The Role of Hydrophobicity in the Stability and pH-Switchability of (RXDX)4 and Coumarin-(RXDX)4 Conjugate β-Sheets.

pH-Switchable, self-assembling materials are of interest in biological imaging and sensing applications. Here we propose that combining the pH-switchability of RXDX (X = Ala, Val, Leu, Ile, Phe) peptides and the optical properties of coumarin creates an ideal candidate for these materials. This suggestion is tested with a thorough set of all-atom molecular dynamics simulations. We first investigate the dependence of pH-switchabiliy on the identity of the hydrophobic residue, X, in the bare (RXDX)4 systems. Increasing the hydrophobicity stabilizes the fiber which, in turn, reduces the pH-switchabilty of the system. This behavior is found to be somewhat transferable to systems in which a single hydrophobic residue is replaced with a coumarin containing amino acid. In this case, conjugates with X = Ala are found to be unstable at both pHs, while conjugates with X = Val, Leu, Ile, and Phe are found to form stable β-sheets at least at neutral pH. The coumarin-(RFDF)4 conjugate is found to have the largest relative entropy value of 0.884 ± 0.001 between neutral and acidic coumarin ordering distributions. Thus, we posit that coumarin-(RFDF)4 containing peptide sequences are ideal candidates for pH-sensing bioelectronic materials.