Evaluation of intrinsic spectroscopic properties of chromophore assemblies by shielding with cyclohexyl base pairs within a DNA duplex.

Here, we investigated spectroscopic behaviors of tetramethylrhodamine (TMR) homo- and hetero-dimers within DNA duplex. In order to shield the chromophores from natural base pairs, we used cyclohexyl base pairs as 'insulators'; these pairs were inserted between the chromophores and nucleobases. When a single TMR moiety was sandwiched between cyclohexyl base pairs, the emission intensity increased by fivefold relative to a TMR between natural base pairs, because electron transfer from nucleobases was suppressed. Next, we inserted two TMRs between the cyclohexyl base pairs and found that they facilitated H-dimer formation of TMR; a distinct hypsochromic shift was induced only when cyclohexyl base pairs were inserted. We further examined quenching behavior of a TMR paired with a quencher dye between cyclohexyl base pairs. Interestingly, fluorescence from TMR was quenched by nitro methyl red more efficiently in the presence of cyclohexyl base pairs than in their absence. This suggests that neighboring natural base pairs disturbed electron or hole transfer between the fluorophore and the quencher. The cyclohexyl base pairs shielded the chromophore pair from the natural base pairs and allowed intrinsic electron transfer.
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