Satohiro Yanagisawa

Selective encapsulation of cesium ions using the cyclic peptide moiety of surfactin: Highly efficient removal based on an aqueous giant micellar system

Cyclic peptide of surfactin (SF) is one of the promising environment-friendly biosurfactants abundantly produced by microorganisms such as Bacillus subtilis. SF is also known to act as an ionophore, wherein alkali metal ions can be trapped in the cyclic peptide. Especially, SF is expected to show high affinity for Cs(+) because of the distinctive cavity size and coordination number. In this study, we reported the specific interaction between SF and Cs(+) and succeeded in the highly efficient removal of Cs(+) from water using giant SF micelles as a natural sorbent. The specific interaction between SF and Cs(+) to form their inclusion complex was revealed by nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopy. We found that SF micelles selectively encapsulate Cs(+), which was suggested by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). A highly effective separation of Cs(+) immobilized on the surface of the SF micelles was also achieved through facile centrifugal ultrafiltration in 91% even in coexisting with other alkali metal ions such as Na(+) and K(+). Thus, the use of the giant micellar system of SF with its high Cs(+) affinity and distinctive assembling properties would be a new approach for the treatment of contaminated soil and water.

pH-induced conformational change of natural cyclic lipopeptide surfactin and the effect on protease activity

The cyclic lipopeptide surfactin (SF) is one of the promising environmental friendly biosurfactants abundantly produced by microorganisms such as Bacillus subtilis. SF shows excellent surface properties at various pH, together with lower toxicity and higher biodegradability than commonly used petroleum-based surfactants. However, the effect of the dissociation degree of SF on self-assembly is still incompletely understood, even though two acidic amino acid residues (Asp and Glu) are known to influence eventual surface and biological functions. Here, we report changes in the secondary structure of SF induced by increased pH, and the effect on protease activity. We found that the β-sheet and β-turn formation of SF are significantly enhanced through increased dissociation of Asp and Glu as revealed by a titration experiment of SF solution to estimate apparent pK1 and pK2 values together with circular dichroism spectroscopy. We also studied the activity of the common detergent enzyme subtilisin in SF solution at above its pK2 (pH 7.6) to understand the role of the dissociation degree in the interaction with the protein. The mixing of SF having a unique cyclic topological feature with subtilisin suppressed the decrease in protease activity observed in the presence of synthetic surfactants such as sodium dodecyl sulfate and polyoxyethylene alkyl ether. Thus, SF has great potential for use in laundry detergent formulations, to improve the stability and reliability of detergent enzymes.