Raman spectroscopy has been combined with site-selective isotopic labeling techniques to obtain structural information on a selected nucleic acid residue in oligonucleotides. In the difference spectrum between the unlabeled and site-selectively labeled oligonucleotides, the Raman signals from the residue at the labeled position show up, while the signals from the residues at unlabeled positions are canceled out. To demonstrate the utility of this new method, we have prepared a self-complementary tetradecamer DNA, d(AGTGCTCGAGCACT)(2), containing single C8-deuterated adenine at position 9 (A9) or 12 (A12). UV (257 nm) resonance Raman difference spectra between the unlabeled and labeled oligonucleotides in solution reveal slightly different microenvironments around A9 and A 12 as expected for a double-stranded helical structure of the tetradecamer DNA. In the presence of an antitumor antibiotic, actinomycin D, which intercalates into the 5'-GC-3' sequence of DNA, the Raman signals of A9 on the 5'-side of the intercalation site become significantly weaker, indicating an increased base stacking with adjacent guanine bases. In contrast, the Raman signals of A12 on the 3'-side are not affected by the binding of actinomycin D. This observation provides the first experimental evidence that the intercalation of actinomycin D induces an asymmetric structural alteration of DNA in solution. (C) 2001 Elsevier Science B.V. All rights reserved.