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pH-Controlled Chiral Packing and Self-Assembly of a Coumarin Tetrapeptide.

McKensie L. Mason,Remy F Lalisse,Tyler J Finnegan,C. Hadad,D. Modarelli,J. Parquette

Published 2019 in Langmuir

ABSTRACT

A coumarin-tetrapeptide conjugate, EFEK(DAC)-NH2 (1), is reported to undergo a pH-dependent interconversion between nanotubes and nanoribbons. An examination of zeta potential measurements, circular dichroism (CD) spectra, and microscopy imaging (TEM and AFM) identified three different self-assembly regimes based on pH: (1) pH 2-5, positively charged, left-handed helical nanotubes; (2) pH 6-8, negatively charged, right-handed helical nanoribbons; and (3) pH 9.0, a monomeric/disassembled peptide. The nanotubes exhibited uniform diameters of 41 ± 5 nm and wall thicknesses of 4.8 ± 0.8 nm; whereas, the nanoribbons existed as either flat or twisted sheets ranging in width from 11-60 nm, with heights of 8 ± 1 nm. The UV-vis and CD spectra of the most common antiparallel, beta-sheet conformation of 1-dimer were simulated at the B3LYP/def2svpd level of theory in implicit water. These studies indicated that transition from nanotubes to nanoribbons was coupled to an M - P helical inversion of the coumarin packing orientation, respectively, within the nanostructures. The assembly process was driven by beta-sheet aggregation and pi-pi interactions leading to the formation of nanoribbons, which progressively wound into helical ribbons and laterally grew into smooth nanotubes as the pH decreased.

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