Assignment of Electronic Transition Moment Directions of Adenine from Linear Dichroism Measurements

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Abstract

The electronic spectra of a series of adenine derivatives have been investigated with respect to the number of electronic transitions, their intensities, and transition moments. The experimental work includes linear dichroism (LD) measurements on samples partially oriented in stretched polymer poly(vinyl alcohol) films, fluorescence anisotropy (FA), and magnetic circular dichroism (MCD). The UV spectra of both 7-methyladenine (7MA) and 9-methyladenine (9MA) are resolved into contributions from five π → π* transitions (I−V). Their polarizations relative to the C4−C5 axis are for 7MA +45° (I, 36 600 cm-1), −16° (II, 39 500 cm-1), −28° (III, 42 600 cm-1), +76° (IV, 45 800 cm-1) and ∼−29° (V, ∼47 700 cm-1); and for 9MA +66° (I, 36 700 cm-1), +19° (II, 38 820 cm-1), −15° (III, 43 400 cm-1), −21° (IV, 46 800 cm-1), and ∼−64° (V, ∼48 320 cm-1). The experimental results are correlated with results from semiempirical INDO/S and ab initio CIS/6-31G(d) and CASPT2 molecular orbital calculations. The transition moments are found to be conserved when the adenine amino group has been substituted with either one or two methyl groups. In addition, LD and MCD spectra for 6-(dimethylamino)-9-ethylpurine, which is soluble in nonpolar solvents, have been measured in stretched polyethylene film and cyclohexane, respectively. The results indicate that the electronic transition moments of the 9-substituted adenine chromophore are essentially the same in a polar and a nonpolar solvent. On the basis of the results for 7MA and 9MA, the reduced LD and absorption spectra of adenine are analyzed in terms of contributions from the 7-H and 9-H tautomers of adenine. By comparison with theoretical and experimental results for purine and 2-aminopurine regarding the position of the lowest n → π* transition, we are able to confidently position the lowest n → π* transition in 9MA very close to the lowest π → π* transition. The proximity of the first 1nπ* and 1ππ* states in adenine might be related to the effecient nonradiative deactivation of the lowest excited 1ππ* state.

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