Dopa (3,4-dihydroxyphenylalanine) is recognized as a key chemical signature of mussel adhesion and has been adopted into diverse synthetic polymer systems. Dopa’s notorious susceptibility to oxidation, however, poses significant challenges to the practical translation of mussel adhesion. Using a Surface Forces Apparatus to investigate the adhesion of Mfp3 (mussel foot protein 3) slow, a hydrophobic protein variant of the Mfp3 family in the plaque, we have discovered a subtle molecular strategy correlated with hydrophobicity that appears to compensate for Dopa instability. At pH 3, where Dopa is stable, Mfp3 slow like Mfp3 fast adhesion to mica is directly proportional to the mol% of Dopa present in the protein. At pH 5.5 and 7.5, however, loss of adhesion in Mfp3 slow was less than half that occurring in Mfp3 fast, purportedly because Dopa in Mfp3 slow is less prone to oxidation. Indeed, cyclic voltammetry showed that the oxidation potential of Dopa in Mfp3 slow is significantly higher than in Mfp3 fast at pH 7.5. A much greater difference between the two variants was revealed in the interaction energy of two symmetric Mfp3 slow films (Ead = −3 mJ/m2). This energy corresponds to the energy of protein cohesion which is notable for its reversibility and pH-independence. Exploitation of aromatic hydrophobic sequences to protect Dopa against oxidation as well as to mediate hydrophobic and H-bonding interactions between proteins provides new insights for developing effective artificial underwater adhesives.
Mussels use a variety of 3, 4-dihydroxyphenyl-l-alanine (DOPA) rich proteins specifically tailored to adhering to wet surfaces. Synthetic polypeptide analogues of adhesive mussel foot proteins (specifically mfp-3) are used to study the role of DOPA in adhesion. The mussel-inspired peptide is a random copolymer of DOPA and N5 -(2-hydroxyethyl)-l-glutamine synthesized with DOPA concentrations of 0–27 mol% and molecular weights of 5.9–7.1 kDa. Thin films (3–5 nm thick) of the mussel-inspired peptide are used in the surface forces apparatus (SFA) to measure the force–distance profiles and adhesion and cohesion energies of the films in an acetate buffer. The adhesion energies of the mussel-inspired peptide films to mica and TiO2 surfaces increase with DOPA concentration. The adhesion energy to mica is 0.09 μJ m−2 molDOPA−1 and does not depend on contact time or load. The adhesion energy to TiO2 is 0.29 μJ m−2 molDOPA−1 for short contact times and increases to 0.51 μJ m−2 molDOPA−1 for contact times >60 min in a way suggestive of a phase transition within the film. Oxidation of DOPA to the quinone form, either by addition of periodate or by increasing the pH, increases the thickness and reduces the cohesion of the films. Adding thiol containing polymers between the oxidized films recovers some of the cohesion strength. Comparison of the mussel-inspired peptide films to previous studies on mfp-3 thin films show that the strong adhesion and cohesion in mfp-3 films can be attributed to DOPA groups favorably oriented within or at the interface of these films.
3, 4-Dihydroxyphenylanine (Dopa)-containing proteins are key to wet adhesion in mussels and possibly other sessile organisms also. However, Dopa-mediated adhesive bonding is a hard act to follow in that, at least in mussels, bonding depends on Dopa in both reduced and oxidized forms, for adhesion and cohesion, respectively. Given the vulnerability of Dopa to spontaneous oxidation, the most significant challenge to using it in practical adhesion is controlling Dopa redox in a temporally- and spatially defined manner. Mussels appear to achieve such control in their byssal attachment plaques, and factors involved in redox control can be measured with precision using redox probes such as the diphenylpicryl hydrazyl (DPPH) free radical. Understanding the specifics of natural redox control may provide fundamentally important insights for adhesive polymer engineering and antifouling strategies.
