We have carried out ab initio electronic structure calculations on a portion of DNA, the results of which provide support for a mechanism that produces single-strand breaks (SSBs) with low-energy electrons. This mechanism involves attaching a low-energy electron (ca. 1 eV) to a π* orbital of a DNA base to form a shape-resonance state. This π* anion then undergoes a sugar-phosphate C-O bond rupture over a small barrier to produce SSBs. In addition to supporting the efficacy of such a mechanism, our results suggest that solvation plays a crucial role in the rate of SSB formation when such very short-lived shape resonances are involved. In particular, they suggest that either the π* anion must be rendered electronically stable by solvation or its detachment lifetime must be several orders of magnitude longer in the solvated species than in the nonsolvated species.

The ultraviolet photoelectron spectrum of O 2 -exhibits 29 resolved vibronic transitions to the three low-lying electronic states of neutral O 2 (X 3 Σ g -, a 1 ∆g, b 1 Σ g + ) from the X 2 Π J (J ) 3/2 and 1/2) spin-orbit states of the anion. A Franck-Condon simulation, using the established molecular constants of the neutral oxygen states, matches every observed feature in the spectrum. The 0-0 origin transition is unambiguously assigned, yielding the electron affinity EA 0 (O 2 ) ) 0.448 ( 0.006 eV. The derived bond dissociation energy is D 0 (O 2 -) ) 395.9 ( 0.6 kJ/mol. Coupled-cluster theory at the CCSD(T)/aug-cc-pVTZ level is used to determine the potential energy curves of O 2 and of O 2 -in its ground state and two excited states, in both the electronically bound and unbound regions. Stabilization methods are employed to characterize the anion curves at bond lengths where their electronic energies lie above that of the ground-state neutral. The calculations confirm that the O 2 -X 2 Π g ground state is adiabatically stable, but the lowest electronically excited states of O 2 -(a 4 Σ u -and A 2 Π u ) are adiabatically unbound with respect to electron detachment. The calculations predict the anionic doublet-quartet splitting to be T e (a 4 Σ u -) -T e (X 2 Π g ) ) 2.40 eV and the first excited doublet at an energy of T e (A 2 Π u ) -T e (X 2 Π g ) ) 3.39 eV. These observations are consistent with electron scattering on O 2 and other experimental data, and they sharply refute recent interpretations of electron-capture detector experiments that EA(O 2 ) ≈ 1 eV, that O 2 -has multiple excited states below the neutral ground-state minimum, and that the doublet-quartet splitting is 0.12 eV [Chen, E. S.; Chen, E. C. M.

Although electrons having enough energy to ionize or electronically excite DNA have long been known to cause strand breaks (i.e., bond cleavages), only recently has it been suggested that even lower-energy electrons (most recently 1 eV and below) can also damage DNA. The findings of the present work suggest that, while DNA bases can attach electrons having kinetic energies in the 1 eV range and subsequently undergo phosphate-sugar O-C sigma bond cleavage, it is highly unlikely (in contrast to recent suggestions) that electrons having kinetic energies near 0 eV can attach to the phosphate unit's P=O bonds. Electron kinetic energies in the 2-3 eV range are required to attach directly to DNA's phosphate group's P=O pi orbital and induce phosphate-sugar O-C sigma bond cleavages if the phosphate groups are rendered neutral (e.g., by nearby counterions). Moreover, significant activation barriers to C-O bond breakage render the rates of both such damage mechanisms (i.e., P=O-attached and base-attached) slow as compared to electron autodetachment and to other damage processes.

The relative stabilities of zwitterionic and canonical forms of neutral arginine and of its protonated derivative were studied by using ab initio electronic structure methods. Trial structures were first identified at the PM3 level of theory with use of a genetic algorithm to systematically vary geometrical parameters. Further geometry optimizations of these structures were performed at the MP2 and B3LYP levels of theory with basis sets of the 6-31++G** quality. The final energies were determined at the CCSD/6-31++G** level and corrected for thermal effects determined at the B3LYP level. Two new nonzwitterionic structures of the neutral were identified, and one of them is the lowest energy structure found so far. The five lowest energy structures of neutral arginine are all nonzwitterionic in nature and are clustered within a narrow energy range of 2.3 kcal/mol. The lowest energy zwitterion structure is less stable than the lowest nonzwitterion structure by 4.0 kcal/mol. For no level of theory is a zwitterion structure suggested to be the global minimum. The calculated proton affinity of 256.3 kcal/mol and gas-phase basicity of 247.8 kcal/mol of arginine are in reasonable agreement with the measured values of 251.2 and 240.6 kcal/mol, respectively. The calculated vibrational characteristics of the low-energy structures of neutral arginine provide an alternative interpretation of the IR-CRLAS spectrum (Chapo et al. J. Am. Chem. Soc. 1998, 120, 12956-12957).

