Abstract
H(2)S is a gasotransmitter with several physiological roles, but its reactivity and short half-life in biological media make its controlled delivery difficult. For biological applications of the gas, hydrogels have the potential to deliver H(2)S with several advantages over other donor systems, including localized delivery, controlled release rates, biodegradation, and variable mechanical properties. In this study, we designed and evaluated peptide-based H(2)S-releasing hydrogels with controllable H(2)S delivery. The hydrogels were prepared from short, self-assembling aromatic peptide amphiphiles (APAs), functionalized on their N-terminus with S-aroylthiooximes (SATOs), which release H(2)S in response to a thiol trigger. The APAs were studied both in solution and in gel forms, with gelation initiated by addition of CaCl(2). Various substituents were included on the SATO component of the APAs in order to evaluate their effects on self-assembled morphology and H(2)S release rate in both the solution and gel phases. Transmission electron microscopy (TEM) images confirmed that all H(2)S-releasing APAs self-assembled into nanofibers above a critical aggregation concentration (CAC) of ∼0.5 mg/mL. Below the CAC, substituents on the SATO group affected H(2)S release rates predictably in line with electronic effects (Hammett σ values) according to a linear free energy relationship. Above the CAC, circular dichroism, infrared, and fluorescence spectroscopies demonstrated that substituents influenced the self-assembled structures by affecting hydrogen bonding (β-sheet formation) and π-π stacking. At these concentrations, electronic control over release rates diminished, both in solution and in the gel form. Rather, the release rate depended primarily on the degree of organization in the β-sheets and the amount of π-π stacking. This supramolecular control over release rate may enable the evaluation of H(2)S-releasing hydrogels with different release rates in biological applications.