Mytilus; byssus; 3, 4-dihydroxyphenylalanine; anti-oxidant; wet adhesion
The holdfast or byssus of Asian green mussels, Perna viridis, contains a foot protein, pvfp-1, that differs in two respects from all other known adhesive mussel foot proteins (mfp): (1) instead of the hallmark L-3,4-dihydroxyphenylalanine (DOPA) residues in mfp-1, for example, pvfp-1 contains C2-mannosyl-7-hydroxytryptophan (Man7OHTrp). (2) In addition, pvfp-1 chains are not monomeric like mfp-1 but trimerized by collagen and coiled-coil domains near the carboxy terminus after a typical domain of tandemly repeated decapeptides. Here, the contribution of these peculiarities to adhesion was examined using a surface forces apparatus (SFA). Unlike previously studied mfp-1s, pvfp-1 showed significant adhesion to mica and, in symmetric pvfp-1 films, substantial cohesive interactions were present at pH 5.5. The role of Man7OHTrp in adhesion is not clear, and a DOPA-like role for Man7OHTrp in metal complexation (e.g., Cu2+, Fe3+) was not observed. Instead, cation–π interactions with low desolvation penalty between Man7OHTrp and lysyl side chains and conformational changes (raveling and unraveling of collagen helix and coiled-coil domains) are the best explanations for the strong adhesion between pvfp-1 monomolecular films. The strong adhesion mechanism induced by cation–π interactions and conformational changes in pvfp-1 provides new insights for the development of biomimetic underwater adhesives.
A silver-releasing antibacterial hydrogel was developed that simultaneously allowed for silver nanoparticle formation and gel curing. Water-soluble polyethylene glycol (PEG) polymers were synthesized that contain reactive catechol moieties, inspired by mussel adhesive proteins, where the catechol containing amino acid 3,4-dihydroxyphenylalanine (DOPA) plays an important role in the ability of the mussel to adhere to almost any surface in an aqueous environment. We utilized silver nitrate to oxidize polymer catechols, leading to covalent cross-linking and hydrogel formation with simultaneous reduction of Ag(I). Silver release was sustained for periods of at least two weeks in PBS solution. Hydrogels were found to inhibit bacterial growth, consistent with the well-known antibacterial properties of silver, while not significantly affecting mammalian cell viability. In addition, thin hydrogel films were found to resist bacterial and mammalian cell attachment, consistent with the antifouling properties of PEG. We believe these materials have a strong potential for antibacterial biomaterial coatings and tissue adhesives, due to the material-independent adhesive properties of catechols.
Marine mussels utilize a variety of DOPA-rich proteins for purposes of underwater adhesion, as well as for creating hard and flexible surface coatings for their tough and stretchy byssal fibers. In the present study, moderately strong, yet reversible wet adhesion between the protective mussel coating protein, mcfp-1, and amorphous titania was measured with a surface force apparatus (SFA). In parallel, resonance Raman spectroscopy was employed to identify the presence of bidentate DOPA–Ti coordination bonds at the TiO2–protein interface, suggesting that catechol–TiO2 complexation contributes to the observed reversible wet adhesion. These results have important implications for the design of protective coatings on TiO2.
Mussels have a remarkable ability to attach their holdfast, or byssus, opportunistically to a variety of substrata that are wet, saline, corroded, and/or fouled by biofilms. Mytilus edulis foot protein-5 (Mefp-5) is one of several proteins in the byssal adhesive plaque of the mussel M. edulis. The high content of 3,4 dihydroxyphenylalanine (Dopa) (~30 mol%) and its localization near the plaque-substrate interface have often prompted speculation that Mefp-5 plays a key role in adhesion. Using the surface forces apparatus, we show that on mica surfaces Mefp-5 achieves an adhesion energy approaching Ead = ~− 14 mJ/m2. This exceeds the adhesion energy of another interfacial protein, Mefp-3, by a factor of 4–5 and is greater than the adhesion between highly oriented monolayers of biotin and streptavidin. The adhesion to mica is notable for its dependence on Dopa, which is most stable under reducing conditions and acidic pH. Mefp-5 also exhibits strong protein-protein interactions with itself as well as with Mefp-3 from M. edulis.
