A day brifging EPR and NMR: two faces of the same "coil" - GIDRM-GIRSE Joint DAY 2024

There is growing interest in industrial formulations using natural ingredients with minimal environmental impact [1]. In this context, biosurfactants from fungi, yeasts, or bacteria are key to developing biocompatible and biodegradable products. Rhamnolipids, in particular, are promising alternatives to synthetic surfactants [2]. They are biotechnologically produced from industrial waste oils and offer advantages such as safety, biocompatibility, biodegradability, and low cost [3].

Rhamnolipids are glycolipids composed of one or two L-rhamnose units linked by an α-1,2-glycosidic bond [4]. The hydrophobic region typically contains β-hydroxy-fatty acids, connected via an ester bond between the β-hydroxy group of the distal chain and the carboxyl group of the proximal chain. The spatial arrangement of the carboxylic group, located far from the sugar moiety, gives rhamnolipids the characteristics of a “two-head” surfactant. However, the behavior of the single moieties and its relevance in the global behavior of the surfactant, remains elusive.

In this contribution, Diffusion Ordered Spectroscopy (DOSY) NMR was used on L-rhamnose to examine its solvation in water. The translational mobilities are affected by both solute-solute interactions, which significantly increase with concentration, and solvent-solute interactions, which provide key insights into the hydration shell of solute molecules. Particularly, the results show a decrease in hydration number as rhamnose concentration increases.

To investigate rhamnolipid self-structuring in water, EPR spectroscopy was used to study their behavior across low and high concentrations. The most notable data were observed at concentrations above 80 wt% rhamnolipid, where significant molecular reorganization was detected, with changes in microviscosity and polarity parameters indicating compact lipid aggregates.

Interestingly, EPR also revealed the presence of endogenous free radicals, correlated with the antioxidant activity of rhamnolipids. The persistent signal, even in the absence of spin probes, suggests the formation of meso structures capable of stabilizing radical species, acting as “redox buffers.”Our study demonstrates that NMR and EPR can be employed in a synergistic manner to elucidate the molecular determinants of rhamnolipid multifunctionality in eco-sustainable formulations. These techniques can be utilized not only to characterize rhamnolipids as surfactants, but also to ascertain their potential as pro- or anti-oxidants, contingent upon external conditions.

References

[1] I. Russo Krauss, R. Esposito, L. Paduano, and G. D'Errico Curr. Opin. Colloid Interface Sci. 70, 101792 (2024)

[2] R. Esposito, I. Speciale, C. De Castro, G. D'Errico, and I. Russo Krauss Int. J. Mol. Sci. 24, 5395 (2023).

[3] R. Esposito, L. Ingenito, D. Cavasso, A. Siciliano, M.L. Alfieri, L. Chiappisi, G. Fragneto, M.F. Ottaviani, M. Guida, L. Paduano, and G. D'Errico J. Mol. Liq. 367, 120547 (2022).

[4] G. Soberón-Chávez. Appl. Microbiol. Biotechnol. 68, 718–725 (2015)

SYNC 2024 - 2nd Symposium for Young Chemists: Innovation and Sustainability

There is a growing interest in industrial formulations based on natural ingredients that have a low environmental impact [1]. In this perspective, the use of biosurfactants derived from fungi, yeasts or bacteria is a key strategy to obtain products that are biocompatible and biodegradable. In particular, rhamnolipids appear to be suitable candidates for replacing synthetic surfactants [2]. They are biotechnologically produced from industrial waste oils, are safe, biocompatible, biodegradable and available at low cost [3]. Although the micellization of rhamnolipids in dilute solutions is well known, their self-aggregation in concentrated mixtures remains largely unexplored.

