Nanoscale dynamics and aging of fibrous peptide-based gels

Abstract

Solutions of the aromatic dipeptide derivative molecule fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF) in dimethyl sulfoxide produce fibrous gels when mixed with water. We study the evolution of density fluctuations of this three-component system using X-ray photon correlation spectroscopy (XPCS) and compare these results to the macroscopic rheology of the gels and optical observations of the microstructure evolution. At the investigated scattering angles, the intensity autocorrelation functions do not follow behavior expected for simple diffusion of individual Fmoc-FF molecules localized within cages of nearest neighbors. Instead, the dynamics are associated with density fluctuations on length scales of ∼10–100 nm arising from disaggregation and reformation of fibers, leading to an increasingly uniform network. This process is correlated with the growth of the elastic modulus, which saturates at long times. Autocorrelation functions and relaxation times acquired from XPCS measurements are consistent with relaxation rates of structures at dynamic equilibrium. This study provides further support to the concept of exploring peptide-based gelators as valence-limited patchy particles capable of forming equilibrium gels.

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

“… 703 − 705 The molecules of 6 form hydrogels by adjusting the pH of the aqueous solution of 6 , 97 , 703 by applying 6 to a silica wafer surface, 706 or by the addition of water to a DMSO solution of 6 . 707 , 708 Despite considerable studies on 6 , 566 , 709 − 715 it was unclear why the mechanical properties reported for the hydrogels of 6 vary significantly, up to 4 orders of magnitude. To address this inconsistency, Adams et al have systematically studied the mechanical properties of hydrogels of 6 prepared using different protocols.…”

Gelation of Fmoc-diphenylalanine is a first order phase transition

“…As the water concentration is decreased, higher Fmoc-FF concentrations are required to see a gel ( Fig. 33 Fmoc-FF is hydrophobic and insoluble in water, but dissolves to very high concentrations in DMSO (volume fractions approaching 0.5), resulting in stable solutions. Furthermore, at fixed Fmoc-FF volume fraction, increasing the amount of water in the system results in increased elasticity.…”

“…6a), with comparisons to literature data for a number of crystallizing systems. 17,33 A second reason the crystals may nucleate more rapidly stems from the quasi-one-dimensional nature of the nuclei. In making these comparisons, we emphasize that we are comparing systems that have the same solubilities at the same average strength of attractions, as indicated in Fig.…”

“…17,32 The fibers are observed to form and fluctuate in local concentration over relatively short periods of time (minutes to hours). 33 Fmoc-FF is hydrophobic and insoluble in water, but dissolves to very high concentrations in DMSO (volume fractions approaching 0.5), resulting in stable solutions. In the presence of water, the peptide backbones form hydrogen bonds necessary to create beta sheets, while the aromatic nature of fluorenyl group and the amino acid side groups allow for p-p stacking interactions, 5 which are highly anisotropic and short-range relative to the size of the Fmoc-FF molecule.…”

Rheo-XPCS

“…A review of this field covering research up to 2011 has described such studies of colloidal gels, block copolymer mesophases, polymer thin films, and other soft materials [1]. More recent examples where XPCS has made connections between rheology and nanoscale dynamics include studies of glassy clay suspensions [2], azopolymer films [3], interfacial nanoparticle gels and glasses [4,5], films of densely branched comb polymers [6], and peptide-based gels [7].…”