Theoretical and Experimental Evidence of Two‐Step Tautomerization in Hypericin. Issue 6 (6th May 2021)

• Record Type: Journal Article
• Authors: Liu, Quan; Wang, Liangxuan; Roldao, Juan Carlos; Adam, Pierre-Michel; Brecht, Marc; Gierschner, Johannes; Wackenhut, Frank; Meixner, Alfred J.
• Is Part Of: Advanced photonics research. Volume 2:Issue 6(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-05-06
• Subjects: hypericin -- intensity autocorrelation -- single molecule microscopy -- tautomerization -- time-dependent density functional theory -- tunneling; Photonics -- Periodicals; 621.36505
• DOI: 10.1002/adpr.202000170
• Languages: English
• ISSNs: 2699-9293

Abstract

Abstract : Hypericin has large potential in modern medicine and exhibits fascinating structural dynamics, such as multiple conformations and tautomerization. However, it is difficult to study individual conformers/tautomers, as they cannot be isolated due to the similarity of their chemical and physical properties. An approach to overcome this difficulty is to combine single molecule experiments with theoretical studies. Time‐dependent density functional theory (TD‐DFT) calculations reveal that tautomerization of hypericin occurs via a two‐step proton transfer with an energy barrier of 1.63 eV, whereas a direct single‐step pathway has a large activation energy barrier of 2.42 eV. Tautomerization in hypericin is accompanied by reorientation of the transition dipole moment, which can be directly observed by fluorescence intensity fluctuations. Quantitative tautomerization residence times can be obtained from the autocorrelation of the temporal emission behavior revealing that hypericin stays in the same tautomeric state for several seconds, which can be influenced by the embedding matrix. Furthermore, replacing hydrogen with deuterium further proves that the underlying process is based on tunneling of a proton. In addition, the tautomerization rate can be influenced by a λ /2 Fabry–Pérot microcavity, where the occupation of Raman active vibrations can alter the tunneling rate. Abstract : Due to the large energy barriers, tautomerization of hypericin is based on tunneling of a proton. Time‐dependent density functional theory (TD‐DFT) calculations show that it is a two‐step process and single molecule experiments allow to study the influence of the local dielectric and photonic environment on the tautomerization rate.