Weak Intrinsic Luminescence in Monomeric Proteins Arising from Charge Recombination

We had earlier reported on the presence of broad UV–vis electronic absorption (250–800 nm) in a monomeric protein rich in charged but lacking aromatic amino acids, referred to as Protein Charge Transfer Spectra (ProCharTS). Specifically, it was shown that the cationic amino/anionic carboxylate head groups of Lys/Glu side chains act as electronic charge acceptors/donors for photoinduced electron transfer either from/to the polypeptide backbone or to each other. In this work, we show that such excitations produce weak intrinsic luminescence in proteins originating from charge recombination. We investigated aqueous solutions of proteins with varying abundance of charged amino acids, like human serum albumin (HuSA) and hen lysozyme, and intrinsically disordered proteins, like PEST fragment of human c-Myc protein, α-synuclein, and dehydrin. The absorbance and luminescence in all protein samples were a linear function of the concentration (0–50 μM) employed, confirming their origin from a monomeric species. The slope of the luminescence/[protein] plot directly correlated with the fraction of charged amino acids present in protein. Specifically, the higher slope in proteins like HuSA was chiefly accounted by a large molar extinction coefficient rather than quantum yield. This coefficient directly correlates with the population of charged side-chain head groups lying in close spatial proximity in the protein, contributed by the three-dimensional (3D) fold of the polypeptide. ProCharTS luminescence parameters appear conserved across proteins. These include overlapping excitation/emission spectra, large Stokes shifts (14 000–3000 cm–¹) that decrease with increasing excitation wavelength, low quantum yields (0.002–0.026) indicating poor radiative recombination efficiency, and multiexponential decays (mean lifetimes = 0.4–2.9 ns).