School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
J Am Chem Soc. 2012 Oct 10;134(40):16452-5. doi: 10.1021/ja3045212. Epub 2012 Sep 20.
The first systematic pico-nanosecond time-resolved spectroscopic study of the firefly emitter oxyluciferin and two of its chemically modified analogues revealed that in the excited state the enol group is more acidic than the phenol group. The 6'-dehydroxylated derivative, in which only the 4-enolic hydroxyl proton is acidic, has an experimentally determined pK(a)* of 0.9 in dimethyl sulfoxide and an estimated pK(a)* of -0.3 in water. Moreover, this compound provided direct evidence that in a nonpolar, basic environment the keto form in the excited state can tautomerize into the enol, which subsequently undergoes excited-state proton transfer (ESPT) to produce enolate ion. This observation presents the first experimental evidence of excited-state keto-enol tautomerization of a firefly fluorophore, and it could be important in resolving the enol-keto conundrum related to the color-tuning mechanism of firefly bioluminescence. The 6'-dehydroxylated form of oxyluciferin adds a very rare case of a stable enol to the family of "super"photoacids.
首次对萤火虫发光体氧杂环丁烷酮及其两种化学修饰类似物进行了皮秒-纳秒时间分辨的系统光谱研究，结果表明，在激发态下，烯醇基团比酚基团更具酸性。6'-去羟基衍生物中，只有 4-烯醇羟基质子具有酸性，在二甲基亚砜中实验测定的 pK(a)*为 0.9，在水中估计的 pK(a)*为-0.3。此外，该化合物提供了直接证据，表明在非极性、碱性环境中，激发态中的酮式可以互变异构为烯醇，随后烯醇发生激发态质子转移（ESPT），生成烯醇化物离子。这一观察结果首次提供了萤火虫荧光团激发态酮-烯醇互变异构的实验证据，这对于解决与萤火虫生物发光颜色调谐机制相关的烯醇-酮难题可能非常重要。氧杂环丁烷酮的 6'-去羟基形式为“超强”酸家族增添了一个非常罕见的稳定烯醇的例子。