Mussel foot proteins (mfps) have been investigated as a source of inspiration for the design of underwater coatings and adhesives. Recent analysis of various mfps by a surface forces apparatus (SFA) revealed that mfp-1 functions as a coating, whereas mfp-3 and mfp-5 resemble adhesive primers on mica surfaces. To further refine and elaborate the surface properties of mfps, the force–distance profiles of the interactions between thin mfp (i.e. mfp-1, mfp-3 or mfp-5) films and four different surface chemistries, namely mica, silicon dioxide, polymethylmethacrylate and polystyrene, were measured by an SFA. The results indicate that the adhesion was exquisitely dependent on the mfp tested, the substrate surface chemistry and the contact time. Such studies are essential for understanding the adhesive versatility of mfps and related/similar adhesion proteins, and for translating this versatility into a new generation of coatings and (including in vivo) adhesive materials.
mussel foot proteins; coatings and adhesives; molecular interactions; surface forces; bioadhesion
Marine and freshwater mussels are notorious foulers of natural and manmade surfaces, secreting specialized protein adhesives for rapid and durable attachment to wet substrates. Given the strong and water-resistant nature of mussel adhesive proteins, significant potential exists for mimicking their adhesive characteristics in bioinspired synthetic polymer materials. An important component of these proteins is L-3,4-dihydroxylphenylalanine (DOPA), an amino acid believed to contribute to mussel glue solidification through oxidation and crosslinking reactions. Synthetic polymers containing DOPA residues have previously been shown to crosslink into hydrogels upon the introduction of oxidizing reagents. Here we introduce a strategy for stimuli responsive gel formation of mussel adhesive protein mimetic polymers. Lipid vesicles with a bilayer melting transition of 37 °C were designed from a mixture of dipalmitoyl and dimyristoyl phosphatidylcholines and exploited for the release of a sequestered oxidizing reagent upon heating from ambient to physiologic temperature. Colorimetric studies indicated that sodium-periodate-loaded liposomes released their cargo at the phase transition temperature, and when used in conjunction with a DOPA-functionalized poly(ethylene glycol) polymer gave rise to rapid solidification of a crosslinked polymer hydrogel. The tissue adhesive properties of this biomimetic system were determined by in situ thermal gelation of liposome/polymer hydrogel between two porcine dermal tissue surfaces. Bond strength measurements showed that the bond formed by the adhesive hydrogel (mean = 35.1 kPa, SD = 12.5 kPa, n = 11) was several times stronger than a fibrin glue control tested under the same conditions. The results suggest a possible use of this biomimetic strategy for repair of soft tissues.
Hierarchical biological materials such as bone, sea shells, and marine bioadhesives are providing inspiration for the assembly of synthetic molecules into complex structures. The adhesive system of marine mussels has been the focus of much attention in recent years. Several catechol-containing polymers are being developed to mimic the cross-linking of proteins containing 3,4-dihydroxyphenylalanine (DOPA) used by shellfish for sticking to rocks. Many of these biomimetic polymer systems have been shown to form surface coatings or hydrogels, however bulk adhesion is demonstrated less often. Developing adhesives requires addressing design issues including finding a good balance between cohesive and adhesive bonding interactions. Despite the growing number of mussel mimicking polymers, there has been little effort to generate structure-property relations and gain insights on what chemical traits give rise to the best glues. In this report, we examined the simplest of these biomimetic polymers, poly[(3,4-dihydroxystyrene)-co-styrene]. Pendant catechol groups (i.e., 3,4-dihydroxystyrene) were distributed throughout a polystyrene backbone. Several polymer derivatives were prepared, each with a different 3,4-dihyroxystyrene content. Bulk adhesion testing showed where the optimal middle ground of cohesive and adhesive bonding resides. Adhesive performance was benchmarked against commercial glues as well as the genuine material produced by live mussels. In the best case, bonding was similar to cyanoacrylate “Krazy” or “Super” glue. Performance was also examined using low (e.g., plastics) and high (e.g., metals, wood) energy surfaces. Adhesive bonding of poly[(3,4-dihydroxystyrene)-co-styrene] may be the strongest of reported mussel protein mimics. These insights should help us to design future biomimetic systems, thereby bringing us closer to development of bone cements, dental composites, and surgical glues.
