Synthesis and wettability characteristics of model adhesive protein sequences inspired by a marine mussel

Changing polymer catechol content to generate adhesives for high versus low energy surfaces

Adhesive bonding is commonly used to replace mechanical fasteners in many applications. However, the surface chemistry of different substrates varies, making adhesion to a variety of materials difficult. Many biological adhesives are adept at sticking to multiple surfaces with a range of surface chemistries. Marine mussels utilize a catechol moiety within their adhesive proteins to bring about surface binding as well as cohesive cross-linking. Mimicking this functionality in synthetic polymers has yielded high strength adhesives that can attach to both high and low surface energy materials, although not equally well. Here, the amount of catechol within a copolymer system was varied for potential tailoring to specific surfaces. Structure-function studies revealed differing trends of optimal catechol content for high energy aluminum versus low energy polytetrafluoroethylene (Teflon^TM) surfaces. Adhesion strengths were optimized with ∼10 mol% catechol for aluminum and ∼41 mol% for Teflon^TM. Varying the catechol incorporation also resulted in changes to wettability, failure modes, and mechanics on these substrates. When considering performance of the entire bulk material, the different surfaces required an altered adhesive-cohesive balance. Tailoring the composition of polymeric adhesives for different surfaces may aid future manufacturing in cases where joining a variety of materials is required.

Expanding the DOPA Universe with Genetically Encoded, Mussel-Inspired Bioadhesives for Material Sciences and Medicine

Catechols are a biologically relevant group of aromatic diols that have attracted much attention as mediators of adhesion of "bio-glue" proteins in mussels of the genus Mytilus. These organisms use catechols in the form of the noncanonical amino acid l-3,4-dihydroxyphenylalanine (DOPA) as a building block for adhesion proteins. The DOPA is generated post-translationally from tyrosine. Herein, we review the properties, natural occurrence, and reactivity of catechols in the design of bioinspired materials. We also provide a basic description of the mussel's attachment apparatus, the interplay between its different molecules that play a crucial role in adhesion, and the role of post-translational modifications (PTMs) of these proteins. Our focus is on the microbial production of mussel foot proteins with the aid of orthogonal translation systems (OTSs) and the use of genetic code engineering to solve some fundamental problems in the bioproduction of these bioadhesives and to expand their chemical space. The major limitation of bacterial expression systems is their intrinsic inability to introduce PTMs. OTSs have the potential to overcome these challenges by replacing canonical amino acids with noncanonical ones. In this way, PTM steps are circumvented while the genetically programmed precision of protein sequences is preserved. In addition, OTSs should enable spatiotemporal control over the complex adhesion process, because the catechol function can be masked by suitable chemical protection. Such caged residues can then be noninvasively unmasked by, for example, UV irradiation or thermal treatment. All of these features make OTSs based on genetic code engineering in reprogrammed microbial strains new and promising tools in bioinspired materials science.

The effect of molecular composition and crosslinking on adhesion of a bio-inspired adhesive

Catechol-containing polymers with a crosslinked structure were obtained by free radical polymerization. Optimal adhesion properties were obtained at a catechol composition of 5 mol%.

Surface functionalization by decal-like transfer of thermally cross-linked urushiol thin films

We have demonstrated surface functionalization through the decal-like transfer of thermally cross-linked urushiol thin films onto various substrates. Tensile adhesive strength measurements showed that the film adheres strongly to the surface of various substrates including chemically inert materials, such as polyolefins and thermosetting resins, because of the properties of urushiol. Furthermore, the highly cross-linked structure of urushiol made the films mechanically robust. These two properties allowed the fabrication of practicable thin films for indirect surface modification. Actually, the robust thin film served as a scaffold for an Au thin film, which was then bound to various substrates. Surface-texturing of nanodecal was also demonstrated as an application aspects.

