Pulmonary Surfactant Protein SP-B Interacts Similarly with Dipalmitoylphosphatidylglycerol and Dipalmitoylphosphatidylcholine in Phosphatidylcholine/Phosphatidylglycerol Mixtures

Porcine pulmonary surfactant-associated protein SP-B was incorporated into bilayers of chain-perdeuterated dipalmitoylphosphatidylglycerol (DPPG-d 62) and into bilayers containing 70 mol % dipalmitoylphosphatidylcholine (DPPC) and 30 mol % DPPG-d 62 or 70 mol % chain-perdeuterated DPPC (DPPC-d 62) and 30 mol % DPPG. The effect of SP-B on the phase behavior, lipid chain order, and dynamics in these bilayers was examined using deuterium nuclear magnetic resonance (2H-NMR). In both DPPG-d 62 and the mixed lipid system, SP-B is found to have little effect on chain order in the liquid crystalline phase. With 11% (w/w) SP-B present, both bilayer systems display a continuous change from liquid crystal to gel with no evidence of two-phase coexistence near the transition. Despite its limited effect on chain order in these bilayers, SP-B is found to strongly perturb chain deuteron transverse relaxation in the liquid crystal and gel phases of DPPG-d 62 and the DPPC/DPPG (7:3) mixtures. The observation that SP-B associates with the bilayer in a way which substantially alters the slow motions responsible for transverse relaxation without significantly affecting chain order in either the liquid crystal or gel phases may place some constraints on possible models for that association.