DEL synthesis

DOS-DEL-1²⁷ and the triazine library³ were synthesized following the previously reported conditions. The eight cycle-1 building blocks (i.e., scaffolds) used in DOS-DEL-1 were prepared according to the reported methods,^27,50 and the remainder of the building blocks for DOS-DEL-1 and the triazine library were sourced from commercial vendors and used without further purification. The DNA “headpiece” (Figure S2) and DNA cycle tags (Figure S4) were purchased as custom orders from LGC Biosearch Technologies as lyophilized powders with all 5’ ends phosphorylated, and the double-stranded oligonucleotides containing primerbinding sites (Figure S3) were purchased as custom orders from IDT as lyophilized powders with all 5’ ends phosphorylated. For both libraries, the DNA headpiece was additionally extended using Fmoc-15-amino-4,7,10,13-tetraoxapentadecanoic acid, the “AOP linker.”

DOS-DEL-1 was synthesized according to protocols adapted from previously reported conditions²⁷ (modifications detailed below). Following each synthesis step, the material was purified by ethanol precipitation and reverse-phase ISCO (RediSep Rf Gold C18 columns) rather than semi-preparative reverse-phase HPLC. The cycle-4 tag (i.e., library identifier) and closing primer (Figures S5, S6) were installed in a single ligation step rather than two separate steps. Following ligation of the cycle-4 tag and closing primer, the final screening construct was additionally purified using the Model 491 prep cell (Bio-Rad).

The triazine library was synthesized according to protocols adapted from previously reported conditions for “DEL-A”³ (modifications detailed below). The library was constructed using a set of 78 Fmoc-amino acids as the cycle-1 building blocks, 290 amines as the cycle-2 building blocks, and 250 amines as the cycle-3 building blocks; these building blocks differed from those used by Clark et al..³ Following ligation of the cycle-4 tag and closing primer, the final screening construct was purified using the Model 491 prep cell (Bio-Rad).