Chemistry Reference
In-Depth Information
With the advancement of medical technology and pharmacological therapy, the importance of individualized
medicine has been increasing. Though molecular-targeting agents are effective remedies for particular
illnesses and are well marketed in the world, they often have serious side-effects. It is important to conduct
reliable diagnoses and medical treatments; not only gene analysis for gene polymorphism, but also for the
analysis of individual drug metabolizing ability. For this reason, though therapeutic drug monitoring (TDM)
is an indispensable analytical technique for better healthcare, it has important problems. Because several
drugs are monitored by the RPLC method, waste solutions and exposure to patients or medical staff are
serious problems.
A chromatography system that is independent of organic solvents is a very important and potential
analytical method from the view points of the green chemistry. In this chapter, we review a green analytical
method for bioseparation.Environmental-responsive chromatography is focused on in particular as a future
analytical method for multidiscipline research fields. This analytical system utilizes a temperature-responsive
polymer, poly(
N
-isopropylacrylamide) (PNIPAAm) and modified surfaces such as the HPLC stationary
phase with an aqueous mobile phase.
20.2 Environmental-responsive polymers
'Intelligent polymers' respond to various physical and chemical stimuli, such as heat, light, pH and the
addition of chemical compounds, and change their physicochemical properties rapidly and reversibly [3-8].
These functional polymers are also known as 'smart', 'stimuli responsive' or 'environmental responsive'
polymers. PNIPAAm is one of the most representative polymers that exhibits thermally reversible soluble-
insoluble change in the vicinity of its lower critical solution temperature (LCST) at 32°C in aqueous solution
[9-10] (see Figure 20.1). PNIPAAm undergoes a sharp phase separation and a coil-globule transition of the
polymer chain around its LCST. In water, PNIPAAm chains show an expanded conformation below the
LCST due to strong hydration between the amido group in PNIPAAm and surrounding H
2
O molecules. In
contrast, PNIPAAm changes to a compact form due to strong dehydration, followed by forming macroscopic
aggregation above the LCST. Additionally, it has also been confirmed that the effect of PNIPAAm precipitation
by the salt concentration exists. The LCST is decreased with increasing the salt concentration, while sharp
soluble-insoluble changes are maintained. The LCST remarkably shifted to 20°C in a 1 M NaCl solution.
Lowering of the LCST by the addition of salt should, therefore, be due to the acceleration of dehydration, that
is, salting out [11].
Based on such unique features, PNIPAAm has been utilized for drug delivery systems, bioconjugates,
cell culture dishes and tissue engineering for regenerative medicine [12-16]. Furthermore, studies on the
PNIPAAm-modified surface, such as evaluating the intermolecular force between the PNIPAAm surface
and proteins by atomic force microscope are being actively carried out. By copolymerization with
NIPAAm and different monomers, the obtained synthetic copolymer's physicochemical property can be
controlled. The composition rate of monomers significantly influences the LCST. The introduction of a
hydrophobic monomer to the PNIPAAm copolymer causes an increase in LCST compared to that of the
PNIPAAm homopolymer. For instance, if butyl methacrylate (BMA), which is widely used as a
hydrophobic molecule, is adjusted, the 1, 3, 5 mol
proportion of the PNIPAAm, LCSTs of synthetic
copolymers increase to 30, 25 and 20°C, respectively [17]. On the contrary, using a hydrophilic monomer,
such as acrylic acid (AAc) as a compositional unit of PNIPAAm copolymer, its transition temperature is
increased compared to 32°C [18].
There have been several reports concerning the pH dependence of LCST. Hoffman
et al
. examined the
LCST of a copolymer composed of NIPAAm and AAc, and showed that it increased with an increase of
%
