© 2002 by The Society for Integrative and Comparative Biology
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Adhesion à la Moule1
1 Marine Science Institute, Department Molecular, Cell & Developmental Biology, University of California, Santa Barbara, California 93106
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 SYNOPSIS INTRODUCTIONADHESIVE STRENGTH OF MUSSEL...PROTEIN DIVERSITY IN ADHESIVE...PROTEIN ADSORPTION AS A...CONCLUSIONSReferences |
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Mussels owe their sessile way of life in the turbulent intertidal zone to adaptive adjustments in the process and biochemistry of permanent attachment. These have understandably attracted scientific interest given that the attachment is rapid, versatile, tough and not subverted by the presence of water. The adhesive pads of mussel byssus contain at least six different proteins all of which possess the peculiar amino acid 3, 4-dihydroxyphenylalanine (DOPA) at concentrations ranging from 0.1 to 30 mol %. Studies of protein distribution in the plaque indicate that proteins with the highest levels of DOPA, such as mefp-3 (20 mol %) and mefp-5 (30 mol %), appear to predominate at or near the interface between the plaque and substratum. Although the presence of DOPA in proteins has traditionally been associated with cross-linking via chelate-mediated or covalent coupling, recent experiments with natural and synthetic DOPA-containing polypeptides suggest that cross-link formation is not the only fate for DOPA. Intact DOPA, particularly near the interface, may be essential for good chemisorption to polar surfaces. Uniformly high DOPA oxidation to cross-links leads to interfacial failure but high cohesive strength, while low DOPA oxidation results in better adhesion at the expense of cohesion. Defining the adaptations involved in balancing these two extremes is crucial to understanding marine adhesion.
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INTRODUCTION |
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SYNOPSISINTRODUCTIONADHESIVE STRENGTH OF MUSSEL...PROTEIN DIVERSITY IN ADHESIVE...PROTEIN ADSORPTION AS A...CONCLUSIONSReferences |
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There is an age of innocence in the pursuit of every scientific discipline. After almost 2,500 years, the science of bioadhesion in most respects has yet to emerge from it. With very few exceptions, most examples of bioadhesion have yet to be reduced to a simple set of principles in chemical and mechanical engineering. Mussel attachment was the subject of one of the earliest recorded observations of bioadhesion. Aristotle (transl. 1910)
noted that the holdfast in the fan mussel (Pinna) consisted of a robust bundle of fibers with sticky tips. The term byssus (Greek ß
O
for flax linen) was accidentally coined by him for the holdfast (van der Feen, 1949) and has since gained universal acceptance. As a model system for bioadhesion, byssal attachment has always had popular appeal. Mussels are common and their attachment is easy to watch; formation of byssus is rapid, robust, and relatively simple in that proteins are secreted into the ventral groove of the foot and molded there prior to release as a new byssal thread. Further details on byssus formation mostly in the blue mussel, Mytilus edulis, can be found in numerous reviews on the subject (Brown, 1952; Price, 1983; Waite, 1983, 1992).
Byssal appeal also has a technological cachet in that mussels attach their byssus to virtually any solid surface. Such versatility intrigues engineers. Add to this the fact that adhesive bonding is occurring in a turbulent saline environment, and it assumes much more than a passing interest. It is appropriate perhaps to point out that water poses some serious challenges to adhesive bonding, and many manufacturing processes go to great lengths to exclude water from surfaces to be adhesively bonded (Cayless, 1991). The chemistry of industrial adhesive bonding is generally of two types: highly energetic (covalent or chelate) or a collection of weaker noncovalent interactions. While the former can be exploited in specific cases where much is known about the chemistry of the adhesive and adherend, most types of adhesion rely on extensive noncovalent interactions across the interface of the adhesive and adherend. These interactions include charge-charge, hydrogen bond, dipole-dipole, induced dipole-dipole, and nonpolar couplings, among others. The latter three are among those often grouped together as van der Waals forces. Expressions for the interactive energies of these interactions have been formulated for all but the H-bonds and reflect that all depend inversely on the dielectric constant (or sometimes the square of the dielectric) of the medium in which they occur (Israelachvili, 1985). Given that the dielectric constant of air is about 1 at 25°C, in water at the same temperature it is about 80. In other words, other factors being equal, the best interfacial interactions that an adhesive might muster in water with an adherend, would be only 1/80 what they are in air. Water, however, presents an additional challenge. The previous scenario presupposes that adhesive molecules can actually make contact with the adherend surface. In those instances where even this is not possible, water becomes a "weak boundary layer" (Schonhorn, 1981). It has no monopoly as a spoiler of adhesion: dirt, oil, grease, slime and oxides pose similar hazards. All amount to a predicament similar to adhesive bonding underwater. When there is no direct contact between the adhesive and adherend, bond performance is at the mercy of the cohesive strength within the boundary layer itself. Thus even a residual boundary layer can easily become the weakest "link in the chain" (Holubka et al., 1984). Given that they attach to hard surfaces underwater, mussels appear to be able to remove weak boundary layers including water from surfaces and, once attached, their adhesion is not subverted by the dielectric constant of the medium all around them. This is remarkable for reasons cited if their adhesion relies mostly on noncovalent interactions with the surface. If their adhesion also relies on more energetic interfacial interactions, it is even more remarkable given the range of materials they attach to. Indeed, the latter would suggest that they are able to recognize and respond to different kinds of surfaces.