Facile preparation of mussel-inspired polyurethane hydrogel and its rapid curing behavior

A facile method was found to incorporate a mussel-inspired adhesive moiety into synthetic polymers, and mussel mimetic polyurethanes were developed as adhesive hydrogels. In these polymers, a urethane backbone was substituted for the polyamide chain of mussel adhesive proteins, and dopamine was appended to mimic the adhesive moiety of adhesive proteins. A series of mussel mimetic polyurethanes were created through a step-growth polymerization based on hexamethylene diisocyanate as a hard segment, PEG having different molecular weights as a soft segment, and lysine-dopamine as a chain extender. Upon a treatment with Fe(3+), the aqueous mussel mimetic polyurethane solutions can be triggered by pH adjustment to form adhesive hydrogels instantaneously; these materials can be used as injectable adhesive hydrogels. Upon a treatment with NaIO4, the mussel mimetic polyurethane solutions can be cured in a controllable period of time. The successful combination of the unique mussel-inspired adhesive moiety with a tunable polyurethane structure can result in a new kind of mussel-inspired adhesive polymers.

Design strategies and applications of tissue bioadhesives

In the past two decades tissue adhesives and sealants have revolutionized bleeding control and wound healing. This paper focuses on existing tissue adhesive design, their structure, functioning mechanism, and their pros and cons in wound management. It also includes the latest advances in the development of new tissue adhesives as well as the emerging applications in regenerative medicine. We expect that this paper will provide insightful discussion on tissue bioadhesive design and lead to innovations for the development of the next generation of tissue bioadhesives and their related biomedical applications.

Synthesis of peptide-cellulose conjugate mediated by a soluble cellulose derivative having β-Ala esters

A derivatization of cellulose was investigated for gaining up the solubility for subsequent conjugation reaction. N(β)-Boc-β-Ala, was introduced to the parent cellulose using carbonyl diimidazole. Degree of substitution towards the cellulose hydroxyls was 49%. Subsequent removal of Boc in trifluoroacetic acid (TFA) yielded β-Ala-cellulose·TFA salt. A protected hexapeptide, Boc-Ser(Bzl)-Gly-Tyr(Bzl)-Ser(Bzl)-Gly-Lys(Z) was synthesized via 9 steps of peptide elongation, and the C-end carboxyl groups of the peptide was coupled with the β-Ala-cellulose in a homogenous dimethyl sulfoxide solution, using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide monohydrochloride to ensure the N(β)-selective acylation. The degree of substitution of the protected peptide towards the β-amino groups of β-Ala-cellulose (DS%(/NH(2))) was 52% for 1.0 eq.mol of the protected peptide feed, and in the presence of N-hydroxysuccinimide, DS%(/NH(2)) was increased to 61%. At 4.0 eq.mol feed, almost quantitative conjugation was observed as DS%(/NH(2))=98-99%. Deprotection of the conjugate using thioanisole-TFA resulted in complete removal of Boc, Bzl on Tyr, and Z on Lys, while a very trace amount of Bzl on Ser seemed to be left uncleaved.

Mussel-inspired load bearing metal-polymer glues

A mussel-inspired synthetic adhesive based on dopamine containing methacrylate copolymers was developed to bond polymers to metal surfaces at an adhesion strength of up to 20 MPa for bulk samples.

Enhanced adhesion of dopamine methacrylamide elastomers via viscoelasticity tuning

We present a study on the effects of cross-linking on the adhesive properties of bio-inspired 3,4-dihydroxyphenylalanine (DOPA). DOPA has a unique catechol moiety found in adhesive proteins in marine organisms, such as mussels and polychaete, which results in strong adhesion in aquatic conditions. Incorporation of this functional group in synthetic polymers provides the basis for pressure-sensitive adhesives for use in a broad range of environments. A series of cross-linked DOPA-containing polymers were prepared by adding divinyl cross-linking agent ethylene glycol dimethacrylate (EGDMA) to monomer mixtures of dopamine methacrylamide (DMA) and 2-methoxyethyl acrylate (MEA). Samples were prepared using a solvent-free microwave-assisted polymerization reaction and compared to a similar series of cross-linked MEA materials. Cross-linking with EGDMA tunes the viscoelastic properties of the adhesive material and has the advantage of not reacting with the catechol group that is responsible for the excellent adhesive performance of this material. Adhesion strength was measured by uniaxial indentation tests, which indicated that 0.001 mol % of EGDMA-cross-linked copolymer showed the highest work of adhesion in dry conditions, but non-cross-linked DMA was the highest in wet conditions. The results suggest that there is an optimal cross-linking degree that displays the highest adhesion by balancing viscous and elastic behaviors of the polymer but this appears to depend on the conditions. This concentration of cross-linker is well below the theoretical percolation threshold, and we propose that subtle changes in polymer viscoelastic properties can result in significant improvements in adhesion of DOPA-based materials. The properties of lightly cross-linked poly(DMA-co-MEA) were investigated by measurement of the frequency dependence of the storage modulus (G') and loss modulus (G''). The frequency-dependence of G' and magnitude of G'' showed gradual decreases with the fraction of EGDMA. Loosely cross-linked DMA copolymers, containing 0% and 0.001 mol % of EGDMA-cross-linked copolymers, displayed rheological behavior appropriate for pressure-sensitive adhesives characterized by a higher G' at high frequencies and lower G' at low frequencies. Our results indicate that dimethacrylate cross-linking of DMA copolymers can be used to enhance the adhesive properties of this unique material.

