Interaction between dipalmitoylphosphatidylglycerol and phosphatidylcholine and calcium

Abstract

Pulmonary surfactant, a lipid-protein mixture, reduces surface tension in the lungs. The major lipid of pulmonary surfactant is dipalmitoylphosphatidylcholine (DPPC) and phosphatidylglycerol (PG) is present in significant amounts. We investigated monolayers of dipalmitoylphosphatidylglycerol (DPPG), DPPC and DPPC:DPPG 9:1, 3:7, 5:5 ( mol mol ) contain 1 mol% of the fluorescent probe NBD-PC in the presence and absence of calcium using an epifluorescence microscopic surface balance. Images were acquired and analyzed for monolayers at various surface pressures π including the liquid expanded (LE), liquid condensed (LC) coexistence region. When compressed on a buffered saline subphase of pH 6.9, all monolayers analyzed showed an LE to LC phase transition and could sustain surface pressures up to 70 mN m −1. Calcium at 1.6 mM in a similar subphase caused changes in the LE−LC transitions and the values of π of the DPPG and DPPC:DPPG 7:3, 5:5 monolayers at which collapse occured. In the absence of calcium, the average size of the LC domains and total amount of the LC phase were similar for all monolayers at a number of π. Calcium caused a reduction in the collapse π and an increase in the average size of condensed domains and total area of the LC phase of DPPG, DPPC:DPPG 7:3, 5:5, but not in the monolayers containing DPPC:DPPG 9:1. The low collapse π and increased condensation are consistent with the possibility that lipid association or headgroup dehydration or both is induced by calcium.