CASSCF Investigation of Electronic Excited States of 2-Aminopurine

Published

January 11, 2001

Author(s)

E L. Rachofsky, J B. Ross, Morris Krauss, R Osman

Abstract

2-aminopurine is a highly fluorescent analog of adenine that can be incorporated synthetically into DNA with little perturbation of the native double-helical structure. The sensitive dependence of the quantum yield of this fluorophore on nucleic acid conformation has made it an invaluable probe of DNA structure, dynamics, and interactions. To assist in the development of models for molecular interpretation of fluorescence measurements, the electronic structure of 2-amino-9-methylpurine has been calculated in the ground state and the lowest singlet ππ* and nπ* excited states. These computations employed the complete active space multiconfigurational self-consistent field method (CASSCF) supplemented by multiconfigurational quasi-degenerate perturbation theory (MCQDPT). The predicted energies for both ππ* excitation and emission and nπ* excitation are in good agreement with previous experimental values. The permanent molecular dipoles of the ground and ππ* excited states are similar in magnitude and direction, consistent with experimental observations of weak solvatochromic shifts in ππ* absorption and emission spectra. However, the permanent dipole of the nπ* state is rotated approximately 60 relative to that of the ground state, implying that the nπ* excitation energy will increase in more polar solvents due to the relative destabilization of this state by unfavorably oriented solvent dipoles. This result demonstrates that the blue-shift of ππ* state in polar solvents, which is commonly attributed to the effect of hydrogen bonding, can arise entirely from a general solvent effect. The energy of a radiationless vibronic transition from the ππ* state will increase in more polar solvents, provided that the solvent does not rearrange during the transition. Consequently, the efficiency of fluorescence quenching by vibronic coupling between the ππ* and nπ* states is predicted to decrease significantly in

Proceedings Title

Journal of Physical Chemistry A

Volume

105

Issue

1

Conference Dates

July 23-27, 1998

Conference Location

Torun, PL

Conference Title

Jablonski Centennial Conference on Luminescence and Photophysics

Pub Type

Conferences

Keywords

2-aminopurine, CASSCF, excited states, MCQDPT, solvent shift

Citation

Rachofsky, E. , Ross, J. , Krauss, M. and Osman, R. (2001), CASSCF Investigation of Electronic Excited States of 2-Aminopurine, Journal of Physical Chemistry A, Torun, PL (Accessed October 31, 2024)