Sequence and structural effects on molecular conductance of bioinspired peptoid oligomers
The sequence-structure-function paradigm demonstrates that polymer sequence directly impacts the three-dimensional structure, which in turn dictates biological function—principles we can apply to synthetic polymer design. One class of sequence-defined polymer that has made great progress in attaining biology-like control over sequence and structure are peptoids or N-substituted polyglycines. The sequence-defined modularity of peptoids enables precise control over their structure–function relationships, enabling potential materials applications in energy storage, energy conversion, and biomedical technologies. In Chapter 1, I lay the foundations of the motivations behind studying how helical secondary structure affects molecular charge transport in peptoid oligomers. In Chapter 2, characterizations of electron charge transport in peptoid oligomers uses the scanning tunneling microscopy break-junction (STM-BJ) method build upon previous studies on the effect of sequence and molecular conformation and secondary structure in synthetic organic molecules and peptides on molecular charge transport. Despite recent progress, understanding the role of peptoid sequence and conformation in electron transport has been challenging to study. Here, I synthesize a library of peptoid oligomers and characterize their molecular electronic properties using the scanning tunneling microscope-break junction (STM-BJ) technique. The work in this chapter reveals well-defined electron transport pathways for peptoids lacking secondary structure. In particular, peptoid sequences with aromatic side groups lacking hydrogen bonds and methyl substitutions at the N-Cα position led to well-defined conductance features. This behavior fundamentally differs from electron transport in peptides, where secondary structure derived from hydrogen bonds leads to an enhanced conductance feature compared to peptides with non-helical backbones. All-atom molecular dynamics (MD) simulations are used to understand the conformational heterogeneity of peptoids. The molecular conformations obtained from MD simulations are used in quantum mechanical calculations based on the non-equilibrium Green’s function–density functional theory (NEGF-DFT) formalism, and the results show reasonable qualitative agreement with experiments. Overall, these results reveal new insights into structure-function relationships describing electron transport in peptoid-based electronic materials. In Chapter 3, we move away from single-molecule studies and investigate the role of sequence and secondary structure on a self-assembled monolayer (SAM) of peptoids. This study specifically focuses on peptoid self-assembled monolayers (SAMs) as a model system that still preserves molecular-level information (sequence, side-chain identity, expected conformation) while introducing intermolecular organization. This allows us to probe how secondary structure and intermolecular packing together influence electronic functionality. I synthesized and characterized a library of rationally designed peptoid oligomers with and without helical nanostructures. SAMs were fabricated, and their film morphology and current voltage relationship was characterized. Film morphology is characterized using atomic force microscopy (AFM) scratching experiments and the charge transport properties of these monolayers are characterized using the eutectic gallium indium alloy (EGaIn) soft contact method where we measure the current voltage relationship. Chapter 4 outlines future directions including designing in situ probes for monitoring peptoid conformation and studies of charge transport through two-dimensional peptoid nanosheet assemblies.
peptoid, singe-molecule charge transport
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations