Submitted to: Journal of Surfactants and Detergents
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/19/2005
Publication Date: 6/1/2006
Citation: Zerkowski, J.A., Solaiman, D., Ashby, R.D., Foglia, T.A. 2006. Head group-modified sophorolipids: new cationic, zwitterionic, and anionic surfactants. Journal of Surfactants and Detergents. 9:57-62.
Interpretive Summary: Sophorolipids (SL) are sugar-containing molecules that can be produced by some yeasts. They also contain a fatty acid or "greasy" tail. Yeasts can produce SL when they are fed cheap and abundant agricultural byproducts such as soy molasses or tallow. We are studying SL as part of a project that converts agricultural wastes into valuable materials. The shapes of the SL molecules permit them to function as detergents. Since they are natural products, they hold promise in this regard because they should be benign environmentally, that is, "green" or eco-friendly compounds that could find use in soaps or personal-care products. The naturally occurring form of SL, however, dissolves only poorly in water. This work addresses this problem by chemically attaching amino acids, which are also naturally occurring, to SL. The amino acid units carry positive and/or negative charges, which help the molecules dissolve in water. We have synthesized seven versions of these amino acid/SL adducts, and they do in fact have improved water solubility: they are approximately ten times as water soluble as the unmodified versions. They also behave like detergents, that is, they lower the surface tension of water. These properties will potentially expand the usefulness of SL-class surfactants, leading to the availability of consumer products such as cleaning solutions and cosmetics to benefit the public.
Technical Abstract: Sophorolipids were chemically modified to increase their water solubility as well as to introduce sites at the polar head groups that permit further chemical derivatization. To that end, amino acid-glycolipid conjugates were prepared using carbodiimide-mediated coupling methods. The amino acid units are multifunctional and possess a para-aminobenzoic acid linker. The glycolipid used was a stearic sophoroside. After acidolytic or hydrogenolytic deprotection of the amino acid N-terminus or side chain, water-soluble compounds are obtained that display good surfactant properties. Values for critical micelle concentration are clustered in the range of high 10(E-6) to low 10(E-5) M, and minimum surface tension values are below 40 mN/m. Two of the compounds represent more complicated structural classes, namely gemini and bolaform surfactants.