Amphiphilic surface active triblock copolymers with mixed hydrophobic and hydrophilic side chains for tuned marine fouling-release properties

Two series of amphiphilic triblock surface active block copolymers (SABCs) were prepared through chemical modification of two polystyrene-block-poly(ethylene-ran-butylene)-block-polyisoprene ABC triblock copolymer precursors. The methyl ether of poly(ethylene glycol) [M(n) approximately 550 g/mol (PEG550)] and a semifluorinated alcohol (CF(3)(CF(2))(9)(CH(2))(10)OH) [F10H10] were attached at different molar ratios to impart both hydrophobic and hydrophilic groups to the isoprene segment. Coatings on glass slides consisting of a thin layer of the amphiphilic SABC deposited on a thicker layer of an ABA polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene thermoplastic elastomer were prepared for biofouling assays with algae. Dynamic water contact angle analysis, X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) measurements were utilized to characterize the surfaces. Clear differences in surface structure were realized as the composition of attached side chains was varied. In biofouling assays, the settlement (attachment) of zoospores of the green alga Ulva was higher for surfaces incorporating a large proportion of the hydrophobic F10H10 side chains, while surfaces with a large proportion of the PEG550 side chains inhibited settlement. The trend in attachment strength of sporelings (young plants) of Ulva did not show such an obvious pattern. However, amphiphilic SABCs incorporating a mixture of PEG550 and F10H10 side chains performed the best. The number of cells of the diatom Navicula attached after exposure to flow decreased as the content of PEG550 to F10H10 side chains increased.

Gecko-inspired surfaces: a path to strong and reversible dry adhesives

The amazing adhesion of gecko pads to almost any kind of surfaces has inspired a very active research direction over the last decade: the investigation of how geckos achieve this feat and how this knowledge can be turned into new strategies to reversibly join surfaces. This article reviews the fabrication approaches used so far for the creation of micro- and nanostructured fibrillar surfaces with adhesive properties. In the light of the pertinent contact mechanics, the adhesive properties are presented and discussed. The decisive design parameters are fiber radius and aspect ratio, tilt angle, hierarchical arrangement and the effect of the backing layer. Also first responsive systems that allow thermal switching between nonadhesive and adhesive states are described. These structures show a high potential of application, providing the remaining issues of robustness, reliability, and large-area manufacture can be solved.

Design of 11-residue peptides with unusual biophysical properties: induced secondary structure in the absence of water

A series of oligopeptides with beta-forming and adhesive properties, were synthesized and analyzed for adhesion shear strength, secondary structure, and association properties. The sequences contained related hydrophobic core segments varying in length from 5 to 12 residues and flanked by di- or tri-lysine segments. Three remarkable peptides consisting of just 11 residues with hydrophobic core sequences of FLIVI, IGSII, and IVIGS flanked by three lysine residues gave the highest dry adhesion shear strength and displayed unusual biophysical properties in the presence and absence of water. KKKFLIVIKKK had its highest adhesion strength at 2% (w/v) at pH 12.0 and showed the highest adhesion strength after exposure to water (water resistance). Both KKKIGSIIKKK and KKKIVIGSKKK, at 4% (w/v) at pH 12.0, displayed nearly identical dry shear strength values to that with the FLIVI core sequence. The peptide with IGSII core, however, displayed a lower water resistance and the latter, IVIGS, showed no water resistance, completely delaminating upon soaking in water. These are the smallest peptides with adhesive properties reported to date and show remarkable adhesion strength even at lower concentrations of 0.2% (w/v), which corresponds to 1.6 mM. The FLIVI containing peptide adopted a beta-sheet secondary structure in water while the IGSII- and IVIGS-containing sequences folded similarly only in the absence of water. Analytical ultracentrifugation studies showed that when the FLIVI sequence adopts beta-structure in aqueous solution, it associates into a large molecular weight assembly. The random coils of IGSII and IVIGS showed no tendency to associate at any pH.