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ADHESIVE STRENGTH OF MUSSEL BYSSUS |
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SYNOPSISINTRODUCTIONADHESIVE STRENGTH OF MUSSEL...PROTEIN DIVERSITY IN ADHESIVE...PROTEIN ADSORPTION AS A...CONCLUSIONSReferences |
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There are reports of mussel tenacity, of byssal tensile strength, and of adhesive tensile strength. Some caution is required to keep these concepts separate. Tenacity denotes the force required to dislodge an attached mussel (Bell and Gosline, 1997
). It is an interesting biological capability that necessarily focuses on the whole animal but does not generally make adjustments for the varying numbers of threads present in each animal. The tensile strength of a byssal thread pertains to the length of thread located between the stem (animal end) and the attachment plaque or pad (substratum end). The strength of the entire byssus is expected to be proportional to the number of threads times the average strength of each thread (Bell and Gosline, 1996). Finally, the adhesive tensile strength is deduced from forces applied directly to byssal plaques (Crisp et al., 1985; Young and Crisp, 1981). Clearly, the second two strengths are subsets of mussel tenacity and can in fact define tenacity when they become the weak link of the system. Force to break or ultimate tensile strength measured in newtons/m2 (N/m2) or pascals (Pa) is increasingly criticized as an inferior measure of adhesion. The energy to break is proposed to be a better measure because it denotes the actual work of adhesion in joules/m3. This can be determined, for example, by calculating the area under the stress-strain curve of a sample loaded to failure. The notion of adhesive strength persists in many biological situations because measuring extension to break is not always feasible in field tests (see Flammang, 2002).
The most comprehensive investigation of byssal plaque adhesion was by Crisp and his colleagues at Menai Bridge during the 1980s (Crisp et al., 1985; Young and Crisp, 1981). Not only did it establish that plaque adhesion varied seasonally, but also that it was defined by the critical surface energy of the substratum (Table 1). Thus, weakest adhesion (about 0.15 x 105 Pa) as measured by the force or load to failure was observed on nonpolar surfaces such as paraffin and polytetrafluoroethylene (PTFE), whereas stronger adhesion occurred on polar surfaces e.g., glass and slate (nearly 1 MPa). Given the range of loads to failure, the relative magnitudes are more important than any absolute magnitude, which on glass, for example, exhibited maxima as high as 100 times the mean load (Young and Crisp, 1981). Significantly, Crisp et al. (1985) were also the first to record the mode of failure in byssal plaques. This was not a comprehensive failure analysis since chemical analysis of the fractured surfaces was not performed. Notwithstanding this, failure on nonpolar surfaces involved primarily "peeling" (interfacial mode) whereas on polar surfaces plaque tearing (cohesive failure) was common. Thus, it might reasonably be concluded from this that mussel fouling could be reduced by the use of materials with low critical surface energies. Similar conclusions were drawn from other studies (e.g., Ohkawa et al., 1999) based on avoidance behavior of mussels on nonpolar surfaces. In fact these observations have done little to remedy marine fouling, because when low energy surfaces are the only ones available, mussels can compensate for the lower strength per plaque by making more threads. Nonetheless when given a choice of substrates, the avoidance results suggest that mussels may be able to detect different surfaces with their foot and choose more promising polar substrata. Clearly water as a weak boundary layer and the dielectric constant of aqueous media do not prevent byssal adhesion on any surface tested, but it remains to be shown how these are specifically mitigated.
