DFT studies on the pairing abilities of the one-electron reduced or oxidized adenine-thymine base pair

Using density functional theory (DFT), the hydrogen bonds making up the adenine-thymine (A-T) base pair are found to increase in total energy upon one-electron oxidation or reduction by 10.9 and 13.3 kcal mol(-1), respectively. Due to unsymmetric changes in the H-bond lengths, this strengthening affects an expansion of the base pair length (N1'-N9) by similar to0.27 Angstrom. In the oxidized pair, A(.+)-T, deprotonation from N-6, and with the reduced pair, A(.-)-T, protonation on N3 or N7 lead to base pairs which have similar base pairing energies as their parent A-T, i.e., the stabilization by the change in oxidation state is annihilated by (de)protonation. The calculated proton affinities of A(.-)-T are large enough to explain its protonation by H2O, which involves heterolytic bond cleavage of a water molecule. The N1 protonated electron adduct of A is a powerful H-bond donor; it is able to mismatch with cytosine (-28.9 kcal mol(-1)). In DNA this could compete with the "legitimate" guanine-cytosine pairing. The pairing abilities of 2-aminopurine, an "unnatural" isomer of A, used as a fluorescent probe in DNA assemblies, are calculated to resemble those of A closely

Photo- and radiation-chemical production of radical cations of methylbenzenes and benzyl alcohols and their reactivity in aqueous solution

Radical cations of methylated benzenes and benzyl alcohols were generated by photoionization and by reaction with the oxidant SO4(.-) in aqueous solution. The photoionization requires two 248 nm photons. The lifetimes and absorption spectra of the radical cations produced were determined by time-resolved conductance and optical detection, and the reaction products were measured by GC. As expected, the radical cation lifetimes increase strongly with increasing number of additional methyl groups, and so does the ratio of deprotonation from a methyl or hydroxymethyl group vs. addition (of water) to a ring position. In the case of toluene the radical cation appears to have a chemical lifetime tau of 10-100 ps less than or equal to tau less than or equal to 20 ns, i.e., longer than it takes for an ion pair to separate into the free (solvated) ions, and it reacts predominantly by addition of water to the ring rather than by deprotonation from the methyl group. A further observation is that, as compared to methoxylated analogues, the methylated benzyl alcohol radical cations are much more reactive, such that OH--induction of side-chain fragmentation, as often required with methoxylated benzyl alcohol-type radical cations, is not necessary

DNA-base radicals. Their base pairing abilities as calculated by DFT

The guanine radical cation (G(.+)) deprotonates at its N1-site to give the neutral radical G(-H)(.). On the basis of DFT calculations this species is able to pair not only with cytosine (C), but also with thymine (T), adenine (A) and guanine (G), giving new base pair structures formed by the N1- site acting as a hydrogen bond acceptor. The deprotonated radical cation, G(-H)(.), forms two new (strong) hydrogen bonds with C, resulting in a new, non-classical, base pair. In contrast, pairing between G(-H)(.) and T is relatively poor. G(-H)(.) forms two hydrogen bonds with A with a similar hydrogen bond energy as the (natural) A-T base pair and it bonds with G and even with itself (G(-H)(.)) forming three or two hydrogen bonds, respectively. The former has practically the same bonding energy as the (natural) G-C base pair. In summary, as compared to G, G(-H)(.) loses any specificity for C since it readily forms pairs with all the DNA bases. The deprotonated radical cation of 7,8-dihydro-8-oxoguanine, the neutral radical 8OG(-H)(.), shows very similar base pairing behavior to G(-H)(.)

Photoionization of β-carotene via electron transfer from excited states to chlorinated hydrocarbon solvents. A picosecond transient absorption study

Time-resolved photoinduced (lambda(exc) 266, 355 and 532 nm) formation of the radical cation of beta-carotene in the chlorinated solvents n-butylchloride, dichloromethane, 1,2- dichloroethane, chloroform and carbon tetrachloride, was studied with 5-10 ps time resolution. The radical cation is formed via electron transfer from the electronically excited solute to the solvent and is characterized by an absorption band with maximum at 900-950 nm. The process is efficient (phi > 0.12) and fast (t < 10 ps), which excludes involvement of the triplet state. It is suggested that the electron transfer proceeds from either the vibrationally excited singlet 2A(g)(-) (l greater than or equal to 2) or from the "forbidden" 1B(u)(-) state. The formation times of the carotene radical cation in the chlorinated hydrocarbon solvents are on the order of their diffusion times