Structural parameters governing excited-state properties of self-assembling π-conjugated peptides: a classical and quantum study*

Peptides that self-assemble are of significant interest for use in the fabrication of biocompatible nano-aggregates. Non-natural π-conjugated subunits may be embedded into the peptide backbone, leading resulting β-sheet-like ribbons formed from self-assembly to have electronic and photophysical properties. Alteration of the amino acid composition of the peptides can have a significant impact on the measured absorption spectra of aggregates, but the exact geometric origin of these changes is unknown. In order to probe these composition-induced alterations, we utilize time-dependent density functional theory calculations to study the excited state properties of peptide configurations extracted from molecular dynamics simulations. We identify geometric variables describing π-conjugated core geometries that appear to be determinative of the wavelength at which the absorption spectrum reaches its peak. When applied to MD simulations, the resulting regression model is shown to be in qualitative agreement with experiment, laying the foundation for in silico prediction of absorption properties of peptide aggregates.