(496a) Photophysical Characterization of New Intrinsic Charge Transfer States in Proteins

The intrinsic optical properties of proteins (i.e., absorbance at 280 nm and fluorescence around 340 nm) are known to originate from the electronic transitions of aromatic amino acids, such as tryptophan. As aromatic amino acids are present in nearly all proteins, intrinsic absorbance and fluorescence have limited use in protein characterization. Recently however, a new weak intrinsic absorbance signal has been uncovered in the visible region, even in proteins which lack aromatic amino acids. This previously overlooked visible absorbance signal has been theorized to arise from charge transfer states which form between charged amino acids within the protein. The unique interactions between charged amino acids likely affect this new intrinsic absorbance signal, and could be used to identify specific protein structures or sequences. Despite experimental observation of this new optical state, the exact photophysical mechanism has not yet been elucidated spectroscopically. Here, we characterize this newly uncovered optical state using advanced spectroscopy techniques in three highly charged proteins. The absorbance spectra of each protein was carefully measured using an integrating sphere to minimize contributions from scattering. We then used transient absorption spectroscopy to fully characterize the energy transfer within each protein, uncovering states with charge transfer character, consistent with theoretical simulations. By fully understanding this intriguing new charge transfer state, protein absorbance could eventually find use as an intrinsic probe for in vivo optical microscopy, or as a new method for the identification of specific protein conformations.