A Molecular Chameleon with Fluorescein and Rhodamine Spectroscopic Behaviors

A new class of fluorescein/rhodamine hybrids with two spirolactone rings was reported to exhibit dual-output fluorescent behaviors independently. Isolation and characterization for two diastereomers, trans-RhOH and cis-RhOH, have been made and their X-ray crystal structures determined. In a basic environment, the spirolactone ring on the hydroxyl side will be opened to give a fluorescein-like optical output with the lowest absorptions at 485 and 530 nm emission. On the other hand, a rhodamine-like optical output, i.e., 528 nm absorption and 575 nm emission, will be switched on by a H+ or a Hg2+ ion, attributed to the spirolactone ring opening on the amino side. In a methanol−buffer system with different pH values, the corresponding pKa values for the hydroxyl and amino groups were determined as 5.7 and 2.3, respectively. Selective Hg2+-sensing properties have also been discussed, and log Ks values of about 3.60 and 3.73 were determined. Confocal microscopic images of Caenorhabditis elegans incubated with RhOH were found to show enhanced fluorescent intensity with a Hg2+ ion, demonstrating the potential application of RhOH for in vivo biological imaging.

本研究报道了一类带有两个螺内酯环(spirolactone ring)的新型荧光素(fluorescein)/罗丹明(rhodamine)杂合物，其展现出独立的双输出荧光行为。现已完成两种非对映异构体(diastereomers) trans-RhOH与cis-RhOH的分离与表征，并测定了它们的X射线晶体结构(X-ray crystal structure)。在碱性环境中，羟基侧的螺内酯环会开环，产生类荧光素的光学响应，其最低吸收波长为485 nm，发射波长为530 nm。另一方面，氨基侧的螺内酯环开环可触发类罗丹明的光学响应（即528 nm吸收与575 nm发射），该过程可由H+或Hg²+离子诱导开启。在不同pH值的甲醇-缓冲液体系中，测得羟基与氨基的解离常数(pKa)分别为5.7和2.3。本文还探讨了其对Hg²+的选择性传感性能，测得的络合稳定常数对数(log Ks)约为3.60与3.73。将RhOH与秀丽隐杆线虫(Caenorhabditis elegans)共孵育后的共聚焦显微图像显示，加入Hg²+离子后荧光强度显著增强，表明RhOH有望用于活体生物成像。