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.

本研究报道了一类含两个螺内酯环（spirolactone）的新型荧光素/罗丹明杂合体，其可独立呈现双输出荧光特性。本研究完成了两种非对映异构体trans-RhOH与cis-RhOH的分离与表征，并解析了其X射线晶体结构。在碱性环境下，羟基侧的螺内酯环会发生开环反应，产生类荧光素的光学信号：其最低吸收峰位于485 nm，发射峰位于530 nm。另一方面，氨基侧的螺内酯环开环后则会开启类罗丹明的光学信号——其吸收峰为528 nm，发射峰为575 nm，该过程可被氢离子（H+）或汞离子（Hg²+）触发。在不同pH值的甲醇-缓冲液体系中，测得羟基与氨基的酸度系数（pKa）分别为5.7和2.3。本研究还探讨了该杂合体对Hg²+的选择性传感性能，测得其结合常数对数（log Ks）约为3.60与3.73。共聚焦显微镜成像结果显示，经RhOH孵育的秀丽隐杆线虫（Caenorhabditis elegans）在添加Hg²+后荧光强度显著增强，证实了RhOH用于活体生物成像的应用潜力。