Proteome-wide epitope mapping of antibodies using ultra-dense peptide arrays.

Antibodies are of importance for the field of proteomics, both as reagents for imaging cells, tissues, and organs and as capturing agents for affinity enrichment in mass-spectrometry-based techniques. It is important to gain basic insights regarding the binding sites (epitopes) of antibodies and potential cross-reactivity to nontarget proteins. Knowledge about an antibody's linear epitopes is also useful in, for instance, developing assays involving the capture of peptides obtained from trypsin cleavage of samples prior to mass spectrometry analysis. Here, we describe, for the first time, the design and use of peptide arrays covering all human proteins for the analysis of antibody specificity, based on parallel in situ photolithic synthesis of a total of 2.1 million overlapping peptides. This has allowed analysis of on- and off-target binding of both monoclonal and polyclonal antibodies, complemented with precise mapping of epitopes based on full amino acid substitution scans. The analysis suggests that linear epitopes are relatively short, confined to five to seven residues, resulting in apparent off-target binding to peptides corresponding to a large number of unrelated human proteins. However, subsequent analysis using recombinant proteins suggests that these linear epitopes have a strict conformational component, thus giving us new insights regarding how antibodies bind to their antigens.

Intramolecular sensitization of photocleavage of the photolabile 2-(2-nitrophenyl)propoxycarbonyl (NPPOC) protecting group: photoproducts and photokinetics of the release of nucleosides.

Novel photolabile protecting groups based on the 2-(2-nitrophenyl)propoxycarbonyl (NPPOC) group with a covalently linked thioxanthone as an intramolecular triplet sensitizer exhibit significantly enhanced light sensitivity under continuous illumination. Herein we present a detailed study of the photokinetics and photoproducts of nucleosides caged with these new protecting groups. Relative to the parent NPPOC group, the light sensitivity of the new photolabile protecting groups is enhanced by up to a factor of 21 at 366 nm and is still quite high at 405 nm, the wavelength at which the sensitivity of the parent compound is practically zero. A new pathway for deprotection of the NPPOC group proceeding through a nitroso benzylalcohol intermediate has been discovered to complement the main mechanism, which involves beta elimination. Under standard conditions of lithographic DNA-chip synthesis, some of the new compounds, while maintaining the same chip quality, react ten times faster than the unmodified NPPOC-protected nucleosides.

Recent highlights on photolithic oligonucleotide array in situ synthesis.

Light directed synthesis of high-density oligonucleotide microarrays is currently performed using either ortho-nitro-benzyl-type [MeNPOC] (Pease, A.C.; Solas, D.; Sullivan, E.J.; Cronin, T.M.; Holmes, C.P.; Fodor, S.P.A. Proc. Natl. Acad Sci U.SA. 1994, 91, 6333.) or ortho-nitrophenylethyl-type [NPPOC] (Hasan, A.; Stengele, K.P.; Giegrich, H.; Cornwell, P.; Isham, K.R.; Sachleben, R.A.; Pfleiderer, W.; Foote, R.S. Tetrahedron 1997, 53, 424Z) protecting groups as the 5'-O-carbonate ester of the phosphoramidite building block. The synthesis cycle uses a combinatorial approach attaching one specific base per cycle, thus as many as 100 cycles need to be run to make an array of 25-mers. Time needed for deprotection/activation of the growing oligo chain determines overall manufacturing time and consequently also cost. In this report we demonstrate the development of photoprotected posphoramidite monomers for light directed array synthesis with increasing sensitivity to the UV light used. If combined with maskless array synthesis, this technology allows for synthesis of arrays with >780,000 different 25-mer oligonucleotides in about one hour and allows for high flexibility in array design and reiterative redesign. The arrays synthesized show high quality and reproducibility in our standard hybridization based assay.