We report a facile approach to the synthesis of acetonide and Fmoc protected 3,4-dihydroxyphenylalanine (DOPA), Fmoc-DOPA(acetonide)-OH. By protecting the amino group of DOPA with a phthaloyl group and the carboxyl group as a methyl ester, acetonide protection of the catechol of DOPA derivative was realized in the presence of p-toluenesulfonic acid. Following removal of protecting groups, the intermediate was converted to Fmoc-DOPA(acetonide)-OH, which was successfully incorporated into a short DOPA-containing peptide, derived from marine tubeworm cement proteins Pc1 and Pc2.
DOPA; Fmoc-DOPA(acetonide)-OH; Acetonide
The extensible byssal threads of marine mussels are shielded from abrasion in wave-swept habitats by an outer cuticle that is largely proteinaceous and approximately fivefold harder than the thread core. Threads from several species exhibit granular cuticles containing a protein that is rich in the catecholic amino acid 3,4-dihydroxyphenylalanine (dopa) as well as inorganic ions, notably Fe3+. Granular cuticles exhibit a remarkable combination of high hardness and high extensibility. We explored byssus cuticle chemistry by means of in situ resonance Raman spectroscopy and demonstrated that the cuticle is a polymeric scaffold stabilized by catecholato-iron chelate complexes having an unusual clustered distribution. Consistent with byssal cuticle chemistry and mechanics, we present a model in which dense cross-linking in the granules provides hardness, whereas the less cross-linked matrix provides extensibility.
The macroscale properties of polymer-matrix composites depend immensely on the quality of the interaction between the reinforcement phase and the bulk polymer. This work presents a method to improve the interfacial adhesion between metal-oxides and a polymer matrix by performing surface-initiated polymerization (SIP) by way of a biomimetic initiator. The initiator was modeled after 3,4-dihydroxy-L-phenylalanine (dopa), an amino acid that is highly concentrated in mussel foot adhesive proteins. Mechanical pull out tests of NiTi and Ti-6Al-4V wires from poly (methyl methacrylate) (PMMA) were performed to directly test the interfacial adhesion. These tests demonstrated improvements in maximum interfacial shear stress of 116% for SIP-modified NiTi wires and 60% for SIP-modified Ti-6Al-4V wires over unmodified specimens. Polymer chain growth from the metal oxides was validated using x-ray photoemission spectroscopy (XPS), ellipsometry, scanning electron microscopy (SEM), and contact angle analysis.
A. Polymer-matrix composites (PMCs); B. Fiber/matrix bond; B. Interfacial strength; B. Surface treatments; B. Interphase
The adhesive proteins secreted by marine mussels form a natural glue that cures rapidly to form strong and durable bonds in aqueous environments. These mussel adhesive proteins contain an unusual amino acid, 3,4-dihydroxy-L-phenylalanine (DOPA), which is largely responsible for their cohesive and adhesive strengths. In this study, we incorporated DOPA into diblock and triblock polymers and developed a membrane contact experiment to assess the adhesive interactions of these materials with TiO2 and tissue surfaces. In a typical experiment a micrometer-thick DOPA-functionalized elastomeric membrane is attached to the end of a cylindrical glass tube. Application of a positive pressure to the tube brings the membrane into contact with the surface of interest. The negative pressure needed to separate the membrane from the substrate is a measure of the strength of the adhesive interaction. The test confirms previous results obtained with TiO2 substrates. Because the membrane geometry is well suited for rough or chemically heterogeneous surfaces, it is ideal for studies of tissue adhesion. DOPA was found to give strong adhesion to tissue surfaces, with the strongest adhesion obtained when the DOPA groups were oxidized while in contact with the tissue surface.
