Molecular Recognition of Oligopeptides and Protein Surfaces - Designing Receptors for the Next Generation of Biosensors - page 70

the substrate was fixed by strong electrostatic interactions between host and guest at
both sides.
These experiments clearly demonstrate the potential of small but focused
libraries, which allow the binding properties of all members to be determined.
A large and random library would not have elucidated the subtle differences of the
binding properties, as shown in Fig. 4 . By characterizing all possible host-guest
pairs it was possible to gain a more detailed insight into the binding event than
would have been the case for a large and random library, in which only a few
selected hit structures are isolated and characterized. Of course, due to the
limitations of small libraries with regard to their structural and functional diversity,
the library has to be carefully designed to give the correct answer to the problems
addressed [ 12 ].
1.2 Di- and Multivalent Receptors for Peptide Recognition
Unlike linear receptors (vide supra) so-called tweezer receptors contain two arms
which are bridged via a template. Depending on the linker structure this kind of
molecules is able to adopt a preorganized, yet flexible cavity in which the substrate
is held, as if by a pair of molecular tweezers [ 13 ]. When correctly designed, the
second side chain is able to form additional attractive interactions to the substrate,
thus increasing the complex stability and the substrate selectivity. The second arm
may either be identical to the first, forming symmetrical tweezer receptors, or
completely different in nonsymmetrical tweezers. This scheme may of course be
further expanded to receptors with three or more arms. In general the aim of such a
multivalent approach is to strengthen the complex stability by means of simulta-
neous interactions between multiple complementary functionalities between host
and guest. Optimally the binding affinity is then higher than the sum of the
corresponding monovalent interactions. This approach is also widely made use of
in nature, be it in bacterium cell [ 8 ], antibody-antigen interactions [ 14 ], or the
interplay of transcription factors with multiple sites of DNA [ 15 ]. In the following
sections this chapter will be limited to the discussion of di- and trivalent receptors.
Pioneering work performed in the early 1990s by Still involved the design of a
library comprising 10,000 (10 4 ) peptidosteroidal receptors for enkephalin-like
peptides (13, Fig. 5 )[ 16 ]. The two arms were linked via a steroidal cheno-12-
deoxycholic acid template and differ in their amino acid sequence. The nonsym-
metric substitution was achieved by making use of the different chemical reactivity
of the two hydroxyl groups at positions C3 and C7. With the help of a two-color
two-substrate assay, receptors could be identified which were able to selectively
bind to one of the two differently labeled substrates: blue-dye-linker- L -Tyr-Gly-
Gly- L -Phe- L -Leu-OH and red-dye-linker- L -Tyr- D -Ala-Gly- L -Phe- L -Leu-OH in
chloroform, which only differ by one amino acid. Thus, by selecting beads which
are stained by merely one color only those receptors which are selective for one of
the two substrates could be isolated. When utilizing a flexible backbone, instead of