Tautomeric equilibria of iso-guanine and related purine analogs

Nucleobase pairs in DNA match hydrogen-bond donor and acceptor groups on the nucleobases. However, these can adopt more than one tautomeric form, and can consequently pair with nucleobases other than their canonical complements, possibly a source of natural mutation. These issues are now being re-visited by synthetic biologists increasing the number of replicable pairs in DNA by exploiting unnatural hydrogen bonding patterns, where tautomerism can also create mutation. Here, we combine spectroscopic measurements on methylated analogs of isoguanine tautomers and tautomeric mixtures with statistical analyses to a set of isoguanine analogs, the complement of isocytosine, the 5th and 6th “letters” in DNA.

DNA中的碱基对（nucleobase pair）依赖核碱基（nucleobase）上的氢键供体与受体基团的相互匹配完成配对。然而，这类核碱基可呈现多种互变异构体形式，进而能够与非标准互补的核碱基发生配对，这或许是自然突变的潜在来源。

目前，合成生物学家正重新审视此类问题——他们通过利用非天然氢键模式来拓展DNA中可复制碱基对的数量，而这类非天然碱基体系中的互变异构现象同样可能诱发突变。

本研究结合了针对异鸟嘌呤（isoguanine）互变异构体及其互变异构混合物的甲基化类似物的光谱测量，以及针对一系列异鸟嘌呤类似物的统计分析；该系列异鸟嘌呤类似物的互补碱基为异胞嘧啶（isocytosine），二者共同构成了DNA的第5和第6个"碱基字母"。