DMPG gel-fluid thermal transition monitored by a phospholipid spin labeled at the acyl chain end

Low ionic strength aqueous dispersion of dimyristoyl phosphatidylglycerol (DMPG) presents a rather peculiar gel-fluid thermal transition behavior. The lipid main phase transition occurs over a large temperature interval (ca. 17 degrees C), along which several calorimetric peaks are observed. Using lipids spin labeled at the acyl chain end, a two-peak electron spin resonance (ESR) spectrum is observed along that temperature transition region (named intermediate phase), at three different microwave frequencies: L-, X- and Q-bands. The intermediate phase ESR spectra are analyzed, and shown to be most likely due to spin labels probing two distinct types of lipid organization in the DMPG bilayer. Based on the ESR spectra parameters, a model for the DMPG intermediate phase is proposed, where rather fluid and hydrated domains, possibly high curvature regions, coexist with patches that are more rigid and hydrophobic.