The protonation reactions
for catechin and epicatechin
have been studied at 25�C, I 0.10M (KCl). Equilibrium
constants Kn for the protonation steps of
the anions L4- have been determined from potentiometric
and/or spectrophotometric measurements. The logKn values for the epimers
differed significantly only for logKl and
logK4 (values for catechin 13.26 � 0.05,
11.26 � 0.06, 9.41 � 0.02, 8.64 � 0.01, and epicatechin
13.40 � 0.05, ll.23 � 0.06, 9.49 � 0.02, 8.72 � .01, n = 1-4 respectively).
These differences are ascribed to specific solvation
effects or ion associations between K+ and the ligand anions.
Epimerization was studied by h.p.l.c., and was found
to be minimal over the experimental time scale at pH c. 13.5. The ring on which each
stepwise protonation occurs was elucidated by reference to log Kn values measured for the model compounds catechol, 4-methylbenzene-1,2-diol, protocatechuic
acid and 3',4'-di-O-methylcatechin. The protonation sequence is B ring, A ring,
B ring and A ring. In contrast to earlier
results, it has been established that the epimerization of catechin
or epicatechin at C2, and the formation of catechinic acid in basic solution, may proceed through a
radical rather than ionic mechanism.