Rhamnolipids Characteristics, Production, Applications, and Analysis - Biosurfactants: Research Trends and Applications - page 79

According to Abdel-Mawgoud et al. (2011), in rhamnolipids, the glycone part
and an aglycone part are linked to each other by the o-glycosidic linkage. The
glycone parts are made of one or two rhamnose moieties linked to each other
through a-1, 2-glycosidic linkage. The 2-hydroxyl group of rhamnose group usu-
ally remains free, though in some homologues, they can be acylated with a long-
chain alkenoic acid.
The aglycone part is composed of one, two, or three saturated or even some-
times mono- or polyunsaturated β-hydroxy fatty acid chains. The length of chains
varies from 8 to 16 carbons. These chains are linked to each other via an ester
bond formed between the β-hydroxyl groups of the distal chain and the carboxyl
group of the proximal chain. Although carboxyl group of the distal β-hydroxy
fatty acid chain usually remains free, in some homologues, this group is esterified
with a short alkyl group and displays the structure of the best known RL congener,
α-l-rhamnopyranosyl-α-l-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate,
which is typically symbolized as Rha-Rha-C 10 -C 10 (Abdel-Mawgoud et al., 2011).
Mulligan (2005) categorized rhamnolipids into four general types: type I
(R1), l-rhamnosyl-β-hydroxydecanoyl-β-hydroxydecanoate, with a molecular
mass of 504 Da; type II (R2), l-rhamnosyl-β-l-rhamnosyl-β-hydroxydecanoyl-
β-hydroxydecanoyl-β hydroxydecanoate, with a molecular mass of 660 Da; type III
(R3), one rhamnose attached to β-hydroxydecanoic acid; and type IV (R4), two
rhamnoses attached to β-hydroxydecanoic acid (Figure 3.5) (Mulligan, 2009).
As noted previously, many researchers such as Deziel et al. (1999) and
Lotfabad et al. (2010) stated that the type of substrate being used for the growth of
P. aeruginosa has a great impact on the composition of the produced rhamnolipids.
For example, the experiments of Deziel et al. (1999) showed that rhamnolipids pro-
duced by P. aeruginosa strain using mannitol or naphthalene as a carbon source
have shown different percentages of congeners and homologues. Using mannitol
as the carbon source increases the percentage of the rhamnolipids with two rham-
nose groups and two 3-hydroxydecanoic acid groups. On the other hand, using
naphthalene increased the production of rhamnolipids with two rhamnose groups
and one 3-hydroxydecanoic acid group. Investigations conducted by Deziel et al.
(1999) and Mulligan (2009) showed that in the mixture of rhamnolipids produced by
P. aeruginosa , the presence of 28 different rhamnolipid congeners and up to 7 homo-
logues can be identified. Also according to Lotfabad et al. (2010), Pseudomonas sp.
grown on different carbon sources has been reported to produce rhamnolipid mix-
tures of 4-28 different homologues. Furthermore, there have been some reports on
other bacterial species that are able to produce different congeners and homologues
of rhamnolipids (Abdel-Mawgoud et al., 2010).
Moreover, many researchers reported that P. aeruginosa strains are able to pro-
duce a variety of mixtures of homologues and congeners of rhamnolipids under dif-
ferent environmental conditions (Abdel-Mawgoud et al. 2010; Dubeau et al., 2009;
Nguyen and Sabatini, 2011; Ochsner et al. 1994; Van Gennip et al. 2009). According
to Mulligan (2009), parameters such as fermenter design, pH, nutrient composition,
substrate, and temperature dictate the composition and the structure of produced
rhamnolipids (Mulligan, 2009).