ -endo/anti.