UV-light-induced hydrogen transfer in guanosine-guanosine aggregates.

Aggregates of a lipophilic guanine (G) derivative have been studied in n-hexane by femtosecond-to-microsecond UV-visible broadband transient absorption, stationary infrared and UV-visible spectroscopy and by quantum chemical calculations. We report the first time-resolved spectroscopic detection of hydrogen transfer in GG aggregates, which leads to (G-H)(·) radicals by means of G(+)G(-) charge transfer followed by proton transfer. These radicals show a characteristic electronic spectrum in the range 300-550 nm. The calculated superimposed spectrum of the species that result from NH⋅⋅⋅N proton transfer agrees best with the experimental spectrum.

Excited state proton transfer is not involved in the ultrafast deactivation of Guanine-Cytosine pair in solution.

Different derivatives of Guanine (G) and Cytosine (C), which sterically enforce the Watson-Crick (WC) conformer, have been studied in CHCl(3) by means of broad-band transient absorption spectroscopy. Our experiments rule out the involvement of an Excited State Proton Transfer (ESPT), which dominates the excited state decay of GC in the gas phase. Instead, the ultrafast dynamics via internal conversion occurs in a polar environment mainly by relaxation in the monomer moieties. Time-dependent density functional theory (TD-DFT) calculations in solution indeed indicate that population transfer from the bright excited states toward the charge transfer state is not effective in CHCl(3) and a noticeable energy barrier is associated with the ESPT reaction. ESPT is therefore not expected to be a main deactivation route for GC pairs within DNA.

Fourier transform infrared spectroscopy of 2'-deoxycytidine aggregates in CDCl3 solutions.

We investigated the self-aggregation of 2'-deoxy-3',5'-bis(tert-butyldimethylsilyl)-cytidine dC(TBDMS)(2) in CDCl(3) solutions by Fourier transform infrared (FT-IR) spectroscopy and report the formation of larger aggregates than dimers in this solvent for the first time. The hydrogen bonding patterns in these complexes, which occur with increasing concentration may serve as a model for DNA super-structures such as triplexes. From the IR spectra, wavelength dependent absolute extinction coefficients of the monomer, dimer as well as a contribution from larger clusters which are supposedly trimers are deduced on the basis of a simple deconvolution method. Our results are supported by RI-B3LYP/TZVP calculations within the conductorlike screening model framework, to account for solvent effects in the ab initio calculations.

Fourier transform infrared spectroscopy of 1-cyclohexyluracil aggregates in CDCl(3) solutions.

We reinvestigated the self-aggregation of 1-cyclohexyluracil (1CHU) in CDCl(3) solutions. Wavelength dependent absolute extinction coefficients of the monomer and of dimers are presented on the basis of a simple deconvolution method. Two isomeric dimer structures coexist in a solution with similar abundance. Our results are supported by RI-B3LYP/TZVP calculations within the conductorlike screening model framework to account for solvent effects in the ab initio calculations.

Structural assignment of adenine aggregates in CDCl3.

We reinvestigated the self-association of 9-substituted adenine derivatives in CDCl3 solutions and present the infrared spectra of 9-ethyladenine and N-methyl-9-ethyladenine and its aggregates in the spectral regions between 1500 and 1800 cm(-1) and between 2700 and 3600 cm(-1). Wavelength dependent absolute extinction coefficients of the monomer and dimers are presented on the basis of a simple deconvolution method. Comparison of the deconvoluted dimer spectra with quantum chemical calculations allows for a structural assignment of the two dimer structures that coexist in 9-ethyladenine/CDCl3 solutions. In contrast, the dimer spectrum of N-methyl-9-ethyladenine is dominated by a single isomer.