Chemistry Reference
In-Depth Information
(a)
(c)
CH
2
CH
CH
2
CH
CH
2
CH
O
O
O
l
n
m
NH
NH
C(CH
3
)
3
OH
H
3
C
CH
3
Heat
Temperature
responsive
Hydrophobic
Cation
exchange
(b)
CH
3
CH
2
CH
CH
2
C
CH
2
CH
Cool
O
O
O
n
m
l
NH
O
(CH
2
)
3
CH
3
NH
(CH
2
)
3
N(CH
3
)
2
H
3
C
CH
3
Analyte
Electric
charge
Temperature
responsive
Hydrophobic
Anion
exchange
Hydrophilic
charge
Hydrophobic
non-charge
Figure 20.9
Chemical structures of dual pH- and temperature-responsive terpolymer,
P(NIPAAm-co-AAc-co-tBAAm) (a), and P(NIPAAm-co-BMA-co-DMAPAAm) (b).
The concept of environmental-responsive chromatography is shown in (c).
achieved on a column of these terpolymer thin hydrogel modified surfaces, as compared to uncharged control
binary copolymers of NIPAAm and
t
BAAm. Although hydrophobic interactions effect the separation of
angiotensin subtypes, combined electrostatic and hydrophobic interactions result in a more pronounced
retention. At temperature below the terpolymer LCST, hydrophobic interactions predominated, and minimal
changes in the electrostatic interactions were supported by a light shift in the apparent AAc carboxylate p
K
a
values (Figure 20.9c). Above the LCST, electrostatic interactions were dramatically reduced as a result of the
decreased charge densities of the polymer modified stationary phase. Therefore, the peptide retention times
were also reduced, exhibiting a maximum at near 30-35°C. These anionic temperature-responsive polymer
modified surfaces are good candidates for the improved separation of bioactive peptides under exclusively
aqueous conditions. Furthermore, this developed method was also applied small molecular-weight compounds,
such as intracerebral hormones [41]. Serotonin (p
K
a
4.7) are
important chemical modulators of the biological clock of vertebrates, and are clinically used to treat jet lag
and sleep diseases. Usually, in the isocratic elution of these samples containing solutes with a wide range of
polarity, it is sometimes difficult to achieve the desire resolution time. It may be necessary to use gradient
elution where the volumes of an organic solvent, the composition of the mobile phase, or some other property
of the solvent (e.g., pH or ionic strength) are changed during the separation. In such analysis, the retention
behavior changes by increasing the temperature are not observed. Figure 20.10 shows chromatograms of
analytes that were obtained at 10, 30 and 50°C using a P(NIPAAm-
co
-AAc-
co
-
t
BAAm) hydrogel-modified
column at pH 6.0. Because analytes are dissociated forms at pH 6.0, the electrostatic interaction between
them and the polymer modified stationary phase should cause increased retention below the LCST. The
polymer chain is in an extended form in aqueous media, and the AAc moiety should act as a cation-exchange
=
4.9, 9.8) and its metabolite melatonine (p
K
a
=
