Vitamin B6 Enzyme Models - Artificial Enzymes - page 45

Murakami et al. [19] developed an additional vitamin B6 model system with a bind-
ing site. They synthesized an octopus cyclophane ( 27 ) as a functional model of the
protein matrix of transaminase. This cyclophane formed a hydrophobic cavity in water
where PLP could be noncovalently incorporated. Alkylamines having various hydro-
phobic chains were employed as substrates, in place of -amino acids, to evaluate the

hydrophobic effect on the Schiff base-forming equilibrium. The Schiff base formation
constant was found to depend markedly on the chain length of a substrate in the pre-
sence of 27 , indicating that the octopus cyclophane can be utilized as an effective ho-
loenzyme model capable of forming a ternary complex.
With the binding group in hand, it is interesting to learn how to induce stereose-
lectivity in the transamination. The earliest example ( 23 ) afforded amino acids with
some stereoselectivity, because of the chirality of the cyclodextrin unit [15]. Tryptophan
and phenylalanine formed by the reaction with 23 showed preferable formation of the
L- enantiomers at L / D ratios of 2:1 and 5:1, respectively. More selectivity is expected if
the proton is delivered by a chirally mounted basic group, as in the enzyme. Thus,
compound 28 , carrying both a pyridoxamine and an ethylenediamine unit attached
to

a

-CD on neighboring primary methylene groups, was prepared and studied for
its ability to form amino acids from keto acids with chiral selectivity [20]. Although
Tabushi et al. reported quite good selectivities, it has proven impossible in our labora-
tory to duplicate these findings. However, in some alternate approaches we have pro-
duced optical induction with related catalysts ( 29 ), but not in high 90% selectivities
[21].

b