Chemistry Reference
In-Depth Information
3.2
Homogeneous Polymer Biocatalysts
3.2.1
Fabrication of Macromolecules with Strong Affinities for Ligands
Endeavors toward this goal have been strongly guided by extensive earlier studies of
binding of small molecules by proteins [4]. Serum albumin is extraordinary among
non-enzymatic proteins in manifesting strong affinities for small molecules of widely
different structure. Molecularly oriented and thermodynamic studies early disclosed
the energetic quantities associated with its multiple, stepwise interactions. Associated
free energies and entropies revealed the apolar and electrostatic interactions involved
in the binding. Furthermore, changes in affinities in successive binding steps of a
specific small molecule reflect macromolecular conformational adaptations.
Thus one might expect flexible, water-soluble synthetic polymers with suitable side
chains to show affinities for small molecules. We have examined, over several decades,
the binding ability of poly(vinylpyrrolidone), polyvinylpyridine, polylysine, polyacryla-
mide, poly(isopropylacrylamide), poly(vinylimidazole), poly(vinylmethyloxazolidi-
none), poly(vinylmethyloxazolidinone-vinylimidazole), poly(vinylpyrrolidone-vinyli-
midazole), poly(vinylpyrrolidone-vinyl alcohol), poly(vinylpyrrolidone-maleic anhy-
dride), poly(vinylmethyloxazolidinone-maleic anhydride), and poly(2-methylami-
noethyl methylacrylate-methacrylic acid). Other investigators have studied similar syn-
thetic polymers [5-12]. In our experience no water-soluble polymer binds small mo-
lecules with an avidity comparable to serum albumin.
These polymers have high intrinsic viscosities (e.g. about 22 ml g
-1
for polyvinyl-
pyrrolidone), which indicate that the macromolecules are swollen and extended in
water. In contrast, serum albumin, with an intrinsic viscosity near 4 ml g
-1
, must
be relatively compact. Such a relatively compact conformation might be obtained
with a water-soluble polymer by introducing cross-linkages or by using a highly
branched matrix. Indeed, the latter has proved to be particularly fruitful.
An interesting polymer constrained to a relatively compact conformation is poly-
ethylenimine (PEI), which, as usually prepared, gives a highly branched rather
than a linear macromolecule. Figure 3.1(A) shows the structure of a segment of
this polymer. Approximately 25% of its nitrogens are primary amines, 50% second-
ary, and 25% tertiary [13]. The polymer branching may be represented schematically
(Figure 3.1B).
The primary amine groups form very suitable loci for the attachment of apolar
groups. We have, therefore, prepared several derivatives with different side chains
attached to a portion of the primary amine groups. These modified polymers show
remarkable binding properties [14]. Figure 3.2 illustrates the tremendously greater
extent of binding by the acylpolyethylenimines as compared with serum albumin.
At a free ligand concentration of 10
-5
M
, the lauroyl derivative of polyethylenimine
binds over 100 moles of small dye molecule, the hexanoyl derivative about 10, and
the butyryl about 1 (Figure 3.2), whereas studies with bovine albumin lead to values
just below 1.
