Selection of aptamers from nucleic acid libraries by in vitro evolution represents a powerful method of identifying high-affinity ligands for a broad range of molecular targets. Nevertheless, a sizeable fraction of proteins remain difficult targets due to inherently limited chemical diversity of nucleic acids. We have exploited synthetic nucleotide modifications that confer protein-like diversity on a nucleic acid scaffold, resulting in a new generation of binding reagents called SOMAmers (Slow Off-rate Modified Aptamers). Here we report a unique crystal structure of a SOMAmer bound to its target, platelet-derived growth factor B (PDGF-BB). The SOMAmer folds into a compact structure and exhibits a hydrophobic binding surface that mimics the interface between PDGF-BB and its receptor, contrasting sharply with mainly polar interactions seen in traditional proteinbinding aptamers. The modified nucleotides circumvent the intrinsic diversity constraints of natural nucleic acids, thereby greatly expanding the structural vocabulary of nucleic acid ligands and considerably broadening the range of accessible protein targets.ince the advent of SELEX (Systematic Evolution of Ligands by EXponential enrichment) 22 years ago (1, 2), aptamers have been described that bind specifically and with high affinity to many different types of targets, including proteins, peptides, and small molecules (3). Binding interactions between aptamers and their targets are characterized by shape complementarity, polar contacts, hydrogen bonding interactions, and chargecharge interactions (3-5). Other than base stacking interactions, hydrophobic contacts, which are known to make key contributions to protein-protein interactions (6-8), have been notably limited, reflecting the lack of such moieties in nucleic acid libraries typically used in SELEX.We have recently shown that augmenting the diversity of randomized libraries with functional groups absent in natural nucleic acids can dramatically improve the success rate of SELEX, especially against difficult protein targets (9, 10). We have named this unique class of binding reagents SOMAmers (Slow Off-rate Modified Aptamers), to account for their distinct composition and binding properties. Among the different types of modifications we have tested, functional groups with hydrophobic character have typically yielded SOMAmers with the highest binding affinity. Although the contribution of such functional groups to the outcome of SELEX experiments has been quite apparent (9), the structural basis for the effect of these "side chains" on folding and binding has been unclear. Here, we report two cocrystal structures of related SOMAmers bound to a protein target, platelet-derived growth factor B (PDGF-BB), solved at a resolution of 2.2 Å and 2.3 Å. The structures elucidate the striking impact of the hydrophobic aromatic functional groups in creating novel intramolecular motifs and their extensive participation in shaping the contact surface with the native protein. By combining nucleic acid secondary st...

S4Can J Surg/J can chir 2020;63(2 Suppl 1) next period of time, in their future projections. Recovery planning for resumption of surgical services after the pandemic should start now and be communicated on a go-forward basis as the health system stabilizes; this planning should include an impact analysis of the projected long-term health care costs and human resource needs caused by delayed cancer treatment.