Block Copolymers; DOPA; Marine Mussel Mimetics; Membrane; Tissue Adhesion
Marine mussels anchor to a variety of surfaces by secreting liquid proteins that harden and form water-resistant bonds to a variety of surfaces. Studies have revealed that these mussel adhesive proteins contain an unusual amino acid, 3,4-dihydroxy-L-phenylalanine (DOPA), which is believed to be responsible for the cohesive and adhesive properties of these proteins. To separate the cohesive and adhesive roles of DOPA, we incorporated DOPA into the midblock of poly(methyl methacrylate)–poly(methacrylic acid)–poly(methyl methacrylate) (PMMA–PMAA–PMMA) triblock copolymers. Self-assembled hydrogels were obtained by exposing triblock copolymer solutions in dimethyl sulfoxide to water vapor. As water diffused into the solution, the hydrophobic end blocks formed aggregates that were bridged by the water-soluble midblocks. Strong hydrogels were formed with polymer weight fractions between 0.01 and 0.4 and with shear moduli between 1 and 5 kPa. The adhesive properties of the hydrogels on TiO2 surfaces were investigated by indentation with a flat-ended cylindrical punch. At pH values of 6 and 7.4, the fully protonated DOPA groups were highly adhesive to the TiO2 surfaces, giving values of ≈2 J/m2 for the interfacial fracture energy, which we believe corresponds to the cohesive fracture energy of the hydrogel. At these pH values, the DOPA groups are hydrophobic and have a tendency to aggregate, so contact times of 10 or 20 min are required for these high values of the interfacial strength to be observed. At a pH of 10, the DOPA groups were hydrophilic and highly swellable, but less adhesive gels were formed. Oxidation of DOPA groups, a process that is greatly accelerated at a pH of 10, decreased the adhesive performance of the hydrogels even further.
Mussels attach to solid surfaces in the sea. Their adhesion must be rapid, strong, and tough, or else they will be dislodged and dashed to pieces by the next incoming wave. Given the dearth of synthetic adhesives for wet polar surfaces, much effort has been directed to characterizing and mimicking essential features of the adhesive chemistry practiced by mussels. Studies of these organisms have uncovered important adaptive strategies that help to circumvent the high dielectric and solvation properties of water that typically frustrate adhesion. In a chemical vein, the adhesive proteins of mussels are heavily decorated with Dopa, a catecholic functionality. Various synthetic polymers have been functionalized with catechols to provide diverse adhesive, sealant, coating, and anchoring properties, particularly for critical biomedical applications.
adhesion energy; byssus; Dopa; mussel foot proteins; wet adhesion
The byssus of marine mussels has attracted attention as a paradigm of strong and versatile underwater adhesion. As the first of the 3,4-dihydroxyphenylalanine (Dopa)-containing byssal precursors to be purified, Mytilus edulis foot protein-1 (mefp-1) has been much investigated with respect to its molecular structure, physical properties, and adsorption to surfaces. Although mefp-1 undoubtedly contributes to the durability of byssus, it is not directly involved in adhesion. Rather, it provides a robust coating that is 4-5 times stiffer and harder than the byssal collagens that it covers. Protective coatings for compliant tissues and materials are highly appealing to technology, notwithstanding the conventional wisdom that coating extensibility can be increased only at the expense of hardness and stiffness. The byssal cuticle is the only known coating in which high compliance and hardness co-exist without mutual detriment; thus, the role of mefp-1 in accommodating both parameters deserves further study.
adhesion; byssus; coating; cuticle; hardness; mussel foot protein-1; Mytilus
Two-component hydrogels formed with star polyethylene glycol amine and linear dextran aldehyde polymers (PEG:dextran) show promise as tissue-specific surgical sealants. There is however a significant loss of adhesion strength to soft tissues following PEG:dextran swelling, which may limit material ability to appose disjoined tissues and prevent leakage from surgical sites. We covalently incorporated the modified amino acid l-3,4-dihydroxyphenylalanine (l-DOPA) into PEG:dextran to enhance post-swelling sealant performance. l-DOPA is an essential component of marine animal adhesive plaques and has been used to confer wet adhesion in synthetic materials. As both PEG:dextran cohesion and adhesion are mediated by aldehyde-amine interactions, l-DOPA side-groups make it a potent network modulator with potential to affect multiple material properties. Following one hour submersion in aqueous media, PEG:dextran doped with 3 mM l-DOPA/M aldehyde on average swelled 50.3% less, had 287.4% greater stiffness and had 53.6% greater functional adhesion strength compared to the neat hydrogel. Increased concentrations of l-DOPA up to 11 mM l-DOPA/M aldehyde similarly curtailed swelling and mitigated property loss with hydration, but sacrificed initial functional adhesion strength, material modulus and biocompatibility. Taken together, these data support tailored l-DOPA conjugation as a promising approach to enhance the clinical performance of PEG:dextran sealants.