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Bond strengths of synthetic industrial and specialty adhesives often far exceed those listed in Table 1, e.g., 100 MPa for epoxies in carbon fiber composites (Kinloch, 1987
). With a level "playing field," however, in which all adhesives are applied to wet surfaces and cured underwater, the values in Table 1 remain impressive. A recent technical bulletin from the US Department of Defense (1996) soliciting proposals on underwater adhesive technology described its goal as the development of adhesives with modest tensile strengths ranging from 0.2 MPa to 0.7 MPa following a 1 hr set time. This is well within the range of mussel adhesive strength and many other marine adhesives. |
PROTEIN DIVERSITY IN ADHESIVE PLAQUES |
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The search for molecules responsible for byssal adhesion began with Brown's (1952)
landmark paper on Mytilus byssus. Here, she first alluded to the peculiar occurrence of 3, 4-dihydroxyphenylalanine- (DOPA) containing proteins in byssus. It took nearly 30 years for further progress. Possibly all the precursor proteins in the plaques are DOPA containing. The six most abundant known plaque proteins (Table 2) have DOPA contents ranging from 0.1 mol % in preCol-D (a byssal collagen) to 30 mol % in mefp-5. Despite the common presence of DOPA, the primary sequences of plaque proteins are quite different. All exhibit repeated sequence motifs that can be long or short. Mefp-2 has eleven repeats of the longest motif, an epidermal growth factor (EGF) domain about 45 residues in length (Inoue et al., 1995). In mefp-1 there are about eighty repeats of a decapeptide sequence in which the prolines (3 in 10) are modified to trans-4-hydroxyproline and trans-2, 3-cis-3, 4-dihydroxyproline (Laursen, 1992; Taylor et al., 1994). PreCol-D has 175 Gly-X-Y repeats in its collagen domain and six polyalanine clusters in the flanking silk-like domains (Qin et al., 1997). The shortest recurrent sequences, Arg/Arg-Asn-Tyr and Tyr-Lys occur in mefp-3 and -5, respectively (Papov et al., 1995; Waite and Qin, 2001). Most of the Arg in mefp-3 is modified to 4-hydroxyarginine (Papov et al., 1995).
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The molecular diversity of the adhesive plaques complicates assignation of adhesion to any particular protein(s). A common method for localizing molecules in vivo is immunohistochemistry. This is based on the well-known fact that the high affinity and specificity of antibody-antigen recognition can be used to reveal the distribution of an antigen such as a protein in tissue thin sections. The interpretation of immunohisto-chemically stained plaque sections has been difficult. There are at least two reasons for this: 1) Some of the proteins (mefp-1, preCol-D, mefp-3) are very poor antigens (this is apparent from many failed attempts to raise specific polyclonal antisera), and 2) the chemical maturation of byssus may involve significant removal or shielding of precursor epitopes. Perhaps the first problem will be technically rectified by the use of nanoparticulate diamond powder such as that reported for mefp-1 (Kossovsky et al., 1995
). The second remains a serious limitation that was particularly well illustrated by Anderson and Waite (2000). Specific high titer polyclonal antibodies were raised to recombinant dpfp-1, a protein precursor of Dreissena polymorpha byssal threads. Using confocal microscopy, the antibodies were shown to bind strongly to nascent thread material in the foot groove. However, no binding was observed in mature threads or proteins extracted from mature threads. Clearly other technologies must be developed to identify the functional distribution of proteins in plaques.
