A Theoretical Investigation of the Physical Reason for the Very Different Luminescence Properties of the Two Isomers Adenine and 2-Aminopurine

Abstract

The geometry of the ground state and the first singlet excite state of adenine and 2-aminopurine was calculated with three different quantum chemical methods: AM1, CIS/6-31G, and CASSCF/6-31G. Three possible deactivation mechanisms or reactions of excited molecule were considered: the pseudo Jahn-Teller distortion, excited-state tautomerism, and formation of so-called twisted intramolecular charge transfer (TICT) states. Different mechanisms for the nonradiative decay are operative for the two isomers. The geometrically relaxed excited state of adenine has n f π* character, while it has π f π* character for 2-aminopurine. The state crossing that occurs during the excited-state relaxation of adenine opens up an effective nonradiative deactivation channel not present for 2-aminopurine. Tautomerism in the excited state might explain the difference in luminescence quantum yield upon DNA binding for 2-aminopurine. The excited state of the 7H tautomer of adenine undergoes a large geometry change during the relaxation, and the final geometry of the n f π* state resembles a TICT state, but with only little charge transfer.

Three-State Conical Intersections in Nucleic Acid Bases

“…The molecule is almost planar, but the amino group is pyramidalized (∠H 10a N 10 C 6 N 1 ) 15°, ∠H 10b N 10 C 6 C 5 ) -14°), as was found in previous theoretical studies. 44,[50][51][52][53][54][55] The geometry is shown in Figure 3 and is in good agreement with previous theoretical results. This figure includes only bond lengths, but a more complete list of internal coordinates is given as Supporting Information.…”

Gas-Phase IR Spectroscopy of Nucleobases

“…For 4-aminopyrimidine, surface hopping calculations identified two conical intersections (5,33,(80)(81)(82)(83)(84)(85)(86)(87)(88)(89)(90)(91)(92): deformation at the C2 position leads to deactivation of the excited state with a lifetime, τ*, of 1 ps and deformation at the C5=C6 bond with a τ* of 400 fs. Immobilizing the latter with a 5 membered ring forms adenine with a conical intersection due to the C2 deformation, τ* of 1 ps and dynamics similar to that in guanine.…”

Electronic Structure of Adenine and Thymine Base Pairs Studied by Femtosecond Electron−Ion Coincidence Spectroscopy

“…Out-of-plane deformation of the six-membered ring can lead to conical intersections between nπ*, ππ*, and the ground state with very small barriers. 21,[24][25][26] A conical intersection to a quasi-dissociative πσ* may also play a role, 8,11 especially at higher excitation energies or in polarizable environments. 16,[27][28][29][30] Time-resolved photoelectron spectroscopy (TRPES) experiments comparing adenine with 9-methyl-substituted adenine 11 found the same biexponential time constants, 0.1 and 1.2 ps, as ion yield studies.…”