Synthesis of PNA Oligomers

The N-(2-aminoethyl)glycine PNA (aegPNA) monomers were purchased from ASM Research Chemicals. PNA monomer J was synthesized following the reported method (35 (link),47 (link)). Synthesis of PNA oligomers was carried out on 4-methylbenzhydrylamine hydrochloride (MBHA∙HCl) polystyrene based resin. The original loading value 1.5–1.7 mmol/g of this solid support was reduced to 0.35 mmol/g, using acetic anhydride as the capping reagent. (Benzotriazol-1-yloxy)tripyrrolidinophosphonium hexafluorophosphate (PyBop) and N,N-Diisopropylethylamine (DIPEA) were used as the coupling reagent and Boc strategy was followed during oligomer synthesis. After sequential deprotection of t-Boc group and coupling of aeg/modified PNA monomers on solid support, final cleavage of the oligomers were done by using ‘high-low trifluoroacetic acid (TFA)-trifluoromethanesulfonic acid (TFMSA)’ method. Oligomers were then precipitated with diethyl ether, dissolved in water and purified by RP-HPLC method using water-CH₃CN-0.1% TFA as the mobile phase. Sample crystallization matrix α-cyano-4-hydroxycinnamic acid (CHCA) was used in matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) to characterize the oligomers.