Aldehyde; Biocompatibility; Hydrogel; l-3,4-dihydroxyphenylalanine (l-DOPA); Tissue adhesive
Aqueous biocompatible tribosystems are desirable for a variety of tissue-contacting medical devices. L-3,4-dihydroxyphenylalanine (DOPA) and lysine (K) peptide mimics of mussel adhesive proteins strongly interact with surfaces and may be useful for surface attachment of lubricating polymers in tribosystems. Here, we describe a significant improvement in lubrication properties of poly(dimethylsiloxane) (PDMS) surfaces when modified with PEG-DOPA-K. Surfaces were characterized by optical and atomic force microscopy, contact angle, PM-IRRAS, and X-ray photoelectron spectroscopy. Such surfaces, tested over the course of 200 rotations (~8m in length), maintained an extremely low friction coefficient (μ) (0.03±0.00) compared to bare PDMS (0.98±0.02). These results indicate the potential applications of PEG-DOPA-K for the modification of device surfaces. Extremely low μ values were maintained over relatively long length scales and a range of sliding speeds without the need for substrate pre-activation and in the absence of excess polymer in aqueous solution. These results were only obtained when DOPA was bound to lysine (modification with PEG-DOPA did not have an effect on μ) suggesting the critical role of lysine in obtaining a lowered friction coefficient.
aqueous lubrication; surface modification; DOPA; lysine; tribology
In nature, mussel adhesive proteins (MAPs) show remarkable adhesive properties, biocompatibility, and biodegradability. Thus, they have been considered promising adhesive biomaterials for various biomedical and industrial applications. However, limited production of natural MAPs has hampered their practical applications. Recombinant production in bacterial cells could be one alternative to obtain useable amounts of MAPs, although additional post-translational modification of tyrosine residues into 3,4-dihydroxyphenyl-alanine (Dopa) and Dopaquinone is required. The superior properties of MAPs are mainly attributed to the introduction of quinone-derived intermolecular cross-links. To solve this problem, we utilized a co-expression strategy of recombinant MAP and tyrosinase in Escherichia coli to successfully modify tyrosine residues in vivo.
A recombinant hybrid MAP, fp-151, was used as a target for in vivo modification, and a dual vector system of pET and pACYC-Duet provided co-expression of fp-151 and tyrosinase. As a result, fp-151 was over-expressed and mainly obtained from the soluble fraction in the co-expression system. Without tyrosinase co-expression, fp-151 was over-expressed in an insoluble form in inclusion bodies. The modification of tyrosine residues in the soluble-expressed fp-151 was clearly observed from nitroblue tetrazolium staining and liquid-chromatography-mass/mass spectrometry analyses. The purified, in vivo modified, fp-151 from the co-expression system showed approximately 4-fold higher bulk-scale adhesive strength compared to in vitro tyrosinase-treated fp-151.
Here, we reported a co-expression system to obtain in vivo modified MAP; additional in vitro tyrosinase modification was not needed to obtain adhesive properties and the in vivo modified MAP showed superior adhesive strength compared to in vitro modified protein. It is expected that this co-expression strategy will accelerate the use of functional MAPs in practical applications and can be successfully applied to prepare other Dopa/Dopaquinone-based biomaterials.
