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46 Hageman and Kuehn
K,
BEAD
+ H,O
B-OH
’ OH
=-
BEAD
ADENOSINE
+
O/H ‘OH
NO COMPLEX
ADENOSINE
Kt
& CH,CH,CONH Q-
BEAD +
2 H,O
HOH& A
Fig. 1. Ionization and diester formation, with adenosine, of an immobl-
lized boronic acid. The “BEAD” represents any insoluble matrix to which
the aryl boronic acid is attached. In aqueous solution, only the ionized,
tetrahedral boronate forms appreciable amounts of stable complex.
anion, and thus, the process of diesterification that occurs is necessar-
ily pHdependent. Since most work has been conducted with immobi-
lized 3-aminobenzeneboronic acid, the chemistry of ionization and
complexation for this boronic acid is illustrated below. The pK, of the
boronic acid in Fig. 1 is about 9 (3), unless shifted by the presence of
competing diols on the matrix or by residual positive charges from
pendant amino groups (4) attached to the matrix. Thus, the working
pH for the commercially available boronic acid matrices cannot be
much lower than 8.0-8.5 in order to have lO-25%, respectively, of the
boronic acid in the boronate anion form.
1.1. Variety of Matrices
Quite a variety of matrices, boronic acid derivatives, and coupling
methods have been employed to prepare immobilized boronic acid
chromatography media (Table 1), and recently, several boronic acid
derivatives have been evaluated with respect to their suitability for use