Adhesive strength and curing rate of marine mussel protein extracts on porcine small intestinal submucosa

An adhesive protein extracted from marine mussels (Mytilus edulis) was used to bond strips of connective tissue for the purpose of evaluating the use of curing agents to improve adhesive curing. Specifically, mussel adhesive protein solution (MAPS, 0.5mM dihydroxyphenylalanine) was applied, with or without the curing agents, to the ends of two overlapping strips of porcine small intestinal submucosa (SIS). The bond strength of this lap joint was determined after curing for 1h at room temperature (25 degrees C). The strength of joints formed using only MAPS or with only the ethyl, butyl or octyl cyanoacrylate adhesives were determined. Although joints bonded using ethyl cyanoacrylate were strongest, those using MAPS were stronger than those using butyl and octyl cyanoacrylates. The addition of 25mM solutions of the transition metal ions V5+, Fe3+ and Cr6+, which are all oxidants, increased the bond strength of the MAPS joints. The V5+ gave the strongest bonds and the Fe3+ the second strongest. In subsequent tests with V5+ and Fe3+ solutions, the bond strength increased with V5+ concentration, but it did not increase with Fe3+ concentration. Addition of 250mM V5+ gave a very strong bond.

Langmuir and Langmuir–Blodgett films of proline-rich N-terminal domain peptide of γ-zein

The proline-rich N-Terminal domain peptides of gamma-zein (VHLPPP)(n) with n=1 and 3 (peptides I and II) are shown to form stable Langmuir films at air/water interface and the films have been characterized using surface pressure-molecular area (pi-A), surface potential-molecular area (DeltaV-A) isotherms, respectively. The longer peptide sequence does not show dramatic increase in surface or interfacial properties suggesting that the minimum length of n=1 is sufficient to achieve the necessary surface properties. Brewster angle micrographs also agreed with these results. The high surface-active nature of the peptide suggests a fairly non-polar character at air/water interface and at solid/air interface when coated expresses a high surface energy. Additives such as isopropyl alcohol (IPA) and polyvinyl alcohol (PVA) with the peptides showed more homogenous films at the air/water interface and also improved mechanical and tensile properties. The organized assembly of peptide I at the air/water and solid/air interface suggests that even thin layer of the peptide could play an important role in coating the inner surface of protein body membrane in storage proteins. Composite films of such short peptides with biocompatible polymers may find applications as surface coatings and in biomaterials.

Peptides by Extension at the N- or C-terminii of Lysine

Dipeptides 3a-g, (3a + 3a'), (3d + 3d'), (3l + 3l')a and tripeptides 6a-e, (6b + 6b'), (6e + 6e') incorporating Z(epsilon)-Lys were prepared in high yields (70-95%) and enantiopurity (> or =97%) in partially aqueous acetonitrile solution by coupling using (i) Z(epsilon)-Lys with N-(Z- and Fmoc-aminoacyl)benzotriazoles 1a-g, (ii) Z(epsilon)-Lys with N-Z-dipeptidoylbenzotriazoles 5a-c, and (iii) N-Fmoc(alpha)-Z(epsilon)-l-Lys-Bt 1h and amino acids 2a,c-e. Unnatural dipeptides 3h-j, (3h + 3h') and tripeptides 6f were similarly prepared from Z(alpha)-Lys. Retention of chirality was demonstrated by parallel experiments involving l-Ala, dl-Ala, l-Met, and dl-Met by NMR and HPLC analysis.

