Chiral Separations by Micellar Electrokinetic Chromatography - Chiral Separations by Capillary Electrophoresis - page 219

counterparts through enantioseparations of lorazepam, temazepam, propanolol, and
BOH by MEKC. The methyl, ethyl, and t -butyl esters of glutamic acid-derived poly-
mer, poly-SUG, were synthesized to vary the hydrophobicity, steric hindrance, and
orientation of the amino acid side chain [234]. Changes in the orientation or ioniza-
tion of the headgroup were found to enhance chiral separation of most of the analytes
employed. On the other hand, steric hindrance of the glutamic acid side chain and
bulky t -butyl group either reduced or completely destroyed chiral resolution of some
analytes. Interestingly, the t -butyl group was found to be essential for the chiral
separation of the other compounds. Thibodeaux et al. [235] synthesized polymers
with proline, valine, norvaline, leucine, norleucine, isoleucine, and t -butyl leucine
as headgroups and used them for enantioseparations by MEKC. The authors noted
lower chiral resolution due to the steric hindrance of the side chain of norleucine
and t -butyl leucine, while side chains containing one or two carbons did not sig-
nii cantly affect the resolution. Polymers of alkenoxy amino acid-based surfactants,
poly(sodium N -undecenoxycarbonyl-l-leucinate) (poly-SUCL) and poly(sodium
N -undecenoxycarbonyl-l-isoleucinate) (poly-SUCIL) have been developed and
employed for enantioseparations of binaphthyl derivatives [236], β -blockers [237],

and -blockers with two stereogenic centers [238] by Shamsi and coworkers. A dif-

ference in chiral recognition mechanism was noted. The polymer with isoleucine
headgroup provided better resolution for

β

-blockers with two stereogenic centers.
Chiral resolution of binaphthyl derivatives was nearly equivalent with both polymers
while poly-SUCL provided overall better resolution and a wider migration window.
In order to examine the effects of steric hindrance of the surfactant headgroup,
and number and position of chiral centers in the surfactant molecule on the sepa-
ration performance, chiral separations of seven organic racemates (three benzoin
derivatives, laudanosine, norlaudanosoline, laudanosoline, and chlorthalidone) using
eighteen polymeric single amino acid and dipeptide surfactants were studied [239].
For six of the seven analytes, dipeptide-derived polymeric surfactants provided bet-
ter chiral separation than polymeric single amino acid surfactants. Steric factors
were also found to be important in determining the chiral separation performance.
In general, chiral selectors with more sterically hindered environments provided bet-
ter chiral selectivity for less sterically hindered analytes, while chiral selectors with
less steric hindrance provided better selectivity for analytes with sterically hindered
chiral centers. Mwongela et al. [240] synthesized a new chiral polymeric surfactant,
poly(sodium N -oleyl-l-leucylvalinate) (poly-SOLV) for chiral separations of BOH,
BNA, BNP, benzoin, benzoin derivatives, warfarin, and coumachlor. The selectivity
of the polymer was compared with that of the corresponding monomeric surfactant.
The higher hydrophobicity of the surfactant was found to provide better chiral rec-
ognition for hydrophobic compounds. The superior performance of the polymers
with l-leucyl-valinate headgroup in chiral MEKC was demonstrated by an extensive
study in which the resolution of 58 out of 75 neutral, anionic, and cationic racemates
was achieved [241].
The effects of polymer polydispersity and alkyl chain length on the performance
and chiral resolution were also studied. Tarus et al. [242] polymerized micellar solu-
tions of sodium N -undecenoyl-l-leucinate (SUL) in the presence of alcohols, such as
hexanol and undecenyl alcohol. The polymers thus obtained were found to be larger

β