Label-free optical detection of protein acetylation using UV-vis charge transfer spectra

We report the first spectroscopic detection of protein acetylation in solution using UV-vis Protein Charge Transfer Spectra (ProCharTS). Acetylation is an important post-translational modification (PTM) that modulates diverse cellular processes, yet its detection relies on antibody-based cost-intensive and/or destructive techniques such as mass spectrometry. ProCharTS exploits the reduction of lysine charge post-acetylation to offer an alternate label-free, easily accessible, and cost-effective option to detect the PTM. Using two charge-rich proteins α₃C and α₃W, we demonstrate that the ProCharTS extinction coefficient between 370-800 nm monotonically decreases with increasing degree of chemical acetylation in the proteins. Complementary spectroscopic analysis and molecular dynamics (MD) simulations indicate that the ProCharTS signatures arise independent of secondary structure changes although tertiary interactions weaken post-acetylation. Using a new computational MD and Time-Dependent Density Functional Theory (TDDFT) approach to simulate ProCharTS of whole proteins from their known 3D structure, we assign the observed PTM-induced decrease in intensity to changes in the size, composition and spatial distribution of charged residue clusters. Our joint experimental-computational approach enables us to detect five or more acetylation events per protein with significant scope for further improvements in sensitivity. More broadly, this study presents a new optical mode (ProCharTS ^PTM ) exploiting charge transfer transitions to probe/track charged residue modifications in protein solutions.