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).