Dipole-bound anionic states of HCN, (HF)2, CH3CN, C3H2, C4H2, C5H2, and stretched CH3F are studied using extended one-electron basis sets at the coupled cluster level of theory with single, double, and noniterative triple excitations (CCSD(T)). Orbital relaxation and electron correlation corrections to the Koopmans' theorem prediction of electron binding energy are analyzed, and a physical interpretation of low-order corrections is proposed. It is demonstrated that the second-order dispersion interaction between the loosely bound electron and the electrons of the neutral host should be included into physical models of dipole-bound anions. Higher-order electron correlation corrections are also found to be important, and a slow convergence of the Møller−Plesset series for electron binding energies is documented. Modifications of the potential energy surfaces of the above polar molecules upon electron attachment are studied at the second-order Møller−Plesset level, and Franck−Condon factors for the anion/neutral pairs are calculated. It is predicted that photoelectron spectra of the dipole-bound anions of C4H2 and C5H2 should display vibrational structure.

It has long been assumed that electron correlation effects are relatively unimportant for describing dipolebound anionic states. It is shown here that this assumption is incorrect: high-level electronic structure calculations on the dipole-bound anion states of CH 3 CN, C 3 H 2 , and C 5 H 2 reveal that for these species a large fraction of the electron binding energy derives from the dispersion-type interaction between the loosely bound electron and the neutral molecule. The predicted values of the electron affinities of the dipole bound states of CH 3 CN and C 3 H 2 , 108 and 173 cm Ϫ1 , respectively, are in excellent agreement with the recent experimental results, 93 cm Ϫ1 and 171Ϯ50 cm Ϫ1 , respectively. The predicted value for C 5 H 2 is 614 cm Ϫ1 .

The instability of the zwitterion structure of glycine is significantly reduced by the attachment of an excess electron as a result of which a local minimum develops on the anionic potential energy surface for the zwitterion structure. However, the global anionic minimum, which is lower by 9 kcal/mol, corresponds to a singly hydrogen-bonded nonzwitterion structure. The vertical electron detachment energies for these two dipole-bound zwitterion and nonzwitterion structures are 3175 and 668 cm-1, respectively.

The fragmentation of positively charged gas-phase samples of peptides is used to infer the primary structure of such molecules. In electron capture dissociation (ECD) experiments, very low-energy electrons attach to the sample and rupture bonds to effect the fragmentation. It turns out that ECD fragmentation tends to produce cleavage of very specific types of bonds. In earlier works by this group, it has been suggested that the presence of positive charges produces stabilizing Coulomb potentials that allow low-energy electrons to exothermically attach to σ* orbitals of certain bonds and thus to cleave those bonds. In the present effort, the stabilizing effects of Coulomb potentials due to proximal positive charges are examined for a small model peptide molecule that contains a wide range of bond types. Direct attachment of an electron to the σ* orbitals of eight different bonds as well as indirect σ bond cleavage, in which an electron first binds to a carbonyl CdO π* orbital, are examined using ab initio methods. It is found that direct attachment to and subsequent cleavage of any of the eight σ bonds is not likely except for highly positively charged samples. It is also found that attachment to a CdO π* orbital followed by cleavage of the nitrogen-to-R-carbon bond is the most likely outcome. Interestingly, this bond cleavage is the one that is seen most commonly in ECD experiments. So, the results presented here seem to offer good insight into one aspect of the ECD process, and they provide a means by which one can estimate (on the basis of a simple Coulomb energy formula) which bonds may be susceptible to cleavage by low-energy electron attachment.