Electronic relaxation pathways of the biologically relevant pterin chromophore

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 ¹ππ* state with excess vibrational energy. A sizable fraction of the vibrationally-excited ¹ππ* state population intersystem crosses to populate a receiver ³nπ* state, while another fraction relaxes to populate the ¹ππ* minimum that decays by fluorescence emission to the ground state. The population that reaches the ³nπ* state either internally converts to the ³ππ* 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.