Ostwald's rule of stages governs structural transitions and morphology of dipeptide supramolecular polymers

The self-assembly of molecular building blocks into nano- and micro-scale supramolecular architectures has opened up new frontiers in polymer science. Such supramolecular species not only possess a rich set of dynamic features as a consequence of the non-covalent nature of their core interactions, but also afford unique structural characteristics. Although much is now known about the manner in which such structures adopt their morphologies and size distributions in response to external stimuli, the kinetic and thermodynamic driving forces that lead to their transformation from soluble monomeric species into ordered supramolecular entities have remained elusive. Here we focus on Boc-diphenylalanine, an archetypical example of a peptide with a high propensity towards supramolecular self-organization, and describe the pathway through which it forms a range of nano-assemblies with different structural characteristics. Our results reveal that the nucleation process is multi-step in nature and proceeds by Ostwald's step rule through which coalescence of soluble monomers leads to the formation of nanospheres, which then undergo ripening and structural conversions to form the final supramolecular assemblies. We characterize the structures and thermodynamics of the different phases involved in this process and reveal the intricate nature of the transitions that can occur between discrete structural states of this class of supramolecular polymers.