The Three Possible 2-(Pyrenylethynyl) Adenosines: Rotameric Energy Barriers Govern the Photodynamics of These Structural Isomers

• Published in: Chemphyschem 2016-05, Vol.17 (9), p.1369-1376
• Main Authors: Reuss, Andreas J., Grünewald, Christian, Braun, Markus, Engels, Joachim W., Wachtveitl, Josef
• Format: Article
• Language: English
• Subjects: Adenosine - chemistry; density functional calculations; fluorescent probes; Isomerism; Molecular Structure; photophysics; Quantum Theory; Spectrometry, Fluorescence; thermodynamics; time-resolved spectroscopy
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201500958

This article presents a comprehensive study of the photophysics of 2‐(2‐pyrenylethynyl) adenosine and 2‐(4‐pyrenylethynyl) adenosine, which are structural isomers of the well‐established fluorescent RNA label 2‐(1‐pyrenylethynyl) adenosine. We performed steady‐state and ultrafast transient absorption spectroscopy studies along with time‐resolved fluorescence emission experiments in different solvents to work out the interplay of locally excited and charge‐transfer states. We found the ultrafast photodynamics to be crucial for the fluorescence decay behavior, which extends up to tens of nanoseconds and is partially multi‐exponential. These features in the ultrafast dynamics are indicative of the rotational energy barriers in the first excited state. Let′s tackle fluorescence from various angles: The ultrafast photodynamics and excited‐state lifetimes of three pyrenylethynyl‐labeled 2‐adenosines with different linkage positions of the pyrene moiety are governed by the symmetry of the linkage position and the associated rotamer enthalpies. The linkage positions in the picture are—from left to right and with increasing symmetry—1, 4, and 2.