Understanding electron transport in biological systems critically relies on characterizing structure-function relationships in peptides and proteins. In this work, we study charge transport in short peptide sequences using a combination of experiments and computational modeling, which serves as a first step in understanding electron transport in complex protein assemblies. Here, we use single-molecule charge transport experiments, molecular dynamics (MD) simulations, and non-equilibrium Green’s function-density functional theory (NEGF-DFT) calculations to understand the role of primary amino acid sequence and secondary structure on charge transport in peptides. Our results show an unexpected bimodal distribution of molecular conductance for oligopeptides with different primary sequences. Gaussian mixture modeling (GMM) is used to show that this two-state molecular conductance behavior arises due to the conformational flexibility of the peptide backbone. The low-conductance state arises due to an extended peptide structure, whereas the high-conductance state arises due to the secondary structure (beta turn or 3₁₀ helix) of the peptide. MD simulations further characterize this conformation-induced charge transport behavior in flexible peptide backbones. Principal components analysis (PCA) is used to identify peptide conformations with the largest amount of structural variation in MD simulations. Molecular conformations from MD simulations are used in NEGF-DFT calculations, which provide good agreement with experiments in revealing conformation dependent changes in molecular charge transport. Overall, this work opens new avenues to understand charge transport in long peptide and protein sequences which will aid in the development of next-generation bioelectronic materials.

Figure: Understanding the role of secondary structure on charge transport in peptides using single molecule techniques

Nicholas Jackson

University of Illinois Urbana-Champaign

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