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50 Hageman and Kuehn
three cases listed in the Introduction will be briefly described. It should
be stressed that the capacity of di- or triols to complex with a boronate
is strongly influenced by configurations of the alcohols.
1.2. Glycoproteins
The capacity of a boronate to complex with a neutral carbohy-
drate depends strongly on the stereochemistry of the diols and, conse-
quently, on whether a sugar is in a pyranose or furanose form. Because
the boronate can be expected to alter the equilibrium between these
two forms, a knowledge of the binding strength of boronates to sugars
with free anomeric carbons will not be an infallible guide to the capac-
ity of proteins having these bound sugars to be retained on a boronate
matrix. For the most part, however, binding data for free sugars is all
that is available. Table 3 gives most of the currently available binding
data for sugars and related compounds to the boronate group or
borate group.
By far the best-studied glycoprotein that is retained on a boronate
matrix is glucosylated hemoglobin (see below). This is an atypical glyco-
protein because a free glucose reacts with an amino terminus of the
hemoglobin and undergoes an Amadori rearrangement, presenting
an arabitol-like structure and an adjacent positive charge to the
boronate anion. Note 5 cites other more typical glycoproteins that can
bind to a boronate matrix.
1.3. Boronate-Sensitive Enzymes
A number of classes of enzymes have been reported to be inhib
ited by aryl and alkylboronic acids. In practice, serine proteinases have
been purified by their affinity for matrices bearing benzeneboronate.
In principle, quite a number of other enzymes might be purified by
use of appropriate boronate matrices. Table 4 summarizes the rela-
tively few examples of immobilized boronate inhibitors that have been
used for afftnity chromatography and suggests other examples where
specific enzymes might be purified with matrices containing appro-
priate boronic acids. Quite a number of hydrolases can be expected to
be retarded on a matrix containing a benzeneboronate, even though
more potent boronate inhibitors can usually be synthesized. The reader
may refer to the references given in Table 4 to prepare specific
boronic acids not commercially available.