title: Four Bases Score a Run: Ab Initio Calculations Quantify a Cooperative Effect of H-Bonding and π-Stacking on the Ionization Energy of Adenine in the AATT Tetramer.
abstract: Benchmark calculations of the lowest ionized state of the (A:T)2 (mixed adenine-thymine) cluster at the geometry taken from the DNA X-ray structure are presented. Vertical ionization energies (IEs) computed by the equation-of-motion coupled-cluster method with single and double substitutions are reported and analyzed. The shift in IE relative to the monomer (A) is -0.7 eV. The performance of the widely used B3LYP, ωB97X-D, and M06-2X functionals with respect to their ability to describe energetics and the character (localization versus delocalization) of the ionized states is also investigated. The shifts in IEs caused by H-bonding and stacking interactions are analyzed in terms of additive versus cooperative effects. It is found that the cooperative effect accounts for more than 20% of the shift in IE relative to the monomer. The cooperative effect and, consequently, the magnitude of the shift are well reproduced by the hybrid quantum mechanics/molecular mechanics scheme in which neutral thymine bases are represented by point charges.
authors: †Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, United States.
authors: Ksenia B
authors: KB
authors: Bravaya
authors: †Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, United States.|‡Department of Chemistry, University of California, Berkeley, California 94720, United States and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors: Evgeny
authors: E
authors: Epifanovsky
authors: †Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, United States.
authors: Anna I
authors: AI
authors: Krylov
keywords: DNA bases; EOM-IP-CCSD; H-bonding; electronic structure; ionized states; photoionization; pi-stacking