Emergence of a New Self-Replicator from a Dynamic Combinatorial Library Requires a Specific Pre-Existing Replicator

Abstract

Our knowledge regarding the early steps in the formation of evolvable life and what constitutes the minimal molecular basis of life remains far from complete. The recent emergence of systems chemistry reinvigorated the investigation of systems of self-replicating molecules to address these questions. Most of these studies focus on single replicators and the effects of replicators on the emergence of other replicators remains under-investigated. Here we show the cross-catalyzed emergence of a novel self-replicator from a dynamic combinatorial library made from a threonine containing peptide building block, which, by itself, only forms trimers and tetramers that do not replicate. Upon seeding of this library with different replicators of different macrocycle size (hexamers and octamers), we observed the emergence of hexamer replicator consisting of six units of the threonine peptide only when it is seeded with an octamer replicator containing eight units of a serine building block. These results reveal for the first time how a new replicator can emerge in a process that relies critically on the assistance by another replicator through cross-catalysis and that replicator composition is history dependent.

Scheme 1. (a) A dynamic Combinatorial Library of Differently Sized Macrocyclic Disulfides Is Formed upon Oxidation of a Threonine Containing Peptide Functionalized Dithiol; (b) Selective Formation of Replicator 1₆ upon Cross-Seeding; (c) Schematic Representation of the Tentative Mechanism through Which Replicator 4₈ Gives Rise to Replicator 1₆

Figure 1

Kinetic profile of a dynamic combinatorial library made from building block 1 (3.8 mM in 50 mM borate buffer, pH 8.1) stirred at 1200 rpm and (a) kept under ambient conditions, (b) 80% oxidized and kept under an inert atmosphere; kinetic profile of a dynamic combinatorial library made from an 80% oxidized solution of building block 1 (3.8 mM in 50 mM borate buffer, pH 8.1) seeded with (c) 4₈ (cross-seeding), (d) 1₆ (self-seeding), stirred at 1200 rpm.

Figure 2

(a) CD spectra and (b) maximum thioflavin T fluorescence emission intensity (at 492 nm) of nonseeded and seeded (10 mol %) libraries made from building block 1 (3.8 mM in 50 mM borate buffer, pH 8.1): i, stirred; ii, inert atm.; iii, nonagitated; iv, seeded with 4₈; v, seeded with 5₈; vi, seeded with 2₆; vii, seeded with 3₆; viii, seeded with 4₆. (c) TEM micrographs of the library corresponding to Figure 1c. Scale bars are 100 nm.

Figure 3

Kinetic profile of a nonagitated dynamic combinatorial library made from building block 4 (3.8 mM in 50 mM borate buffer, pH 8.1) 80% oxidized (a) nonseeded (b) after addition of 10 mol percent of preformed 1₆ as seed. (c) Kinetic profile of a library made by mixing peptide 1 and peptide 4 (3.42 mM in 1 and 0.38 mM in 4) in borate buffer (pH 8.1, 50 mM) to form a 3.8 mM library. The library was then oxidized to 80% with freshly prepared perborate solution (80 mM) and stirred at 1200 rpm under an inert atmosphere.