Mussel adhesive protein; Dopa; Dopaquinone; In vivo modification; Tyrosinase; Co-expression; Escherichia coli
Surgical repair of a discontinuity in traumatized or degenerated soft tissues is traditionally accomplished using sutures. A current trend is to reinforce this primary repair with surgical grafts, meshes, or patches secured with perforating mechanical devices (i.e., sutures, staples, or tacks). These fixation methods frequently lead to chronic pain and mesh detachment. We developed a series of biodegradable adhesive polymers that are synthetic mimics of mussel adhesive proteins (MAPs), composed of 3,4-dihydroxyphenylalanine (DOPA)-derivatives, polyethylene glycol (PEG), and polycaprolactone (PCL). These polymers can be cast into films, and their mechanical properties, extent of swelling, and degradation rate can be tailored through the composition of the polymers as well as blending with additives. When coated onto a biologic mesh used for hernia repair, these adhesive constructs demonstrated adhesive strengths significantly higher than fibrin glue. With further development, a pre-coated bioadhesive mesh may represent a new surgical option for soft tissue repair.
biomimetic material; tissue adhesive; surgical mesh; multiblock copolymer; DOPA
Biotechnological approaches to practical production of biological protein-based adhesives have had limited success over the last several decades. Broader efforts to produce recombinant adhesive proteins may have been limited by early disappointments. More recent synthetic polymer approaches have successfully replicated some aspects of natural underwater adhesives. For example, synthetic polymers, inspired by mussels, containing the catecholic functional group of 3,4-L-dihydroxyphenylalanine adhere strongly to wet metal oxide surfaces. Synthetic complex coacervates inspired by the Sandcastle worm are water-borne adhesives that can be delivered underwater without dispersing. Synthetic approaches offer several advantages, including versatile chemistries and scalable production. In the future, more sophisticated mimetic adhesives may combine synthetic copolymers with recombinant or agriculture-derived proteins to better replicate the structural and functional organization of natural adhesives.
Chemical cross-linking is an attractive approach to map peptide-protein and protein-protein complexes. Previously, we explored 3,4-dihydroxylphenylalanine (DOPA) as a protein cross-linking agent upon periodate oxidation.(Burdine, L.; Gillette, T. G.; Lin, H-J.; Kodadek, T. J. Am. Chem. Soc. 2004, 126, 11442–11443) We report here a study on the chemistry of DOPA-protein cross-linking. First, using a peptide nucleic acid (PNA) templated system, we identified the α-amino, ε-amino of Lys, imidazole of His, and thiol of Cys as functional groups capable of attacking DOPA ortho-quinone. Second, we demonstrated that periodate-induced DOPA-protein cross-linking could be carried out efficiently at neutral pH in the presence of excess aliphatic 1,2-diols such as ethylene glycol, lactose, and ATP. This result indicated that DOPA-protein cross-linking and 1,2-diol oxidative cleavage proceed via different mechanisms and, that carbohydrates will not interfere with this process when carried out in crude cell extracts or on intact cells.
The sandcastle worm Phragmatopoma californica, a marine polychaete, constructs a tube-like shelter by cementing together sand grains using a glue secreted from the building organ in its thorax. The glue is a mixture of post-translationally modified proteins, notably the cement proteins Pc-1 and Pc-2 with the amino acid, 3,4-dihydroxyphenyl-L-alanine (DOPA). Significant amounts of a halogenated derivative of DOPA were isolated from the worm cement following partial acid hydrolysis and capture of catecholic amino acids by phenylboronate affinity chromatography. Analysis by tandem mass spectrometry and 1H NMR indicates the DOPA derivative to be 2-chloro-4, 5-dihydroxyphenyl-L-alanine. The potential roles of 2-chloro-DOPA in chemical defense and underwater adhesion are considered.
2-chloro-4,5-dihydroxyphenylalanine; Adhesive protein; Cement; Phragmatopoma californica; Sandcastle worm
The cuticle of mussel byssal threads is a robust natural coating that combines high extensibility with high stiffness and hardness. In this study, fluorescence microscopy and elemental analysis were exploited to show that the 3,4-dihydroxyphenyl-l-alanine (dopa) residues of mussel foot protein-1 colocalize with Fe and Ca distributions in the cuticle of Mytilus galloprovincialis mussel byssal threads. Chelated removal of Fe and Ca from the cuticle of intact threads resulted in a 50% reduction in cuticle hardness, and thin sections subjected to the same treatment showed a disruption of cuticle integrity. Dopa-metal complexes may provide significant interactions for the integrity of composite cuticles deformed under tension.