Nucleic acid aptamers have the ability to bind selectively to proteins or other cellular targets, due to their specific 3-dimensional folding patterns and their potential in forming intermolecular interactions with the target. Slow off-rate modified aptamers (SOMAmers) have been developed which contain nucleotides with amino acid side chains which utilise the important hydrophobic interactions between amino acids and protein targets. This acts to increase binding affinity to the target, whilst maintaining the conformational freedom of a DNA strand. This project will investigate the use of a range of hydrophobic amino acid based modifications at the 5-position of uridine using chymotrypsin as a target. The objective is to produce optimised aptamers with selective and high affinity binding to chymotrypsin.