and also induces concentration‐dependent transitions. These results suggest that β‐sheets is the preferred bioactive conformation of surfactin.
Shen et al. [216] have produced from the B. subtilis strain three different deuterated surfactins (one perdeuterated, one with the four leucines perdeuterated, and one with everything except the four leucines perdeuterated) and used them in NR and SANS studies at pH 7.5. As expected, the largest signal in the reflectivity profiles is from the perdeuterated sample. Fitting the layer as a Gaussian distribution normal to the surface, in all three cases the area per molecule at the surface is 147 ± 10 Å2 and the overall thickness of the layer is 14 ± 2 Å. The results also suggest an unusually close‐packed surface layer and that the alkyl chain must be folded back into the leucines of the heptapeptide ring in order to give the observed compactness and the low extent of immersion in the aqueous subphase. Therefore, surfactin adopts a compact globular structure at the surface. The best fit of data was obtained by a core‐shell model in which the core contains the alkyl chain and the four leucines, and the remaining head group, water, and the counterions are at the shell. The aggregation number was 20 ± 4, the overall micelle diameter 50 ± 5 Å, and the radius of the hydrophobic core 22 ± 2 Å.
The structure of other cyclolipopeptides different from surfactin have been reviewed by Kaspar et al. [20]. Among them families of iturins (heptapeptides), fengycin (decapeptides), or locillomycins (nonapeptides) can be mentioned, as well as linear lipopeptides. In comparison to surfactin, much less is known about their physicochemical properties.
1.8 Final Comments
Most of the isolated biosurfactants have not been characterized as deeply as sophorolipids or surfactins reviewed above. In many cases, the tiny amounts obtained prevent more careful studies and, for many of them, the cmc is the only physicochemical variable so far provided. Even the published values for cmc must be handled cautiously as the purity degree of the biosurfactant is low. However, if the biosurfactant belongs to a given family of derivatives, many of their properties can be estimated with different degrees of accuracy. Aggregation number or the change in heat capacity are provided examples in this chapter for classical and sugar surfactants. Conversely, a new measured physical quantity may be checked and compared with published values for similar compounds, although, occasionally, results (such as those obtained for some gemini surfactants) can break an accepted rule. The absence of physicochemical characterization is also related to the origin of research groups involved in their discovery, as they are more interested in biological properties and applications, as following chapters of this book will review. As classical surfactants, biosurfactants are affected by revisions of old theories or by new proposals. Although we have presented the theories and models in their simplest versions, we have also illustrated that concepts largely accepted during decades are nowadays under scrutiny.
Also, the knowledge of the structure/property relationship allows a possible improvement of some properties of a new biosurfactant by enlargement of the alkyl chain, introduction of a hydrophobic or hydrophilic residue, or the synthesis of new structures (gemini, Y‐shaped, bolaamphiphile…). All of them are nowadays well‐known options. The cooperation between chemistry and biology research groups, with a wide range of capacities, can be decisive in the improvement of desired properties and applications. Comments by Menger [217] about host–guest systems, typical examples of supramolecular entities, are valid for stimulating such a cooperation. Emulating his comments, it might be easy to design on paper a new surfactant bearing a wish‐list of optimally oriented capacities and properties. Of course, the risk will be that after spending hours (and money) in the synthesis laboratory, such a molecule does not fulfill our expectations. We have unpublished experience on several negative projects. The task may be facilitated by the ability of researchers to further develop molecules from a biological origin that are the result of evolution. Bile salts (which have been our focus of interest for the last three decades) might be good examples of previous assertions.
The steroid nucleus with some specific organic functions located at certain positions and different orientations and, mainly, its enormous transcendence in living organisms (including human beings) is the result of a billion years of evolution of nature. Although all the human scientific knowledge would not probably be able to design it from zero, we (and others) have been able to modify their hydrophobic/hydrophilic balance by attaching specific residues into the structure. As a result, the formation of initially unexpected supramolecular structures is now well documented. Publications of the potential use of these new derivatives for the formation of gels, resolution of enantiomers, complexing a single water molecule, and synthesis of new antibiotics and antidotes can be found elsewhere [218–222]. These are just examples that pretend to encourage chemical modifications on new biosurfactants that microorganisms provided us.
Acknowledgement
The authors thank the Ministerio de Economía, Industria y Competitividad (Spain) (Project MAT2017‐86109‐P) for financial support.
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