26
Bioorganic Chemistry
this situation is to use a 1 : 1 ratio of in-
coming amino acid to resin-bound amine.
This then requires longer coupling times
to allow the slowest amino acids an op-
portunity to react. Some of the advantages
of solid-phase synthesis are thus lost, as a
largeexcesscanno
tbeemp
loyedtodr
ive
the reaction.
An alternative to the use of mixtures of
amino acids during coupling is the use
of so-called split and mix synthesis. This
method will work
only
with the method
of iterative deconvolution. Using an ex-
ample of a dipeptide library with three
possible amino acids, a typical split and
mix synthesis would begin by separating
the resin to be used into three equal por-
tions (Fig. 20). To each of these portions,
a single speci±c amino acid would be at-
tached (in the example these are G, P,
and T). Because an individual amino acid
is being used, it can be used in excess
to drive the reaction to completion. (This
is not possible when using a mixture of
amino acids, without running the risk of
more reactive amino acids overpopulating
the product mixture.) When these reac-
tions are complete, these three separate
portions of resin are combined and mixed
thoroughly, and again separated into three
separate portions. Again, to each of these
portions, a single speci±c amino acid is at-
tached. These three portions are then again
mixed thoroughly and separated into ran-
dom mixtures. The resultant beads have
fully random amino acids at each position
and can then be speci±cally modi±ed with
a speci±c amino acid for use in an itera-
tive deconvolution strategy. The difference
between this approach and that of using
mixtures of amino acids is the manner in
which the random positions were gener-
ated. In the case of split and mix synthesis,
the random positions are generated by us-
ing individual amino acids reacting with,
and
then
mixing the resultant beads. The
method of amino acid mixtures generates
random positions by using mixtures of
amino acids reacting at each position.
Both these methods for generating ran-
dom positions can be effective and have
strengths and weaknesses. Split and mix
synthesis allows one to avoid the differ-
ential reactivity problem that can exist
with random mixtures by allowing large
excesses to react. Random mixtures on the
other hand are easier to manipulate. In ad-
dition, for logistical reasons, only they can
+
G
+
G
G
G-G
P-G
T- G
G-P
P-P
T- P
G-T
P-T
T- T
P
T
G
P
T
G
P
T
G
R1-R2
Result:
resin with
mixtures at
positions
R1 and R2
+
P
P
+
T
T
+
P
+
T
Three equal
portions
of beads
Individually
react with
specific
amino acid
Mix beads
and split into
three portions
React each portion
with a specific
amino acid
Mix all
portions
Fig. 20
Using the split and mix approach to generate resin with a bound mixture of all possible
dipeptides using the amino acids G, P, and T.
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