Pulmonary surfactant-associated protein SP-B has little effect on acyl chains in dipalmitoylphosphatidylcholine dispersions

Synthetic human pulmonary surfactant-associated protein SP-B has been interacted with chain-perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62) in aqueous dispersions, and the dispersions were investigated by magnetic resonance spectroscopy. The protein caused only small perturbations of the deuterium magnetic resonance spectra in the gel and liquid-crystal states. In an amount of 11% by weight in DPPC, it produced a small reduction in the magnitude of the first moments of the spectra in the gel and a small increase (approximately 5%) in their magnitude in the liquid crystal. In the liquid crystal the protein was observed to cause a similar effect on all portions of the acyl chain, as observed by its proportional shifting of splittings obtained from "dePaked" spectra. Using data from circular dichroism spectra, the protein was found to be about 45% alpha-helical in methanol and in DPPC dispersions. alpha-Helical content was not significantly changed by the presence of 2 mM calcium or by the packing state of the acyl chains. The presence of the protein enhanced the adsorption rate of lipid into the air-water interface when dispersions of lipids or lipid plus SP-B were injected below the interface. The results could be consistent with the protein interacting with the lipid near the head groups or arranging itself around the edges of bilayer discs, or a combination of the two orientations.