Four years ago, we began exploring the potential of using matrix-assisted laser desorption/ionization mass spectrometry with time-of-flight (MALDI-TOF) to this end. When a natural consortium of proteins is pickled using a matrix such as sinapinic acid, then dried and subjected to high vacuum laser irradiation, proteins in the neighborhood of laser-excited matrix are desorbed and ionized (Floriolli et al., 2000). Given that the penetration of the UV laser does not go deeper than 200–300 nm into porous organic membranes (Strupat et al., 1994), MALDI MS may be able to provide useful information about proteins near the plaque/substratum interface. With an infrared laser this depth could be increased ten-fold. Accordingly, plaque prints soaked in matrix and dried were subjected to MALDI so that the desorbed proteins could be detected according to their mass/charge ratio (Floriolli et al., 2000). Mefp-3 and -5 were desorbed from the plaque prints of Mytilus edulis and M. galloprovincialis byssus (Fig. 1; Floriolli et al., 2000; Warner and Waite, 1999). While such data may seem to confirm that fp-3 and fp-5 are present at or near the interface of the plaque and adherend, there are limits to the interpretation given the peculiarities of MALDI TOF. For example, other proteins may be present but fail to desorb due to cross-linking, Proteins may not ionize easily, or their interaction with matrix may be suboptimal. Numerous factors contribute to protein desorption and ionization. Suffice it say that mefp-3 is detected in plaque prints from all tested surfaces. Given that twenty or more mefp-3 variants may be present in the foot of an individual mussel (Warner and Waite, 1999), we were intrigued whether the deposition of certain variants was especially tailored to particular surfaces such as glass, polyethylene, acrylic and steel. Although each mussel has a different preference for the suite of mefp-3 variants used in attachment, they apparently do not deposit the variants in a surface specific manner (Floriolli et al., 2000). It is still possible, however, that posttranslational modification of mefp-3 variants is adjusted in a surface-dependent manner. This is under active investigation. Mefp-5, in contrast, is only abundant in prints from stainless steel (Fig. 1). Taken at face value these data suggest that mussels may be able to tailor adhesive chemistry to surface type.
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PROTEIN ADSORPTION AS A MEASURE OF ADHESION |
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Given that a mussel's ability to attach to a variety of surfaces underwater begs imitation, an inspired biomimetic line of research would require knowing which proteins and modifications contribute directly to adhesion and how. This depends on a straightforward and relevant assay for measuring adhesion of proteins. There are many methods for testing the strength of adhesive bonds. The following conditions are necessary for meaningful testing of byssal adhesive proteins: 1) Materials to be joined should be bonded and loaded in water or seawater as appropriate, 2) The method should be able to test nanomolar amounts of protein, and 3) the method should offer high sensitivity and fine control of force and strain rate. There are only three techniques that satisfy all conditions: atomic force microscopy (AFM), surface force balance, and optical tweezers. To date only AFM has been specifically applied to mussel adhesive proteins, but not in "force" mode, which remains an important goal. Instead, the proteins have been evaluated as thin films using the scanning, tapping or indentation mode of the AFM (Baty et al., 1997
; Hansen et al., 1998).
Probably because of its commercial availability (Cell-TakTM from Becton-Dickinson Bioscience, Bedford, Mass; MAP from Biopolymer AB, Sweden), most studies of mefp adsorption and film formation have employed mefp-1. Although mefp-1 appears to function as a sealant or cuticle in the byssus (Miki et al., 1996; Rzepecki et al., 1992), its basic isoelectric point, flexible conformation (Deacon et al., 1998), and high DOPA content (15–20 mol %) are features shared with mefp-3 and -5, two proteins implicated in plaque adhesion (Fig. 2). Mefp-1 adsorption has been investigated by quartz crystal microbalance (QCM), surface plasmon resonance (SPR), ellipsometry, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and surface enhanced Raman spectroscopy (SERS). SERS suggests that mefp-1 peptides chemisorb to gold and that DOPA functionalities are coordinated in edgewise fashion with the phenoxy groups toward the gold surface (Ooka and Garrell, 2000). This interesting observation coincides with conclusions in an earlier study using a germanium surface (Olivieri et al., 1989), but requires more detailed structural analyses to be conclusive. Edgewise adsorption is consistent with the thin-layer electrochemical studies of polyphenols chemisorbed to platinum (Soriaga and Hubbard, 1983) and various spectroscopic analyses of pyrocatechol chemisorbed to titanium oxide (Rodriguez et al., 1996), iron and manganese oxides (McBride, 1986; McBride and Wesselink, 1988) and aluminum oxides (Kummert and Stumm, 1980; McBride and Wesselink, 1988). Stability constants of organic chelates on mineral or metal surfaces are thought to approximate levels measured by solution chemistry (Kummert and Stumm, 1980). Solution studies of iron (III)-binding by DOPA groups in mefp-1 suggest stability constants in excess of 1040 at 0.1 M NaCl, pH 7.5 and 25°C (Taylor et al., 1994b, 1996). Similar affinities are predicted for pyrocatechol binding of Al (III), Si (IV), and Ti (IV) with somewhat lower values for transition metals (see Table 2 in Waite, 1990b). If the DOPA functionalities of mefp-1 are indeed mobilized to chelate metals/metal oxides during chemisorption to steel, for instance, it follows that adsorbed mefp-1 should retard corrosion. This has indeed been demonstrated (Hansen et al., 1995).
