Abstract
Self-assembling peptides are a promising class of biomaterials with desirable biocompatibility and versatility. In particular, the oligopeptide (RADA)4, consisting of arginine (R), alanine (A), and aspartic acid (D), self-assembles into nanofibers that develop into a three-dimensional hydrogel of up to 99.5% (w/v) water; yet, the organization of water within the hydrogel matrix is poorly understood. Importantly, peptide concentration and polarity are hypothesized to control the internal water structure. Using variable temperature deuterium solid-state nuclear magnetic resonance (2H NMR) spectroscopy, we measured the amount of bound water in (RADA)4-based hydrogels, quantified as the non-frozen water content. To investigate how peptide polarity affects water structure, five lysine (K) moieties were appended to (RADA)4 to generate (RADA)4K5. Hydrogels at 1 and 5% total peptide concentration were prepared from a 75:25 (w/w) blend of (RADA)4:(RADA)4K5 and similarly analyzed by 2H NMR. Interestingly, at 5% peptide concentration, there was lower mobile water content in the lysinated versus the pristine (RADA)4 hydrogel. Regardless of the presence of lysine, the 5% peptide concentration had higher non-frozen water content at temperatures as low as 217 ± 1.0 K, suggesting that bound water increases with peptide concentration. The bound water, though non-frozen, may be strongly bound to the charged lysine moiety to appear as immobilized water. Further understanding of the factors controlling water structure within hydrogels is important for tuning the transport properties of bioactive solutes in the hydrogel matrix when designing for biomedical applications.
Highlights
Self-assembling peptide hydrogels offer a wide range of potential biomedical applications, ranging from drug delivery platforms [1,2], fillers for treating osteoarthritis [3], engineered matrices for neuronal cultures [4,5,6,7], to hemostatic agents [8]
The biocompatible nanofibers are simple to synthesize and form a hydrogel of net neutral charge that can respond to external stimuli at physiological conditions [13,14,15,16]. (RADA)4 has the potential for minimally invasive therapies, protein delivery, and sustaining
Analyzing the water structure in the peptide matrix is fundamental to understanding the internal transport properties of hydrogels for biomedical applications
Summary
Self-assembling peptide hydrogels offer a wide range of potential biomedical applications, ranging from drug delivery platforms [1,2], fillers for treating osteoarthritis [3], engineered matrices for neuronal cultures [4,5,6,7], to hemostatic agents [8]. The limited understanding of the peptide self-assembly process at the molecular level impedes the practical application of hydrogel biomaterials. Of particular significance are the bound water shells that solvate the peptide nanofibers of the hydrogel. These layers of hydration may lead to an increase in the viscosity experienced by diffusing molecules within the hydrogel and affect macroscopic release kinetics, as illustrated by the Stokes–Einstein equation. By controlling the hydrogel’s internal water structure, it may be possible to alter the diffusion rate for releasing absorbed solutes from the hydrogel
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