Low‐Temperature Crystallization for Separating Monoacetylated Long‐Chain Sophorolipids: Characterization of Their Surface‐Active and Antimicrobial Properties

Abstract

AbstractPseudohyphozyma (formerly Rhodotorula) bogoriensis synthesizes long‐chain‐length (22‐carbon chain) sophorolipids (22:0‐SL) that are variously acetylated at the 6′‐ or/and 6″‐carbons of the sophorose unit. In this paper, we describe a low‐temperature crystallization protocol that preferentially separates the 6′‐monoacetylated 22:0‐SL (6′‐Ac1‐22:0‐SL) from a parental mixture of 22:0‐SL containing a majority (64.7%) of the 6′,6″‐diacetylated moiety (6′,6″‐Ac2‐22:0‐SL), as deduced from high‐pressure‐liquid‐chromatography evaporative‐light‐scattering‐detection (HPLC‐ELSD) and LC/Q‐TOF‐MS analyses. Tensiometry measurements using the Wilhelmy plate method yielded minimum‐surface‐tension (SFTmin) and critical‐micelle‐concentration (CMC) values of 34.6 ± 1.0 mN m−1 and 0.014 mM, respectively, for 6′‐Ac1‐22:0‐SL (CRYSTAL) fraction and 34.9 ± 1.0 mM m−1 (SFTmin) and 0.018 (CMC) for the hexane precipitate (Hx‐PRCP) fraction containing a high concentration (89.2%) of the diacetylated 6′,6″‐Ac2‐22:0‐SL after crystal removal. In contrast, the SFTmin and CMC of the well‐studied 16‐18 carbon (C16‐18)‐SL of Starmerella bombicola were 35–37.2 mN m−1 and 0.05‐(>0.3) mM, respectively. Individually, the purified CRYSTAL and Hx‐PRCP fractions exhibited a similar degree of strong growth‐inhibition activity against Cutibacterium (formerly 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, showed that, depending on the bacteria and strains tested, the CRYSTAL fraction was either slightly better than or equally effective as the Hx‐PRCP fraction in inhibiting cell growth.