Synthesis and Application of Rosin-Based Surfactants - Rosin-based Chemicals and Polymers - page 165

5.4.5 Chiral Separation
Because there are many chiral carbon atoms in the pine resin acids and their
derivatives, they have been widely used in the separation technology. Pine resin acids
and amines are used as chiral reagents for the resolution of isomers of biological
compounds. For example, dehydroabietic acid is used as a reagent for separating
chiral amines and dehydroabietylamine is used as a reagent to separate chiral acids.
These separation reactions usually depend on salt formation reactions, and according
to their solubility in organic solvents, resolution of the biological isomers can be
obtained by recrystallisation. With the development of separation technology, micellar
electrokinetic chromatography (MEKC), as an electrokinetic separation technique
[70], has become one of the most popular techniques in the field of separation science
due to its high resolving power and capability of separating both ionic and neutral
compounds. One of the attractive applications of MEKC is enantiomer separation
[71]. In MEKC, chiral surfactants have been used as a pseudostationary phase for
chiral separation. Rosin is a naturally occurring enantiomeric diterpenic acid, which
is an excellent starting material for preparing chiral surfactants because of its wide
availability and special stereostructure.
Wang and co-workers reported [33] a new type of anionic chiral surfactant (SMA,
[A02, see Scheme 5.16 ]), which was used for the enantioselective MEKC separation of
amino acid enantiomers derivatised with naphthalene-2,3- dicarboxaldehyde (NDA-
d/l -AA). Under the conditions selected, two pairs of tested amino acid enantiomers,
including NDA- d/l -trptophan and NDA- d/l -kynurenine were resolved. On the other
hand, SMA showed high aqueous solubility and low CMC, which simplified the
MEKC methodology, and avoided the use of a comicellar phase system and organic
solvents for chiral separation [33].
Zhao and co-workers reported [49] a new type of zwitterionic chiral surfactant,
DHAMAP, (Z10, see Scheme 5.28 ) which was used to perform chiral separation
of d/l -amino acids by capillary electrophoresis (CE). Six pairs of tested amino acids
enantiomers including NDA- d/l -tryptophan (NDA- d/l- Trp), NDA- d/l -phenylalanine
(NDA- d/l -Phen), NDA -d/l -kynurenine (NDA- d/l -Kyn), NDA- d/l -β-phenylalanine
(NDA- d/l -β-Phen), NDA- d/l -4-methylphenylalanine (NDA- d/l -4-M-Phe) and NDA-
d/l -arginine were well-resolved ( Figure 5.4 ). All compounds were fully separated
by using a chiral running buffer consisting of a rosin-based surfactant DHAMAP.
However, separation was not obtained if the running buffer did not contain DHAMAP.
DHAMAP is an amphoteric chiral surfactant; alterations of buffer pH can affect
the charge on the analyte and the chiral pseudophase, thus influencing the chiral
separation.