RUI: Photoacidity in bioactive, natural coumarins

Calvin University, Grand Rapids MI

Investigators

Abstract

In this project funded by the Chemical Structure, Dynamics & Mechanisms B Program of the Chemistry Division, Professor Mark Muyskens of the Department of Chemistry and Biochemistry at Calvin University, and his team of undergraduate students, are investigating the properties of a broad class of bio-protective natural substances known as coumarins. Widely found in plants, coumarins have been shown to have antiviral, antifungal, anti-inflammatory, antioxidant, and antibacterial properties, some of which may be linked to their proton donating ability, more commonly known as acidity, and their interaction with light. Planned research includes a combination of computational and experimental studies. The main goal of this project is to measure the acidity of a selected group of hydroxycoumarins in their electronic ground and excited states. Ground states are the resting state for a molecule; excited states are produced by absorption of a photon of light and are short-lived. Some types of coumarins are photoacids, meaning that they become much stronger acids once they absorb light. These substances can show pH-dependent fluorescence, a process in which excited states re-emit a photon of light. The mechanisms by which coumarins react with light, the structure-property relationships, and the extent to which we can use computer models to understand and predict coumarin acidity are expected products of this investigation. Results of this research have the potential to explain the beneficial role of coumarins in plants, and possibly extend that role also to humans. Coumarins also have potential applications as biosensors to reveal what is happening inside living cells. The project is well suited to provide research opportunities for undergraduate students. Outreach activities to high school and middle school students, particularly supporting students from groups underrepresented in chemical fields, are also part of the project activities. The main goals of this project are to measure the ground state and excited state acidity of a representative group of hydroxycoumarins. These results will be used to create a structure-activity relationship between acidity and photoluminescence function of the coumarin. The project involves a comprehensive study using steady-state and time-resolved fluorescence techniques to determine the photoacid properties of the selected coumarins, which comprise three groups: mono-hydroxycoumarins, di-hydroxycoumarins and coumarins structurally similar to the natural substance scopoletin. Full characterization of a photoacid involves determining both the ground state and excited state acidity. A novel aspect of the project is the approach to determine experimentally the excited state acid properties using fluorescence spectroscopy. Furthermore, time-resolved fluorescence provides direct observation of the photodynamics of excited state molecules as well as a means to determine excited state rate constants for the proton transfer process. Additional products of this study are the fluorescence quantum yield and excited state lifetime measurements for the target compounds. The experimental observations coupled with computational modeling using density functional theory offer a more thorough understanding of the photoacid and fluorescence characteristics of coumarins. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.