| In order to facilitate the discovery of receptors for complex oligosaccharides capable of operating under physiological conditions, a combinatorial approach was taken towards the solid-phase synthesis and evaluation of a oligoboronic acid library. To obtain this library, a resin bound polyamine backbone was desired to provide the necessary scaffold for the alkylation with a 2-bromomethylphenylboronic ester to give the 2-(N,N′-dialkylamino)phenylboronic acid units required for saccharide binding. The polyamine library was prepared via the exhaustive amide reduction of a peptide (i.e., a polyamide) library using borane-tetrahydrofuran complex as the reducing agent. A method for reducing polystyrene-bound peptides was developed which employs iodine in a buffered medium to cleave the borane-amine adducts that form after the reduction. This method was found to be mild enough for use on peptides linked to polystyrene supports via the highly acid labile trityl linker. To encode each split-pool library, an efficient partial termination synthesis was developed during the preparation of the peptide library that allowed single beads, expressing a single compound, to be decoded by electrospray mass spectrometry coupled to liquid chromatography. This decoding method was found effective for both polystyrene and TentaGel® bound polyamine libraries, and for TentaGel ® bound triboronic acids. Screening of a 289-membered, resin bound triboronic acid library was performed in on-bead assay against a fluorescently labeled Lewis-b glycoside. However, the binding of Lewis-b to any member of the library was never observed, perhaps due to the poor binding affinities that cannot be visually detected. This study has therefore led to a new approach which examines smaller, tri- and diboronic acid parallel libraries in solution phase screenings against simpler disaccharides.; Since natural polyamines are known to be biologically active, our synthetic polyamine libraries were also evaluated as selective receptors for polyanionic targets. Using a polystyrene bound tetramine library, selective receptors for trisulfonated dyes and a synthetic peptide were discovered via on-bead assays. These preliminary studies will eventually lead to the screening of the polyamine libraries against biologically significant molecules such as oligonucleotides, ion channel proteins and polysulfated saccharides including heparin. |
