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

Fourier transform surface enhanced Raman scattering spectroscopy (FT-S ERS) has been first applied to characterize the triple stranded helix d(C+T)(8)-d(AG)(8) . d(CT)(8) (pH 4.5) (tripler) 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 tripler adsorbed on the silver electrode yields five in tense and one weak SERS bands located at 241, 839, 1189, 1293, 1643, a nd 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 o ligo-DNA duplex observed by Koglin and Sequaries (Top, Curr. Chem., 13 4 (1989) 1). In the case of the tripler, the occurrence of the 839 cm( -1) band with concomitant disappearance of the adsorbed adenine ring-b reathing band at 734 cm(-1) of the duplex indicates that the helical s tructure of the tripler near the surface of the silver electrode is we ll preserved and exhibits conformational rigidity, We observed a desor ption process of the tripler from the silver electrode when the electr ode approached -0.9 V (potential of zero charge of the tripler) vs. Ag /AgCl. This indicates that the highly negatively charged tripler shows a more favorable absorption on a highly positively charged silver sur face. The structure of the tripler has also been studied by Fourier tr ansform infrared (FTIR) and Raman spectroscopy, The vibrational spectr a obtained clearly revealed that the conformation of the sugar moiety of the purine strand and one pyrimidine strand in the tripler is C-2-e ndo/anti, whereas that of the other pyrimidine strand is C-3-endo/anti .