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-B_(1–25), a 25-residue polypeptide surrogate of human lung surfactant protein B. Our experiments consist of intersecting microjets of aqueous SP-B_(1–25) solutions with O_3/O_2/H_2O/N_2(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-B_(1–25). We found that O-atoms from both O_3 and ·OH are incorporated into the reactive cysteine Cys_8 and Cys_(11) and tryptophan Trp_9 components of the hydrophobic N-terminus of SP-B_(1–25) 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-B_(1–25), these transformations will impair its role as a surfactant.