2H NMR studies of the effect of pulmonary surfactant SP-C on the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine headgroup: a model for transbilayer peptides in surfactant and biological membranes.

Surfactant protein C (SP-C) was isolated from solvent extracts of porcine pulmonary surfactant by gel filtration chromatography. The surfactant protein was combined with dipalmitoylphosphatidylcholine deuterated at the alpha and beta positions of the choline headgroup (DPPC-d4) Deuterium nuclear magnetic resonance spectra were collected as a function of temperature for a series of protein concentrations. The splitting of the alpha-deuteron spectrum in the liquid-crystalline phase was insensitive to temperature but decreased with increasing protein concentration. The response of headgroup conformation to protein concentration was consistent with an interaction between the lipid headgroup dipole and the net positive surface charge associated with the protein. The observed effect per charge on the alpha splitting was less than that reported for singly-charged amphiphiles [Scherer, P. G., & Seelig, J. (1989) Biochemistry 28, 7720-7728] but was similar to that obtained using a multipled-charged amphiphilic polypeptide [Roux, M., Neumann, J.-M., Hodges, R. S., Devaux, P. F., & Bloom, M. (1989) Biochemistry 28, 2313-2321]. This comparison suggests that the charges on SP-C are located near the bilayer surface. The possibility that the headgroup response is sensitive to the degree of clustering of surface charge is discussed. The beta-deuteron splitting in the liquid-crystalline phase decreased with increasing temperature but was relatively insensitive to protein concentration, suggesting that the torsion angle about the C alpha-C beta bond might be sensitive to steric interactions between the lipid headgroup and the protein.