**Acknowledgements**

*Surfactants and Detergents*

**4. Conclusions**

**Figure 7.**

*of this part.*

On this aspect of modeling complex interfacial systems, we can conclude that the development of CG models will open the way to new force fields capable of quantitatively predicting the surface tension and main properties such as the CMC. The prediction of the CMC, already operational for some CG models [51], will require additional adjustments for new molecules with various intramolecular interactions. The development of CG force fields using mesoscale simulation methods [58–60] is an active area of research. Different methodologies coexist to develop these CG interactions: a bottom-up approach consisting in deriving the force field from atomistic simulations and a top-down approach aiming to build the parameters of the model from mapping onto macroscopic properties such as the interfacial tension.

*Density distributions of the water and different parts of the syringafactin molecules at two surfactant concentrations: (a) 4 surfactant molecules and (b) 32 surfactant molecules at each interface. The lipid head is represented by the first three beads of the lipid chain whereas the protein part is represented by a typical bead* 

This work is the first report of a detailed study of biosurfactants produced by *Pseudomona*s and *Xanthomonas* strains isolated from cloud samples. We have used a convenient method to purify these compounds based on adsorption on Amberlite coupled with a puriFlash® chromatographic technique; the different steps were monitored using the pendant drop method. High-resolution LC-MS-MS allowed assigning unambiguously the structure of viscosin, massetolide E, and different syringafactins. The measurements of CMC of viscosin and syringafactin showed that viscosin is a particularly powerful biosurfactant. Finally, two approaches of molecular dynamics were used to model the conformation of these biosurfactants at the water-air interface: an atomistic description for viscosin (CHARMM force field) and a CG model for syringafactin A (MARTINI force field). This last approach is particularly original and promising. To our knowledge, these studies constitute the

first modeling of interfacial properties of such complex biosurfactants.

In addition to fundamental knowledge of biosurfactant properties, this work shows that cloud microorganisms can provide an unexplored source of biosurfactants. Rather few strains, mainly *Pseudomonas*, were shown to produce viscosin, massetolides, and syringafactins, and two new isolates from this genus are described here. We report here the first production of syringafactins by a strain of *Xanthomonas*. Considering that more than 30 strains of our microbial collection isolated from clouds were very active biosurfactant producers (σ ≤ 30 mN.m<sup>−</sup><sup>1</sup>

[27], further investigation is very promising to isolate and study other unusual or

)

**84**

even new biosurfactants.

This work was funded by the French-USA program ANR-NSF SONATA and the French-Slovak Program Stefanik N° 35588ZE.
