Rhamnolipids structure

Figure 10.1. (a) Rhamnolipid from P. aeruginosa ATCC 9027 showing cadmium binding, (b) Structure of the iron-siderophore complex of enterobactin. 

Zhang, Y. Miller, R.M. (1995). Effect of rhamnolipid (biosurfactant) structure on solubilization and biodegradation of n-alkanes. Applied and Environmental Microbiology, 61, 2247-51. 

The autoinducer of the third quorum sensing system in P. aeruginosa is PQS. (Figure 36) PQS is involved in lasB expression, and RhlR is required for PQS activity. The structure of PQS is similar to that of antimicrobial quinolones, but PQS shows no antimicrobial activity. PQS is converted from 2-heptyl-4(lH)-quinolone (HHQ) by PqsH, which is activated by 3-oxo-Cl2-HSL—LasR. PQS regulates the expression of elastase, rhamnolipid, pyocyanin, and LecA. As PQS regulates LasB expression and the synthesis of PQS is regulated by LasR and... 

The rhamnolipid B (70) has the molecular structure of biosurfactants, comprising a hydrophilic part (rhamnose moiety) and a hydrophobic part ((3-hydroxydecanoate moiety) [151]. The biosurfactant property of this glycolipid was supposed to confer the ability to intercalate into and disrupt the zoospore plasma membrane [152,153]. However, other glycolipids such as sophorolipids and trehalose lipids, which have chemical properties similar to the rhamnolipids, did not show zoosporicidal activity at concentrations up to 1000 pg/ml. 

Rhamnolipids, in which one or two molecules of rhamnose are linked to one or two moleeules of 3-hydroxydecanoic acid, are the best-studied glycolipids. Production of rbamnose-containing glycolipids was first described by Jarvis and Johnson [10]. The main glycolipids produced by P. aeruginosa are rhamnolipids of the types 1 and 2, L-rhamnosyl-(3-hydroxydecanoyl-p-hydroxydecanoate and L-ihamnosyl-L-rhamnosyl- 3-hydroxydecanoyl- 3-hydroxydecanoate. Their structures can be seen in Fig. 1. The formation of rhamnolipid types 3 and 4 containing one 3-hydroxydecanoic acid with one and two rhamnose units, methyl ester derivatives of rhamnoUpids 1 and 2, and rhanmolipids with alternative fatty acid chains have also been reported [5]. 

Similarly, naturally derived surfactants extracted from fermentation broths or prepared by partial hydrolysis of natural extracts can contain polysaccharides, proteins, and phospholipids. For example, rhamnolipids and sophorolipids have unique structural features that cause them to deposit on chemically similar surfaces and modify surface energy even at very low concentrations. Clearly, the emergence of biotechnology in the twenty-first century will drive the development of new surfactants from microbial fermentation, and improve the commercial viability of known surfactants from such processes. 

Scheme 1. Structure of four different rhamnolipids (R1-R4) produced by P. aeruginosa.

Figure 14.1 Structure of a rhamnolipid with two rhamnose and two p-hydroxydecanoic acid units.

Other studies [85] have shown that cationic bacitracin, anionic AOT, and nonionic Tween 80 all enhanced cellulose hydrolysis, implying that the charge of the surfactant was not an important consideration. In fact. Tween 80 (0.1%) increased the rate and extent of saccharification by up to 40% [82,85]. The structure of the hydrophilic head group of the surfactant also had little significance [85], Those with a sugar group, sophorolipid and rhamnolipid, worked well, as did bacitracin, which has a peptide hydrophilic group [85]. 

Specifically the 4-deoxyhexosyl disaccharide (50) has been repeated and structurally characterised by X-ray diffraction, and the following 6-deoxyhexosyl compounds have been prepared 2-Q-a-L-fucopyranosyl-D-galactose,2-acetamido-2-deoxy-(3- and 4-Q-a-L-fucopyranosyl)-D-glucose as their trifluoroacetamidopropyl p-glycosides,methyl 3[3-(2-Q-a-L-rhainnopyranosyl-a-L-rhamnopyranosyloxy)decanoyloxy] decanoate (51, the rhamnolipid of Pseudomonas aeruginosa >. and methyl 3-Q-(L-rhamnopyranosyl)-i8-D-... 

Biosurfactants are classified based on their chemical composition and microbial origin. The chemical structure of biosurfactants contains hydrophilic and hydrophobic groups. Amino acids, peptides and polysaccharides can be present as hydrophilic moieties and saturated or unsaturated fatty acids can be present as hydrophobic moieties in the structure. Structure based classification has following major classes of biosurfactants (Guerra-Santos et al., 1987 Kooper and Goldenberg, 1987) (i) Glycolipids (Rhamnolipids, Trehalolipids and Sophorolipids) (ii) Lipopeptide and lipoprotein (iii) Fatty acids, phospholipids and neutral lipids (iv) Pol5mieric biosurfactants, and (v) Particulate biosurfactants. 

Edward, J. R., Hayashi, J. A. Structure of rhamnolipid from Pseudomonas aeruginosa. Arch. Biochem. Biophy. 1965,111,415 21. 

Rhamnolipid 1 and rhamnolipid 3 are the major rhamnolipids produced by using resting cells of Pseudomonas aeruginosa DSM 2874. Two further rhamnolipids that are similar in structure but contain only one hydroxydecanoic acid unit, rhamnolipids 2 and 4, have also been detected. The rhamnolipid production with resting cells is a two-step process. In a first step Pseudomonas aeruginosa cells are produced and harvested. In a second step this biomass is used for the rhamnolipid production under growth-limiting conditions [29],... 

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