Graduate Student, Case Western Reserve University
Using femtosecond laser spectroscopy, I show kinetics of pterin molecules, potential sensitizers
Pterins are a group of molecules that are found in everywhere in nature, harvesting solar energy or helping build neurotransmitters. However, these molecules have collectively been shown to be good at absorbing carcinogenic UV radiation and their presence seems to cause skin bleaching (vitiligo) and possibly even skin cancer. By analyzing the path of the UV energy once it is absorbed by the pterin, we can assess what type of reactions are possible, including photosensitization reactions that could lead to DNA damage. From our experiments, we see multiple species that may damage DNA; either by converting oxygen into a damaging reactive species or by direct reaction with the DNA itself. By comparing multiple similar molecules, I believe that this ability may be general to the chemical family. I hesitate to speculate on the potential applications for this information because it is so foundational, however, pterins used as fluorescent probes for understanding biochemistry and potential cancer treatments already exist, and understanding and being able to maximize the feature of interest will help those and other groups similarly.
Abstract: Pterin derivatives are endogenous photosensitizers common to every biological kingdom and are involved in numerous photobiological processes for which they have received widespread attention. Despite the photobiological relevance of the pterin compounds, their excited-state dynamics have not been investigated rigorously-an undertaking that must begin with a thorough understanding of the photophysics of the common core chromophore, 2-amino-1H-pteridin-4-one (a.k.a. pterin). In this contribution, direct observation of the ultrafast excited-state dynamics of pterin in aqueous borate buffered solution at pH 10.5 are revealed using broadband transient absorption spectroscopy with femtosecond time resolution. The time-resolved experiments are complemented with ground- and excited-state calculations that are performed at the time-dependent density functional level of theory. Excitation of pterin at 350 nm initially populates the electronic (1)ππ* state with excess vibrational energy. A sizable fraction of the vibrationally-excited (1)ππ* state population intersystem crosses to populate a receiver (3)nπ* state, while another fraction relaxes to populate the (1)ππ* minimum that decays by fluorescence emission to the ground state. The population that reaches the (3)nπ* state either internally converts to the (3)ππ* state or abstracts a hydrogen atom from the solvent to form a semi-reduced neutral radical species, both of which decay with rates accelerated by molecular oxygen. The neutral radical can also decay through back hydrogen atom transfer to the solvent, as evidenced by the normal kinetic isotope effect observed for this process. The transient species characterized in this study are expected to participate in a wide-range of photochemical reactions that have been documented in the literature between pterin and other biological molecules.
Pub.: 10 May '17, Pinned: 30 Jun '17