Conformers of guanosines and their vibrations in the electronic ground and excited states, as revealed by double-resonance spectroscopy and ab initio calculations.

Abstract

two-photon ionization. The results showed that nucleobases and complexes composed of paired bases can be studied in great detail in the absence of the external effects of solvent interactions and the collective modes of DNA. Here we show by means of double-resonance laser spectroscopy and ab initio calculations that, in the investigated wavelength range around 290 nm, guanosine (Gs), 2-deoxyguanosine (2-deoxyGs), and 3-deoxyguanosine (3-deoxyGs) each exhibit only the spectrum of one stable isomer, which is stabilized by a strong intramolecular sugar(5-OH) ¥¥¥ enolguanine(3-N) hydrogen bond. Figure 1 shows a possible structure of guanosine (Gs) with guanine in its enol form; atoms which are important for the following discussion are numbered. The R2PI spectra of laser-desorbed Gs, 2-deoxyGs, 3 ± deoxyGs have already been published. [5, 21] In the earlier paper we speculated that two conformers might contribute to the vibronic spectra of Gs and 3deoxyGs. To test this hypothesis we performed UV ± UV spectral hole burning (SHB) measurements. Figure 2 shows that each of the three spectra originates from only one conformer, because every single peak in the R2PI spectrum is reflected in the corresponding SHB spectrum. For comparison the R2PI spectrum of 2,3-diisopropylguanosine is shown as well. Figure 3 shows the IR spectra obtained from IR ± UV measurements with the UV laser fixed to the most intense vibronic transition of the respective guanosine, indicated by asterisks in Figure 2. For comparison the IR spectra of 2,3-diisopropylGs and one enol and one keto guanine (G) tautomer are also shown. In 2,3-diisopropylGs, the OH groups of the sugar moiety in the 2and 3-positions are blocked. Therefore, its IR spectrum does not exhibit the corresponding OH vibrations but agrees well with