Synthesis and properties of a series of bio-based surfactants with different hydrophobic chain length

S. Sato, H. Habe
National Institute of Advanced Industrial Science and Technology (AIST),
Japan

Keywords: green surfactant, glyceric acid, glycerol, biorefinery

Summary:

Glyceric acid (2,3-dihydroxypropanoic acid, GA) is a natural organic compound found in specific plants as a minor constituent, but can be mass-produced biotechnologically from glycerol, a by-product of biodiesel fuel production. We synthesized a series of glycerate-derived green surfactants with different hydrophobic chain length (C6 to C10), i.e., sodium dihexanoylglycerate (diC6GA-Na), sodium dioctanoylglycerate (diC8GA-Na) and sodium didecanoylglycerate (diC10GA-Na) from GA and corresponding acyl chlorides. The chemical structures of the synthesized compounds were confirmed using chemical analysis, e.g., nuclear magnetic resonance spectra and liquid chromatography coupled with mass spectrometry. The obtained diacyl GAs were not soluble in water, whereas their sodium salts were water soluble and had surface tension-lowering properties in aqueous solution. The critical micelle concentration (CMC) was determined, and the CMC was found to be significantly dependent on the hydrophobic chain length and decreased with increasing of the carbon number in the alkyl chain; the values of diC6GA-Na, diC8GA-Na and diC10GA-Na were 2.92 mM, 0.82 mM and 0.23 mM, respectively. By contrast, the surface tension at the CMC were 33.9 mN/m, 25.5 mN/m and 27.9 mN/m, respectively. The inhibitory effects of 10 mM diC6GA-Na, diC8GA-Na and diC10GA-Na (above the CMC) on trypsin activity was determined by measuring tryptic hydrolysis of Nα-benzoyl-DL-arginine-4-nitroanilide (BAPNA), i.e., the absorption spectrum of p-nitroaniline (410 nm), a tryptic hydrolysate of BAPNA. One unit of enzyme activity was defined as the amount of enzyme that produced 1 micromol of p-nitroaniline per minute, which was calculated using the molar extinction coefficient of p-nitroaniline at 410 nm of 8,800. The specific activity of trypsin with 10 mM diC6GA-Na, diC8GA-Na and diC10GA-Na to hydrolyze BAPNA was 0.55 ± 0.084 U/mg protein, 0.26 ± 0.045 U/mg protein and 0.39 ± 0.10 U/mg protein. Our finding suggests that diC8GA-Na may be a more attractive trypsin inhibitor as well as surfactants for industrial use than both diC6GA-Na and diC10GA-Na.