Replication of Sequence Information in Synthetic Oligomers

Abstract

Conspectus The holy grail identified by Orgel in his 1995 Account was the development of novel chemical systems that evolve using reactions in which replication and information transfer occur together. There has been some success in the adaption of nucleic acids to make artificial analogues and in templating oligomerization reactions to form synthetic homopolymers, but replication of sequence information in synthetic polymers remains a major unsolved problem. In this Account, we describe our efforts in this direction based on a covalent base-pairing strategy to transfer sequence information between a parent template and a daughter copy. Oligotriazoles, which carry information as a sequence of phenol and benzoic acid side chains, have been prepared from bifunctional monomers equipped with an azide and an alkyne. Formation of esters between phenols and benzoic acids is used as the equivalent of nucleic base pairing to covalently attach monomer building blocks to a template oligomer. Sequential protection of the phenol side chains on the template, ester coupling of the benzoic acid side chains, and deprotection and ester coupling of the phenol side chains allow quantitative selective base-pair formation on a mixed sequence template. Copper catalyzed azide alkyne cycloaddition (CuAAC) is then used to oligomerize the monomers on the template. Finally, cleavage of the ester base pairs in the product duplex by hydrolysis releases the copy strand. This covalent template-directed synthesis strategy has been successfully used to copy the information encoded in a trimer template into a sequence-complementary oligomer in high yield. The use of covalent base pairing provides opportunities to manipulate the nature of the information transferred in the replication process. By using traceless linkers to connect the phenol and benzoic acid units, it is possible to carry out direct replication, reciprocal replication, and mutation. These preliminary results are promising, and methods have been developed to eliminate some of the side reactions that compete with the CuAAC process that zips up the duplex. In situ end-capping of the copy strand was found to be an effective general method for blocking intermolecular reactions between product duplexes. By selecting an appropriate concentration of an external capping agent, it is also possible to intercept macrocyclization of the reactive chain ends in the product duplex. The other side reaction observed is miscoupling of monomer units that are not attached to adjacent sites on the template, and optimization is required to eliminate these reactions. We are still some way from an evolvable synthetic polymer, but the chemical approach to molecular replication outlined here has some promise.