Physicochemical characterization of a monorhamnolipid secreted by Pseudomonas aeruginosa MA01 in aqueous media. An experimental and molecular dynamics study

Given the increasing interest in the characterization of new biosurfactants, in this work we have carried out a physicochemical study of a monorhamnolipid (monoRL) produced by Pseudomonas aeruginosa MA01 in aqueous media. The detailed knowledge of the physicochemical properties of these monoRL biosurfactant is of importance for the validation of this particular P. aeruginosa strain as a useful biosurfactant producer. A pKa value for monoRL of 5.9 was consistently obtained, as well as the indication that the presence of one or two rhamnose rings does not have a notorious influence on the pKa of the carboxyl group. The critical micelle concentration (cmc) of the negatively charged monoRL is dependent on the ionic strength, whereas that of the protonated form is not, whereas the charge of the polar head of monoRL has little effect on the surface area. Dynamic light scattering showed that in the vicinity of the cmc structures with an average diameter of 50 nm are present, whereas at concentrations well above the cmc the size increases to about 200 nm. Taken together our results show that monoRL presents a monomer-to-micelle transition, which depends on pH and ionic strength, similar to that described before for the diRL species. However the formation of lamellar vesicles described for diRL at pH 7.4, was not observed here. Molecular dynamics (MD) simulations yielded a similar value for the lateral diffusion coefficient of protonated anionic monoRL, indicating that the negative charge does not affect biosurfactant mobility in the monolayer surface. The radial distribution function value is slightly higher for the protonated monoRL; therefore the number of molecules located at a particular distance is somehow higher in the case of the protonated form. On the other hand, it is clearly obtained that the carboxylate group of the anionic form moves more inside the aqueous phase as compared to the carboxyl group of the protonated form. The results obtained correspond to the expected behaviour for a biosurfactant molecule in relation to the dependence of protonation state and micelle formation, and therefore the molecular dynamics simulation appears to describe properly our molecular systems.