Three-Dimensional Arrangement of Sugar Residues along Helical Polypeptide Backbone. 2. Synthesis of Periodic N-Glycosylated Peptides by Polymerization of Tripeptide Active Esters Containing α,α-Disubstituted Amino Acid

New type of N-glycosylated peptides having periodic sequence of -[X-Gln(beta-D-GlcNAc)-Aib]- [X = L-Glu(OMe), L-Lys(Ac), L-Ala; Aib = alpha-aminoisobutyric acid] were synthesized by polymerization of glycosylated tripeptides with an active ester methods using Cl(-+)H(3)N-L-Glu(OMe)-Gln[beta-D-GlcNAc(Ac)(3)]-Aib-ONp (Np=p-nitrophenyl) (13a), Cl(-+)H(3)N-L-Lys(Ac)-Gln[beta-D-GlcNAc(Ac)(3)]-Aib-ONp (13b), and Cl(-+)H(3)N-L-Ala-Gln[beta-D-GlcNAc(Ac)(3)]-Aib-ONp (13c) as the monomers. Polymerizable glycosylated tripeptides were prepared by stepwise N,N-dicyclohexylcarbodiimide (DCC)/1-hydroxybenzotriazole (HOBt) method. Polymerizations of 13a-c were initiated by triethylamine and proceeded in DMSO at 50 degrees C for 5 days in the presence of 1-hydroxy-7-azabenzotriazole (HOAt) as the activator (conversions were 25-75%). The glycopeptides were deacetylated by hydrazine monohydrate in methanol to afford periodic glycopeptides 14 (12-27 residues) without racemization (yield, 35-89%). CD spectra in methanol, trifluoroethanol, and water of deacetylated glycopolymers 14a, 14b, and 14c showed double minima (206 and 222 nm) of negative Cotton effect indicating that N-glycoside (N-acetyl-d-glucosamine) was arranged three-dimensionally along the alpha-helical peptides in water as well as in organic protic solvents. The helix content depends on the solvent, peptide sequence, and spacer between peptide backbone and sugar. Interaction of the glycopeptides with wheat germ agglutinin (WGA) lectin was investigated by fluorescence measurement.

Coil dimensions of the mussel adhesive protein Mefp-1

To obtain a better understanding of factors controlling cross-linking rates of Mussel adhesive proteins, we study the conformation of the Mussel Adhesive Protein Mefp-1. The dimensions of Mefp-1 in solution are determined by dynamic light scattering. Under physiological conditions, the hydrodynamic radius RH of Mefp-1 is found to be 10.5+/-1.1 nm. Measured Mefp-1 dimensions are compared with theoretical dimensions of Mefp-1 in random coil conformations. We have strong indications that Mefp-1, under dilute and physiological conditions, has a self-avoiding random walk conformation with helix-like deca-peptide segments. With a number of segments of approximately 90, the segment length is found to be 2.7 nm.

Enzymatic cross-linking of a phenolic polymer extracted from the marine alga Fucus serratus

We have shown that a phenolic polymer (PP) extracted from Fucus serratus can be cross-linked using a vanadium-dependent bromoperoxidase (BPO). The methanol extracted PP was adsorbed to a quartz crystal sensor and the cross-linking was initiated by the addition of BPO, KBr, and H2O2. The decreased dissipation upon addition of the cross-linking agents, as measured with the quartz crystal microbalance with dissipation monitoring (QCM-D) method, was interpreted as intramolecular cross-links were formed between different phloroglucinol units in the PP. With surface plasmon resonance, it was shown that no desorption occurred from the sensor surface during the cross-linking. UV/vis spectroscopy verified the results achieved with QCM-D that all components, i.e., BPO, KBr, and H2O2, were necessary in order to achieve intramolecular oxidative cross-linking of the polymer.

