Vibrational Spectroscopy of the G· · ·C Base Pair:  Experiment, Harmonic and Anharmonic Calculations, and the Nature of the Anharmonic Couplings

Abstract

The results of harmonic and anharmonic frequency calculations on a guanine-cytosine complex with an enolic structure (a tautomeric form with cytosine in the enol form and with a hydrogen at the 7-position on guanine) are presented and compared to gas-phase IR-UV double resonance spectral data. Harmonic frequencies were obtained at the RI-MP2/cc-pVDZ, RI-MP2/TZVPP, and semiempirical PM3 levels of electronic structure theory. Anharmonic frequencies were obtained by the CC-VSCF method with improved PM3 potential surfaces; the improved PM3 potential surfaces are obtained from standard PM3 theory by coordinate scaling such that the improved PM3 harmonic frequencies are the same as those computed at the RI-MP2/cc-pVDZ level. Comparison of the data with experimental results indicates that the average absolute percentage deviation for the methods is 2.6% for harmonic RI-MP2/cc-pVDZ (3.0% with the inclusion of a 0.956 scaling factor that compensates for anharmonicity), 2.5% for harmonic RI-MP2/TZVPP (2.9% with a 0.956 anharmonicity factor included), and 2.3% for adapted PM3 CC-VSCF; the empirical scaling factor for the ab initio harmonic calculations improves the stretching frequencies but decreases the accuracy of the other mode frequencies. The agreement with experiment supports the adequacy of the improved PM3 potentials for describing the anharmonic force field of the G...C base pair in the spectroscopically probed region. These results may be useful for the prediction of the pathways of vibrational energy flow upon excitation of this system. The anharmonic calculations indicate that anharmonicity along single mode coordinates can be significant for simple stretching modes. For several other cases, coupling between different vibrational modes provides the main contribution to anharmonicity. Examples of strongly anharmonically coupled modes are the symmetric stretch and group torsion of the hydrogen-bonded NH2 group on guanine, the OH stretch and torsion of the enol group on cytosine, and the NH stretch and NH out-of-plane bend of the non-hydrogen-bonded NH group on guanine.

Gas-Phase IR Spectroscopy of Nucleobases

“…In the quest to develop more comprehensive computational strategies it is useful to be able to compare with both near-and mid-IR data as sensitive tests. Figure 9 shows such a comparison for the GC base pair with the cytosine in the enol form, as depicted in trace (c) in Figure 12 (62). The mid IR part of this spectrum (left panel) was obtained at FELIX and the lower frequencies provide an especially sensitive test for theory.…”

Analysis of the S2←S0 vibronic spectrum of the ortho-cyanophenol dimer using a multimode vibronic coupling approach

“…Anharmonicity along single-mode coordinates in the ground state of hydrogen-bonded systems is a well-known phenomenon and has been the subject of extensive studies. 44 As an example, for the (oCP) 2 system, Fig. 4(a) shows the ground state potentials along the totally symmetric normal mode ν 10 including the harmonic ground state potential and the anharmonic fit to the ab initio single-point energies calculated along Q 10 .…”

The World of Non-Covalent Interactions: 2006

“…Harmonic and anharmonic vibration frequencies were determined for the guanine...cytosine complex and compared with gas-phase IR-UV double resonance spectral data. Harmonic frequencies were obtained at RI-MP2/cc-pVDZ and RI-MP2/TZVPP levels and anharmonic frequencies were obtained by the CC-VSCF method based on improved semiempirical PM3 results 130 . Comparison of the data with experimental results indicates that the average absolute percentage deviations for the method is 2.6% for harmonic RI-MP2/cc/pVDZ, 2.5% for harmonic RI-MP2/TZVPP and 2.3% for the adopted PM3 CC-VSCF.…”