In-cell Fpop

In-cell FPOP uses hydroxyl radicals as brief chemical probes of the cellular environment. These radicals oxidize solvent-accessible amino acid side chains, so residues exposed on a protein surface can acquire modifications that differ when conformation or binding changes accessibility. Mapping modified residues by mass spectrometry therefore connects chemical labeling patterns with structural and interaction changes.

The short laser pulse initiates hydroxyl-radical production by photolyzing hydrogen peroxide, creating a rapid labeling event inside the cell. Because oxidation occurs before the reaction is quenched, the experiment captures a time-limited accessibility pattern rather than allowing labeling to continue indefinitely. This temporal control helps preserve differences associated with protein conformation, binding, and intracellular exposure.

Mass spectrometry identifies which amino acid residues carry oxidative modifications after the cellular reaction. Researchers can then compare residue-level modification patterns between protein states or interaction conditions. A change in the modified-residue map indicates altered accessibility, which can reflect a conformational rearrangement, a binding interface, or the influence of the intracellular environment on the protein.

A basic In-cell FPOP workflow begins with living cells and hydrogen peroxide, followed by exposure to a short laser pulse. Photolysis generates hydroxyl radicals, which react rapidly with accessible side chains. The reaction is then quenched, and the resulting protein material is analyzed by mass spectrometry. The final residue-level pattern supplies the structural readout.

Hydrogen peroxide, a pulsed laser, a quenching step, and mass spectrometric analysis are the central experimental components described for this approach. Living cells provide the near-native setting in which proteins are labeled, while the laser supplies the initiating event and mass spectrometry records the resulting residue modifications. Together, these components connect cellular exposure to biochemical structural information.

Researchers apply In-cell FPOP when they need to compare protein states within cells or examine proteins without removing them from their cellular environment. The method supports analysis of conformation, molecular interactions, and binding interfaces under near-native conditions. It is therefore useful for determining how intracellular conditions influence protein behavior while preserving cellular context during structural measurements.