In this contribution, Electron Paramagnetic Resonance (EPR) spectroscopy is used to study the local structure and dynamics of concentrated aqueous rhamnolipid mixtures using paramagnetic molecular probes. A detailed computational analysis of the EPR spectral components clearly shows a dramatic molecular reorganization of the mixture starting from 80 wt% rhamnolipid, as the microviscosity and polarity parameters indicate the formation of more compact lipid aggregates. These observations suggest a phase transition that is currently being investigated by SAXS for meso-morphological characterization of aggregates. Preliminary results indicate the presence of unstructured ordered phases [4]. The concentrated rhamnolipid mixtures are currently also being studied by rheology, as viscoelastic properties are fundamental for a variety of practical applications [5]. Concentrated samples exhibit shear thinning at 45 °C, suggesting a non-isotropic solution. Notably, as the temperature decreases, shear thinning is not observed at 70%, while it remains present at high concentration.

Interestingly, the presence of endogenous free radicals in the samples is revealed by EPR and correlated with the antioxidant activity of the rhamnolipids. The persistent signal registered even in the absence of spin probes suggests the formation of meso structures capable of stabilizing radical species, thus acting as "redox buffers".

Our results highlight rhamnolipids as promising multifunctional components for eco-sustainable formulations where, in addition to the typical role of surfactants (e.g. foaming or emulsifying agents), they can also act as pro- or anti-oxidants depending on the external conditions.

References 

[1] Hayes G. D., Smith A. G., AOCS Press, 2019, 3-38.
[2] Sarubbo A. L. et al., Biochemical Engineering Journal, 2022, 181, 108-377.
[3] Esposito R. et al., International Journal of Molecular Sciences, 2023, 24 (6), 5395.
[4] Baccile N. et al., Langmuir, 2023, 39 (27), 9273-9289.
[5] Khan M. B. et al., JCIS Open, 2022, 8, 10006 

Il Contributo dei Giovani Chimici in Campania Edizione 2024 

There is a growing interest in industrial formulations based on natural ingredients that have a low environmental impact [1]. Biosurfactants derived from fungi, yeasts or bacteria is a key strategy to obtain products that are biocompatible and biodegradable. In particular, rhamnolipids appear to be suitable candidates for replacing synthetic surfactants [2]. They are biotechnologically produced from industrial waste oils, are safe, biocompatible, biodegradable and available at low cost [3]. Although the micellization of rhamnolipids in dilute solutions is well known, their self-aggregation in concentrated mixtures remains largely unexplored. In this contribution, Electron Paramagnetic Resonance (EPR) spectroscopy is used to study the local structure and dynamics of concentrated aqueous rhamnolipid mixtures using paramagnetic molecular probes. A detailed computational analysis of the EPR spectral components shows a dramatic molecular reorganization of the mixture starting from 80 wt% rhamnolipid, suggest more compact lipid aggregates. These observations suggest a phase transition that is currently being investigated by SAXS for mesomorphological characterization of aggregates. Preliminary results indicate the presence of unstructured ordered phases [4]. The concentrated rhamnolipid mixtures are currently also being studied by rheology, as viscoelastic properties are fundamental for a variety of practical applications [5]. Concentrated samples exhibit shear thinning at 45 °C, suggesting a non-isotropic solution. As the temperature decreases, shear thinning is not observed at 70%, while it remains present at high concentration. The presence of endogenous free radicals in the samples is revealed by EPR and correlated with the antioxidant activity of the rhamnolipids. The persistent signal registered even in the absence of spin probes suggests the formation of meso structures capable of stabilizing radical species, thus acting as "redox buffers". Our results highlight rhamnolipids as promising multifunctional components for ecosustainable formulations where, in addition to the typical role of surfactants (e.g. foaming or emulsifying agents), they can also act as pro- or anti-oxidants depending on the external conditions.

References
[1] Hayes G. D., Smith A. G., AOCS Press, 2019, 3-38
[2] Sarubbo A. L. et al., Biochemical Engineering Journal, 2022, 181, 108-377.
[3] Esposito R. et al., International Journal of Molecular Sciences, 2023, 24 (6), 5395.
[4] Baccile N. et al., Langmuir, 2023, 39 (27), 9273-9289.
[5] Khan M. B. et al., JCIS Open, 2022, 8, 100067