N-H stretching vibrations of guanosine-cytidine base pairs in solution: ultrafast dynamics, couplings, and line shapes

J Phys Chem A. 2013 Feb 7;117(5):845-54. doi: 10.1021/jp309237u. Epub 2013 Jan 29.

Abstract

Dynamics and couplings of N-H stretching vibrations of chemically modified guanosine-cytidine (G·C) base pairs in chloroform are investigated with linear infrared spectroscopy and ultrafast two-dimensional infrared (2D-IR) spectroscopy. Comparison of G·C absorption spectra before and after H/D exchange reveals significant N-H stretching absorption in the region from 2500 up to 3300 cm(-1). Both of the local stretching modes ν(C)(NH(2))(b) of the hydrogen-bonded N-H moiety of the cytidine NH(2) group and ν(G)(NH) of the guanosine N-H group contribute to this broad absorption band. Its complex line shape is attributed to Fermi resonances of the N-H stretching modes with combination and overtones of fingerprint vibrations and anharmonic couplings to low-frequency modes. Cross-peaks in the nonlinear 2D spectra between the 3491 cm(-1) free N-H oscillator band and the bands centered at 3145 and 3303 cm(-1) imply N-H···O═C hydrogen bond character for both of these transitions. Time evolution illustrates that the 3303 cm(-1) band is composed of a nearly homogeneous band absorbing at 3301 cm(-1), ascribed to ν(G)(NH(2))(b), and a broad inhomogeneous band peaking at 3380 cm(-1) with mainly guanosine carbonyl overtone character. Kinetics and signal strengths indicate a <0.2 ps virtually complete population transfer from the excited ν(G)(NH(2))(b) mode to the ν(G)(NH) mode at 3145 cm(-1), suggesting lifetime broadening as the dominant source for the homogeneous line shape of the 3301 cm(-1) transition. For the 3145 cm(-1) band, a 0.3 ps population lifetime was obtained.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Base Pairing
  • Chloroform / chemistry
  • Cytidine / chemistry*
  • Deuterium Exchange Measurement
  • Guanosine / chemistry*
  • Kinetics
  • Solutions
  • Spectrophotometry, Infrared

Substances

  • Solutions
  • Guanosine
  • Cytidine
  • Chloroform