Density Functional Theory Studies of the Extent of Hole Delocalization in One-Electron Oxidized Adenine and Guanine Base Stacks

This study investigates the extent of hole delocalization in one-electron oxidized adenine (A) and guanine (G) stacks and shows that new IR vibrational bands are predicted that are characteristic of hole delocalization within A-stacks. The geometries of A-stacks (A(i); i = 2-8) and G-stacks (GG and GGG) in their neutral and one-electron oxidized states were optimized with the bases in a B-DNA conformation using the M06-2X/6-31G* method. The highest occupied molecular orbital (HOMO) is localized on a single adenine in A-stacks and on a single guanine in GG and GGG stacks located at the 5'-site of the stack. On one-electron oxidation (removal of an electron from the HOMO of the neutral A- and G-stacks) a "hole" is created. Mulliken charge analysis shows that these "holes" are delocalized over two to three adenine bases in the A-stack. The calculated spin density distribution of A(i)(center dot+) (i = 2-8) also showed delocalization of the hole predominantly on two adenine bases, with some delocalization on a neighboring base. For GG and GGG radical cations, the hole was found to be localized on a single G in the stack. The calculated HFCCs of GG and GGG are in good agreement with the experiment. Further, from the vibrational frequency analysis, it was found that IR spectra of neutral and the corresponding one-electron oxidized adenine stacks are quite different. The IR spectra of A(2)(center dot+) has intense IR peaks between 900 and 1500 cm(-1) that are not present in the neutral A2 stack. The presence of A(2)(center dot+) in the adenine stack has a characteristic intense peak at similar to 1100 cm(-1). Thus, IR and Raman spectroscopy has potential for monitoring the extent of hole delocalization in A stacks.