Phase diagram for a lysyl-phosphatidyl glycerol analogue in biomimetic mixed monolayers with phosphatidyl glycerol: insights into the tunable properties of bacterial membranes.

Ion pairing between the major phospholipids of the Staphylococcus aureus plasma membrane (phosphatidylglycerol - PG and lysyl-phosphatidylglycerol - LPG) confers resistance to antimicrobial peptides and other antibiotics. We developed 3adLPG, a stable synthetic analogue which can substitute for the highy-labile native LPG, in biophysical experiments examining the membrane-protecting role of lipid ion pairing, in S. aureus and other important bacteria. Here we examine the surface charge and lipid packing characteristics of synthetic biomimetic mixtures of DPPG and DP3adLPG in Langmuir monolayers, using a combination of complementary surface-probing techniques such as infrared reflection-absorption spectroscopy and grazing-incidence x-ray diffraction. The resultant phase diagram for the ion paired lipids sheds light on the mixing behavior of lipids in monolayer models of resistant phenotype bacterial membranes, and provides a platform for future biophysical studies.