Site-specific, covalent incorporation of Tus, a DNA-binding protein, on ionic-complementary self-assembling peptide hydrogels using transpeptidase Sortase A as a conjugation tool†Dedicated to the memory of Joachim H. G. Steinke.‡Electronic supplementary information (ESI) available: Further experimental data. See DOI: 10.1039/c3sm00131hClick here for additional data file

The site-specific conjugation of DNA-binding protein (Tus) to self-assembling peptide FEFEFKFKK was demonstrated. Rheology studies and TEM of the corresponding hydrogels (including PNIPAAm-containing systems) showed no significant variation in properties and hydrogel morphology compared to FEFEFKFKK. Critically, we demonstrate that Tus is accessible within the gel network displaying DNA-binding properties.

Thermal aggregation of recombinant protective antigen: aggregate morphology and growth rate

The thermal aggregation of the biopharmaceutical protein recombinant protective antigen (rPA) has been explored, and the associated kinetics and thermodynamic parameters have been extracted using optical and environmental scanning electron microscopies (ESEMs) and ultraviolet light scattering spectroscopy (UV-LSS). Visual observations and turbidity measurements provided an overall picture of the aggregation process, suggesting a two-step mechanism. Microscopy was used to examine the structure of aggregates, revealing an open morphology formed by the clustering of the microscopic aggregate particles. UV-LSS was used and developed to elucidate the growth rate of these particles, which formed in the first stage of the aggregation process. Their growth rate is observed to be high initially, before falling to converge on a final size that correlates with the ESEM data. The results suggest that the particle growth rate is limited by rPA monomer concentration, and by obtaining data over a range of incubation temperatures, an approach was developed to model the aggregation kinetics and extract the rate constants and the temperature dependence of aggregation. In doing so, we quantified the susceptibility of rPA aggregation under different temperature and environmental conditions and moreover demonstrated a novel use of UV spectrometry to monitor the particle aggregation quantitatively, in situ, in a nondestructive and time-resolved manner.

Effect of peptide and guest charge on the structural, mechanical and release properties of β-sheet forming peptides

The effect of peptide charge on the self-assembly, gelation behavior, and model drug release profiles has been explored here for three octa-peptides, VEVKVEVK (VEK2), VKVKVEVK (VEK3), and VEVEVKVE (VEK1), that carry a net charge of 0, +2, and -2 at neutral pH, respectively. Transparent, self-supporting hydrogels were found to form above a critical concentration when the peptide charge modulus was >1 and this was independent of the sign of the charge. TEM, SAXS, and shear rheology revealed that there were no differences in hydrogel structure or mechanical properties when the peptides were at the same concentration and carried the same charge modulus. All peptides were found to form dense fibrillar networks formed by β-sheet rich single fibers where lateral aggregation of the fibers occurred and increased with decreasing charge modulus. Such behavior was found to correlate with an increase in hydrogel mechanical properties, demonstrating that fiber lateral aggregation is inextricably linked with the mechanical properties of these hydrogels. Two hydrophilic model drug molecules, namely napthol yellow (NY) and martius yellow (MY), were subsequently incorporated within the VEK1 and VEK3 hydrogels at pH 7 and although they did not effect the self-assembly of the peptide at a molecular level, they did effect the level of lateral fiber aggregation observed and, therefore, the mechanical properties of the hydrogels. The release of each molecule from the hydrogels was monitored over time and shown to be controlled by Fickian diffusion where the diffusion rate, D, was dependent on the ratio between the overall effective charges carried by the peptide, i.e., the fibrillar network, and the overall charges carried by the guest molecules, but independent from the hydrogel concentration and mechanical properties within the ranges investigated. This work highlights the possibility of controlling the rate of release of small drug molecules by manipulating the charges on the guest molecules as well as the charged state of the self-assembling peptide.

