Effect of a dirhamnolipid biosurfactant on the structure and phase behaviour of dimyristoylphosphatidylserine model membranes.

Rhamnolipids are bacterial biosurfactants containing one or two rhamnose rings and a hydrophobic hydrocarbon portion. These compounds are mainly isolated from Pseudomonas spp culture media, and have been shown to present outstanding biological activities. A number of experimental works have shown that the interaction of rhamnolipids with target membranes could play a role in these actions. Therefore the study of the interaction of purified rhamnolipids with the various phospholipid components of biological membranes is of great interest. This paper shows the phase behaviour of mixtures of 1,2-dimyristolylphosphatidylserine (DMPS) with a dirhamnolipid (diRL) fraction produced by P. aeruginosa. This experimental approach has been based on the use of physical techniques such as Differential Scanning Calorimetry (DSC) and Fourier-Transform Infrared Spectroscopy (FTIR). DSC indicated that the presence of increasing concentrations of diRL in the bilayer resulted in a progressive broadening of the gel to liquid-crystalline phase transition of DMPS. In addition a complex thermal behaviour was observed, with the presence of more than one transition at higher concentrations of the biosurfactant, indicating phase separation. FTIR showed that diRL increased the proportion of gauche rotamers of DMPS, thus affecting acyl chain order. The change in the frequency of the carboxylate stretching band of DMPS observed upon interaction with diRL pointed toward changes in the local environment of the polar headgroup of the phospholipid, resulting in a modification of its conformation or insertion within the bilayer. This result was corroborated by the effect of diRL on the carbonyl and phosphate stretching bands of DMPS, showing an increase of the hydration both in the gel and in the liquid-crystalline phase. Molecular Dynamics (MD) simulations gave further support to the experimental results, showing diRL cluster formation as well as an augmented exposition of DMPS to the water layer in the presence of the biosurfactant.