A glycosylated byssal precursor protein from the green mussel Perna viridis with modified dopa side-chains

Foot tissue of the green mussel Perna viridis contains a variety of byssal precursor proteins with the unusual redox-active amino acid, Dopa (-beta-3,4-dihydroxyphenyl-alpha-alanine). Eight proteins were detectable in acidic extracts of the Perna foot by a redox cycling assay with nitroblue tetrazolium. In one of these, however, P. viridis foot protein-1 (Pvfp-1), activity was not due to Dopa, but to another redox-active derivative. Based on specific colorimetric derivatization with Arnow's reagent, ninhydrin and phenylisothiocyanate (Edman), mass spectrometry, the redox-active derivative in Pvfp-1 is not consistent with any known modification. Another uncommon modification of Pvfp-1 involves O-glycosylation of threonine by mannose, glucose or fucose. As in previously characterized fp-1s, the primary sequence of the Pvfp-1 (apparent mass 89 kDa) has two consensus decapeptide motifs; one is APPKPX1TAX2K and the other is APPPAX1TAX2K, where P is Pro/Hyp, and X1 and X2 are difucosylated threonine and a redox sensitive derivative of tyrosine or Dopa, respectively. Of these two unusual residues, X2 is unique to Pvfp-1, whereas O-glycosylated Thr has been previously detected in freshwater mussel fp-1. The sequence homology of Pvfp-1 with the common structural motifs of the fp-1 protein family strongly suggests that the Pvfp-1 functions as the byssal coating (lacquer) protein.

Quinone Cross-Linked Polysaccharide Hybrid Fiber

The present article describes the synthesis of the N-(Lys-Gly-Tyr-Gly)-chitosan using the water-soluble active ester method, the preparation of the N-(Lys-Gly-Tyr-Gly)-chitosan-gellan hybrid fibers, and the reinforcement of the hybrid fibers by enzymatic cross-linking between the N-grafted peptides chains of chitosan. The cationic polysaccharide chitosan was treated with Boc-Lys(Z)-Gly-Tyr(Bzl)-Gly (4-hydroxyphenyl)dimethylsulfonium methyl sulfate ester in DMF-0.15 M acetic acid to incorporate the peptides into the side chain amino groups of chitosan followed by the acidic removals of the Z and Bzl groups. The degrees of N substitution were estimated to be 2.0 and 10 molar % by changing the molar ratios of the amino groups of the parent chitosan and the active ester. The resulting cationic N-(Lys-Gly-Tyr-Gly)-chitosan was spun into the hybrid fibers with the anionic polysaccharide gellan in water. The tensile strengths of the N-(Lys-Gly-Tyr-Gly)-chitosan hybrid fibers were superior to those of the original chitosan-gellan fibers. The mechanical strengths of the hybrid fibers further increased upon enzymatic oxidation using tyrosinase. Based on these results, we concluded that the covalent cross-linking due to the enzyme oxidation between the grafted peptides significantly contributed to reinforcement of the polysaccharide hybrid fibers. The present results afford a new methodology for the reinforcement achieved by the polymer modification inspired by a biological process.

Lysyl oxidase-catalyzed cross-linking and insolubilization reactions of Lys-containing polypeptides and synthetic adhesive proteins

The lysyl oxidase- (LO-) mediated insolubilization reactions of the Lys-containing polypeptides have been examined using poly(L-Lys) with degrees of polymerization (Dps) ranging 1 from 2300, copoly(LysxAlay) (x:y = 1:4, 1:3; 1:1, 2:1, and 3:1), copoly(LysxGlyy) (x:y = 1:1 and 2:1), and synthetic adhesive proteins with sequential repetitive units enriched in the Lys residues, poly(Ala-Lys-Pro-Ser-Tyr-Pro-Pro-Thr-Tyr-Lys), poly(Ala-Gly-Tyr-Gly-Gly-Ala-Lys), and poly(Gly-Gly-Gly-Tyr-Gly-Gly-Tyr-Gly-Lys). All of the substrates were insolubilized by the LO-catalyzed oxidation of the epsilon-amino group in the Lys residues. The Dps of the polypeptide substrates did not affect the kinetic constants, the Km and Vmax values. The Km and Vmax values and the insolubilization rates varied depending on the Lys contents in the substrate polypeptides, which were enriched in Gly and Ala residues. As the Lys content increased, the Km and Vmax values became lower and higher, respectively. The insolubilization rates decreased with increase of the Lys content. The time-dependent changes in the LO-catalyzed aldehyde production, the insolubilization, and remaining LO activity demonstrated that the cross-linking and the insolubilization steps occurred along with LO deactivation, indicating that the enzymatic and chemical processes in the LO-mediated insolubilization occur in order.