Enzymatic catalyzed synthesis and triggered gelation of ionic peptides

We investigate the possibility of using the protease thermolysin to drive the synthesis and gelation of ionic-complementary peptides from nongelling precursors. In this system, short peptide fragments are continuously interconverted to form a dynamic peptide library, which eventually favors synthesis of peptides that are thermodynamically stabilized by molecular self-assembly. Thermolysin was added at a fixed concentration (0.3 mg mL(-1)) to solutions (0-300 mg mL(-1)) of the short tetrapeptide FEFK. Initially, the protease partially hydrolyzed the tetrapeptide into dipeptides in all samples. Subsequently, longer peptide sequences were found to form through reverse-hydrolysis. The stability of the different sequences was found to be dependent on their self-assembling properties. The sequences that self-assembled into antiparallel beta-sheet rich fibers became the stable products for the reverse hydrolysis reaction, while the others formed were unstable and disappeared with increasing incubation time. Ultimately, the main product of the system was octapeptide, which suggests that it represents the thermodynamically favored product of this dynamic library. Its concentration dictated the gelation behavior of the sample, and gels with moduli up to 25 kPa where obtained depending on the initial concentration of tetrapeptide.

Thermo-reversible protein fibrillar hydrogels as cell scaffolds

Hen egg white lysozyme has been exposed to various physical and chemical denaturing environments and the physical properties of the resulting gels have been examined and their potential for use as tissue engineering scaffolds has been explored. Transparent, self-supporting fibrillar hydrogels were obtained when lysozyme was heated at low pH, while opaque, particulate gels were obtained at high pH. No increase in viscosity was observed for lysozyme at pH 7 unless the native state was disrupted by reducing the disulfide bridges. This was achieved by adding 20 mM of the reductant dithiothreitol (DTT). Under these conditions the macroscopic critical gelation concentration, C(gel), was found to be approximately 3.0 mM and mechanical spectra obtained as a function of temperature revealed that the gelling and melting temperatures increased with increasing lysozyme concentration. The mechanical strength of the hydrogel measured as the plateau elastic modulus shows a scaling behavior of G(e) approximately c2.43 for concentrations > or = C(gel), which is in good agreement with the theoretical prediction for densely cross-linked semi-flexible networks. Infrared spectroscopy showed that an alpha-helix to beta-sheet molecular transition occurred during heating resulting in beta-sheet rich fibrils forming through the self-assembly of beta-sheet rich denaturated proteins. Cryo-transmission electron microscopy shows these fibres (6 nm in diameter) exist as single entities at low concentration, and at C(gel) associate to form the junctions of a well defined regular network. Our preliminary cell culture experiments show the gel matrix promotes cell spreading, attachment and proliferation; indicating our lysozyme hydrogels are cytocompatible and they provide a viable support for the cells.

Thermoreversible lysozyme hydrogels: properties and an insight into the gelation pathway

The gelation behaviour of aqueous solutions of hen egg white lysozyme (HEWL) in the presence of 20 mM DTT in the concentration range 0.7 to 4.0 mM has been investigated using microDSC, FTIR, cryoTEM, SANS and oscillatory rheology. The macroscopic critical gelation concentration, Cgel, was found to be ∼ 3.0 mM. The disruption of the disulfide bonds by the DTT and the destabilisation of the protein were found to be a prerequisite for the formation of β-sheet rich fibrils under the mild conditions used in this work. Using our methodology the hydrogels obtained have a pH of 7, hence are suitable for cell culture, and are also thermoreversible. The hydrogel melting temperature was found to increase with increasing concentration and a similar structure was observed across the concentration range investigated. Our results suggest these systems are composed of a well defined regular network where single β-sheet rich fibrils (∼ 3 nm diameter) form initially, then two of these fibrils associate two-by-two to form junctions (∼ 6 nm diameter) and then on cooling further aggregate to form larger bundles of fibres. The network mesh size was found to decrease with increasing concentration. Our results suggest that below Cgel small unconnected gel-like aggregates exist that have a similar structure to the hydrogels obtained above Cgel. Using our data we propose a model for the denaturation and gelation behaviour of our system. During the first heating an α-helix to β-sheet molecular transition for the protein conformation occurs resulting in β-sheet rich fibrils forming through the self-assembly of β-sheet rich denaturated proteins. At high temperature the solution contains β-sheet rich fibrils with dissolved protein. On cooling an increase in the amount of β-sheet was observed via FTIR suggesting that as the temperature is decreased more and more protein forms β-sheet rich fibrils. At the gelation temperature these fibrils associate two-by-two to form the network junctions resulting in the macroscopic gelation of the sample. Our results suggest the network junctions are formed via specific hydrophobic interactions. The hydrogels elastic modulus was found to scale as C2.45 for C > Cgel.

