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In this work, we establish the use of surface-enhanced Raman scattering (SERS) as a label-free analytical technique for the direct detection of G-quadruplex formation. In particular, we demonstrate that SERS analysis allows the evaluation of the relative stability of G quadruplexes that differ for the number of G tetrads and investigate several structural features of quadruplexes, such as the orientation of glycosidic bonds, the identification of distortions in the sugar-phosphate backbone, and the degree of hydrogen-bond solvation. Herein, the fluctuation of the SERS spectra, due to the specific interaction of vibrational modes with the SERS-active substrate, is quantitatively analyzed before and after quadruplex formation. The results of this study suggest a perpendicular orientation of the quadruplexes (with or without the 3'-tetra end linker) with respect to the silver colloidal surface, which opens new perspectives for the use of SERS as a label-free analytical tool for the study of the binding mode between quadruplexes and their ligands.

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During the last forty years, several groups have reported results concerning the oxidative cyclization of polyenes, mostly dienes, with transition-metal-oxo species such as permanganate, ruthenium tetroxide, perruthenate and osmium tetroxide, but only recently has a systematic study of some of these processes been undertaken. The formation in a single step of tetrahydrofuran, polytetrahydrofuran, tetrahydropyran and oxepane products with complete relative stereocontrol, and the unique postulated mechanisms, render these processes very appealing from both the synthetic and theoretical points of view. Recent synthetic applications of these oxidative transformations have highlighted their usefulness. This review summarizes the state of the art in this field, in an attempt to provide a comprehensive view of these processes. Some similarities between the chemistry of ruthenium tetroxide and rhenium(VII)-oxo species are highlighted.