Ultimately, we would like to move beyond using
linear peptides because we feel that they are inherently non-specific
for a single receptor, and in fact, it has been shown that the
amino acid sequence flanking ECD confers little specificity (23).
Therefore, we synthesized and tested cyclic peptide mimics containing
the ECD sequence with the reasoning that cyclization would conformationally
constrain the amino acids. We initially investigated the inhibitory
activity of two cyclic peptide mimics, cyclo(EC2DC1)YNH2, and
cyclo-(D2EC2D1C1)-YNH2 (3,4). We chose to cyclize the peptides
with a thioether linkage because of the simplicity of forming
the linkage in the presence of the cysteine of the ECD sequence
(24).
These cyclic peptides inhibited sperm-egg binding in mice, however,
the IC50's were the same as their linear counterparts: 500 µM
(3,4). We determined the three dimensional structures of these
two cyclic peptide mimics by multi-dimensional solution NMR in
order to evaluate why cyclization did not improve potency of inhibition
(19). The structures revealed that the peptides are structured.
However, the Cb-Cb distance between the E and the D is 9.1 Å
in one peptide and 7.7 Å in the other. These distances are
similar to that of a linear peptide.
We are currently working with Prof. William Lubell at the Université
de Montréal who has developed methodology to provide diverse
templates of predefined geometry that are easily functionalized
with the appropriate peptide sequence, in this case ECD, for peptide
mimetic synthesis.