Carbon-centered radicals in isolated soy proteins

Solid-state electron paramagnetic resonance (EPR) spectroscopy of commercial samples of isolated soy proteins (ISP) revealed a symmetrical free-radical signal typical of carbon-centered radicals (g= 2.005) ranging from 2.96 x 10(14) to 6.42 x 10(14) spins/g. The level of free radicals in ISP was 14 times greater than similar radicals in sodium caseinate, 29 times greater than egg albumin, and about 100 times greater levels than casein. Nine soy protein powdered drink mixes contained similar types of free radicals up to 4.10 x 10(15) spins/g of drink mix, or up to 6.4 times greater than the highest free-radical content found in commercial ISP. ISP samples prepared in the laboratory contained trapped radicals similar to the levels in commercial ISP samples. When ISP was hydrated in 2.3 mM sodium erythorbate or 8.3 mM L-cysteine, frozen and dried, the level of trapped free radicals increased by about 17- and 19-fold, respectively. The ESR spectrum of defatted soybean flakes contained overlapping signals from the primary free-radical peak (g= 2.005) and a sextet pattern typical of manganese-II. The manganese signal was reduced in the laboratory ISP and very weak in the commercial ISP.

Irreversible Collapse of Poly(vinyl stearate) Monolayers at the Air-Water Interface

The collapse of Langmuir monolayers of poly(vinyl stearate) (PVS) at the air-water interface has been investigated by combined measurements of the surface pressure-area isotherms and Brewster angle microscopy (BAM). Atomic force microscopy (AFM) has been used to gain out-of-plane structural information on collapsed films transferred onto a solid substrate by a modified version of the inverse Langmuir-Schaefer deposition method. At high areas per monomer repeat unit, BAM imaging revealed that the films are heterogeneous, with large solidlike domains (25-200 mum in diameter) coexisting with liquidlike domains. Upon film compression, the domains coalesced to form a homogeneous monolayer before the film collapsed at constant pressure, forming irreversible three-dimensional (3D) structures. BAM images showed that two 3D structures coexisted: buckles of varying width extending across the surface and perpendicular to the direction of the compression and dotted islandlike structures. Upon expansion, the film fractured and both 3D protrusions persisted, explaining the marked hysteresis recorded in the Langmuir isotherms. Experiments with AFM confirmed the 3D nature of both protrusions and revealed that many buckles contain substructures corresponding to narrow buckles whose heights are a multiple of a single bilayer. Additionally, many multilayer islands with diameters spanning from 0.2 mum to over 3.5 mum were characterized by varying heights between 2 nm and up to over 50 nm. The key to the formation of the irreversible 3D structures is the presence of large inhomogeneities in the PVS monolayer, and a generalized phenomenological model is proposed to explain the collapse observed.

Thermoreversible Protein Hydrogel as Cell Scaffold

A thermoreversible fibrillar hydrogel has been formed from an aqueous lysozyme solution in the presence of dithiothreitol (DTT). Its physical properties and potential as a tissue engineering scaffold have been explored. Hydrogels were prepared by dissolving 3 mM protein in a 20 mM DTT/water mixture, heating to 85 degrees C and cooling at room temperature. No gel was observed for the equivalent sample without DTT. The elastic nature of the gel formed was confirmed by rheology, and the storage modulus of our gel was found to be of the same order of magnitude as for other cross-linked biopolymers. Micro differential scanning calorimetry (microDSC) experiments confirmed that the hydrogel was thermally reversible and that gelation and melting occurs through a solid-liquid-like first-order transition. Infrared spectroscopy of the hydrogel and transmission electron microscopy studies of very dilute samples revealed the presence of beta-sheet-rich fibrils that were approximately 4-6 nm in diameter and 1 mum in length. These fibrils are thought to self-assemble along their long axes to form larger fibers that become physically entangled to form the three-dimensional network observed in both cryo-scanning electron microscopy (cryo-SEM) and small-angle neutron scattering (SANS) studies. The hydrogel was subsequently cultured with 3T3 fibroblasts and cells spread extensively after 7 days and stretched actin filaments formed that were roughly parallel to each other, indicating the development of organized actin filaments in the form of stress fibers in cells.