Low-temperature crystallization for separating monoacetylated long-chain sophorolipids: characterization of their surface-active and antimicrobial properties

Authors: Solaiman, Daniel; Ashby, Richard - Rick; Nunez, Alberto; Cross, Nicole

Submitted to: Journal of Surfactants and Detergents
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2020
Publication Date: 3/2/2020
Citation: Solaiman, D., Ashby, R.D., Nunez, A., Cross, N.V. 2020. Low-temperature crystallization for separating monoacetylated long-chain sophorolipids: characterization of their surface-active and antimicrobial properties. Journal of Surfactants and Detergents. 23:553-563. https://doi.org/10.1002/jsde.12396.
DOI: https://doi.org/10.1002/jsde.12396

Interpretive Summary: Sophorolipids (SLs) are natural detergent molecules produced by yeast using renewable feedstocks including low or no cost agro-industrial waste products as starting materials. One strain of yeast called Rhodotorula bogoriensis is of special interest because it produces a mixture of unusual SLs containing an extra-long (XL) water-hating (or hydrophobic) tail. We previously showed for the first time that the XL-tail SLs are more potent antibacterial agents against pimple-causing bacteria than the common SLs. Now we found a method to separate out two components (called Ac1-C22-SL and Ac2-C22-SL) from the mixture of the naturally produced XL-tail SLs. We then determined their detergent potency to find for the first time that they are in fact superior than the other common SLs based on an indicator called CMC (stands for “critical-micelle-concentration”). They however, are generally a weaker antibacterial agent than the common SLs against tooth decay- or pimple-causing bacteria. One exception was Ac1-C22-SL that showed the strongest antibacterial activity against pimple-causing bacteria among the common and the XL-tail SLs. The present study thus for the first time taught a method to separate components of XL-SLs and at the same time reported their respective detergency strength and antibacterial potency.

Technical Abstract: Rhodotorula bogoriensis synthesizes long-chain-length (22-carbon chain) sophorolipids (C22-SLs) that are variously acetylated at the 6'- or/and 6 '-carbons of the sophorose unit. In this paper, we describe an isolation protocol that separates the 6'-monoacetylated C22-SL (6'-Ac1-C22-SL) from the mixture containing mainly the 6',6'-diacetylated sophorolipids (6',6'-Ac2-C22-SL), as deduced from HPLC-ELSD and LC/Q-TOF-MS analyses. Tensiometry measurement using the Wilhelmy plate method yielded minimum-surface-tension (SFTmin) and critical-micelle-concentration (CMC) values of 34.6 ± 1.0 mN/m and 0.014 mM, respectively, for 6'-Ac1-C22-SL. For the 6',6'-Ac2-C22-SL fraction, these values were SFTmin = 34.9 ± 1.0 mM/m and CMC = 0.018 mM. In contrast, the SFTmin and CMC of the well-studied medium-chain-length C16-18-SLs of Starmerella bombicola were 35-37.2 mN/m and 0.05-(>0.3) mM, respectively. The 6'-Ac1-C22-SL and 6',6'-Ac2-C22-SL species exhibited a similar degree of strong growth-inhibition activity against Propionibacterium acnes as determined by an agar-plate zone of inhibition assay. Study on the growth inhibition of oral health-related bacteria, i.e., Streptococcus mutans and Lactobacillus acidophilus, by 6'-Ac1-C22-SL and 6',6'-Ac2-C22-SL species showed that, depending on the bacteria and strains tested, the 6'-Ac1-C22-SL is either slightly better than or equally effective as 6',6'-Ac2-C22-SL in inhibiting cell growth.