Photoinduced Electron Transfer Occurs between 2-Aminopurine and the DNA Nucleic Acid Monophosphates: Results from Cyclic Voltammetry and Fluorescence Quenching

Abstract

2-Aminopurine (2AP) is a fluorescent adenine analogue that is useful in part because its substantial fluorescence quantum yield is sensitive to base stacking with native bases in ss- and ds-DNA. However, the degree of quenching is sequence dependent and the mechanism of quenching is still a matter of some debate. Here we show that the most likely quenching mechanism in aqueous solution involves photoinduced electron transfer (PET), as revealed by cyclic voltammetry (CV) performed in aprotic organic solvents. These potentials were used with spectroscopic data to obtain excited-state reduction and oxidation potentials. Stern-Volmer (S-V) experiments using the native base monophosphate nucleotides (NMPs) rGMP, rAMP, rCMP, and dTMP were performed in aqueous solution to obtain quenching rate constants kq. The results suggest that 2AP* can act as either an electron donor or an electron acceptor depending on the particular NMP but that PET proceeds for all NMPs tested.

Differential Fluorescence Quenching of Fluorescent Nucleic Acid Base Analogues by Native Nucleic Acid Monophosphates

“…This is true especially for the pyrimidines. Indeed, the quenching constants for 3MI by pyrimidines are about a factor of 3 lower than that for 6MAP or DMAP (or 2AP for that matter 17 ). This observation is discussed further below.…”

“…The pterdines have lower oxidation potentials than the pteridones, with the range spanning E OX°E XP (6MAP) ) 1.95 V compared to E OX°E XP (6MI) ) 1.65 V. Both pteridones and pteridines have higher oxidation potentials than 2AP. 17 The reduction potentials for the pteridines/pteridones show poorer agreement between experimental and calculated values, so much so that the computations were not predictive for 6MI and 3MI due to the presence of two cathodic peaks in the (DMF) voltammograms. Taking the lowest potential cathodic peak, E RED°E XP , as a basis for comparison, the difference between experimental and calculated potentials is largest for 3MI (320 mV) and smallest for 6MAP (190 mV).…”

“…17 Theoretical one-electron redox potentials of the FBAs were calculated using density functional theory (DFT) on the methylated form using Gaussian 03 (v.c02). 35 The 8-methyl form of the pteridines/pteridones was used instead of the nucleosides in order to reduce computational time.…”

“…We have shown this to be the case for 2AP. 17 Examination of ∆G NBO°a nd ∆G NBR°f or 6MI suggests that 6MI could undergo both NBO or NBR by C and dT, as these NMPs have free energies in the same range, about 0.6 eV (see Table TABLE 3 …”

Exciplexes and conical intersections lead to fluorescence quenching in π-stacked dimers of 2-aminopurine with natural purine nucleobases

“…43–47 These electron transfer processes often involve guanine, which readily oxidizes to behave like an electron donor, since its oxidation potential is the lowest among the natural nucleobases. 43–46,48 Other mechanisms have also been proposed, specifically the involvement of dark states which can be accessed depending on the conformations in DNA. 24,25,35,49 …”