OH-Radical Oxidation of Lung Surfactant Protein B on Aqueous Surfaces.

Air pollutants generate reactive oxygen species on lung surfaces. Here we report how hydroxyl radicals (·OH) injected on the surface of water react with SP-B1-25, a 25-residue polypeptide surrogate of human lung surfactant protein B. Our experiments consist of intersecting microjets of aqueous SP-B1-25 solutions with O3/O2/H2O/N2(g) gas streams that are photolyzed into ·OH(g) in situ by 266 nm laser nanosecond pulses. Surface-sensitive mass spectrometry enables us to monitor the prompt (<10 μs) and simultaneous formation of primary O n -containing products/intermediates (n≤5) triggered by the reaction of ·OH with interfacial SP-B1-25. We found that O-atoms from both O3 and ·OH are incorporated into the reactive cysteine Cys8 and Cys11 and tryptophan Trp9 components of the hydrophobic N-terminus of SP-B1-25 that lies at the topmost layers of the air-liquid interface. Remarkably, these processes are initiated by ·OH additions rather than by H-atom abstractions from S-H, C-H, or N-H groups. By increasing the hydrophilicity of the N-terminus region of SP-B1-25, these transformations will impair its role as a surfactant.