Formal [4 + 2] Cycloadditions of Maleimides on Duplex DNA.

Near-quantitative DNA bioconjugation and detailed mechanistic investigations of reactions involving 5-(vinyl)-2'-deoxyuridine (VdU) and maleimides are reported. According to accelerated reaction rates in solvents with increasing polarity and trends in product stereochemistry, VdU-maleimide reactions proceed via a formal [4 + 2] stepwise cycloaddition. In contrast, 5-(1,3-butadienyl)-2'-deoxyuridine (BDdU) reacts with maleimides in a concerted [4 + 2] Diels-Alder cycloaddition.

A Bright and Ionizable Cytosine Mimic for -Motif Structures.

A new fluorescent cytosine analog "C" containing a -stilbene moiety was synthesized and incorporated into hemiprotonated base pairs that comprise -motif structures. Unlike previously reported fluorescent base analogs, C mimics the acid-base properties of cytosine (p ≈ 4.3) while exhibiting bright (ε × Φ ≈ 1000 cm M) and red-shifted fluorescence (λ = 440 → 490 nm) upon its protonation in the water-excluded interface of C:C base pairs.

[4 + 2] and [2 + 4] cycloaddition reactions on single- and double-stranded DNA: a dual-reactive nucleoside.

Here we report dual reactivity of diene-modified duplex DNA containing 5-(1,3-butadienyl)-2'-deoxyuridine "BDdU". Normal-electron demand [4 + 2] cycloaddition proceeded upon addition of a maleimide, whereas inverse-electron demand [2 + 4] cycloaddition occurred upon addition of a tetrazine to give a novel, photoswitchable product.