Role of Charged Amino Acids in Sullying the Fluorescence of Tryptophan or Conjugated Dansyl Probe in Monomeric Proteins

When Trp/dansyl probe conjugated to a monomeric protein is photoexcited, it is assumed that all emitted fluorescence originates solely from them. In this work, we show that hidden unconventional intrinsic chromophores (called ProCharTS) that originate from confined charge clusters in the protein can contaminate Trp/dansyl emission. Previous work has shown that charge recombination among charge-separated excited states of monomeric proteins, rich in charged residues, can emit weak luminescence (300-700 nm) overlapping with ProCharTS absorption (250-800 nm) and Trp (300-400 nm) and dansyl (400-600 nm) emission. We examine how this overlap taints the fluorescence arising from Trp/dansyl. We compared the effect of dense aqueous solutions of amino acids, Lys/Glu/Asp/Arg/His, on the fluorescence intensity decay/spectrum of N-acetyl-l-tryptophan amide (NATA). Significant broadening on the red side of Trp emission spectrum was observed solely in the presence of lysine, which appeared to be the most potent in altering the mono-exponential fluorescence decay of NATA. Interestingly, NATA in the presence of proteins α3C and dehydrin (DHN1), which are rich in Lys residues, showed substantial deviation from mono-exponential fluorescence decay in contrast to PEST wt and Symfoil-4P pv2, which lack Lys residues. Remarkably, Trp emission spectra among charge-rich proteins like α3W, PEST M1, and DHN1 CW1 were altered on the red side of Trp emission. Emission spectrum of dansyl-labeled human serum albumin (HuSA) was broadened and its fluorescence quenched with gradual addition of excess unlabeled HuSA, which displays bountiful ProCharTS luminescence. Our results unveil the additive influence of ProCharTS luminescence on Trp/dansyl emission with no measurable evidence of energy transfer.