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Mefp-1 adsorption to nonpolar surfaces has been investigated using methyl-terminated alkylthiolated gold and poly-(octadecyl methacrylate); adsorption is patchy and disorganized (Baty et al., 1997
; Höök et al., 2001). Thin layers could be delaminated by low shear using AFM in tapping mode, and drying led to extensive patch perturbation (Baty et al., 1997). Protein adsorbed to polystyrene and pyrolyzed graphite surfaces, however, showed increased durability and resistance to displacement, which was attributed to the increased opportunity for noncovalent 
- and -cation interactions (Baty et al., 1996; Hansen et al., 1998). QCM analysis in parallel with SPR has great diagnostic value because, while SPR detects the amount of protein present in a film, QCM is also sensitive to the amount of solvent trapped or entrained in that film (Höök et al., 2001). SPR and QCM measurement of mefp-1 adsorption to a nonpolar surface in flow cells (Fant et al., 2000) have determined that adsorbed layers on nonpolar surfaces are thicker, include more entrained water, and reflect a higher dissipative component (due to viscous effects) than those adsorbed to silica. QCM and SPR both indicate that on silica mefp-1 is adsorbed in dense rigid layers. Comparative Langmuirian behavior of mefp-1 and -2 was investigated by FTIR on germanium and nonpolar surfaces (Suci and Geesey, 2001). Both proteins have similar surface coverage and adsorption rates indicating, at least on these materials, that the ten fold difference in DOPA content did not offer any special advantages.
It is important to take into account the oxidative instability of DOPA when analyzing films made from adsorbed mefp-1. Indeed studies of the chemical maturation of mefps provide some of the most exciting insights into biofilms. Periodate or mushroom tyrosinase treatment of adsorbed mefp-1 both of which oxidize peptidyl-DOPA to peptidyl-DOPA-quinone, leads to drastic physical changes in film properties. Films become increasingly difficult to dislodge by AFM in tapping mode (Hansen et al., 1998); also, films shrink to roughly half their initial thickness (e.g., 15 nm to 7 nm) and become markedly stiffer (Höök et al., 2001). Such changes are definitely consistent with in situ polymerization or cross-linking.
Details pertaining to the cross-linking chemistry of DOPA proteins continue to be debated. Two covalent pathways are currently entertained: 1) Michael-addition between DOPA-quinone and alkylamines such as lysine, and 2) free radical aryl coupling similar to that occurring in dityrosine or lignin formation. While the first pathway had much popular support in the latter half of the last century (Pryor, 1962; Laursen, 1992) quinone alkylamine adducts have been characterized in only a few proteins, chiefly lysyl oxidases (Wang et al., 1996). Related quinone-imidazole adducts have been isolated from insect cuticle (Xu et al., 1997). Support for aryl coupling, in byssus and other DOPA-containing structures, is growing. McDowell et al. (1999) detected aryl-coupled products resembling diDOPA (Fig. 3) in flow stressed byssal plaques using solid-state 13C-NMR. Mass spectrometric analysis of fp-1 derived decapeptides, oxidized in vitro by periodate or tyrosinase, formed multimers with masses that were consistent with diDOPA and diDOPA-quinone cross-link formation (Burzio and Waite, 2000). Addition of low molecular weight DOPA analogs, i.e., catechols, to the reaction medium during oxidation prevented peptide multimer formation. Instead, peptide mass was seen to increase by the addition of one to six catechols (Burzio and Waite, 2000). In contrast, addition of lysine, glycine or tyrosine did little to perturb or prevent multimer formation by oxidized decapeptides.
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In a recent investigation of mefp-1 adsorption to polyvinylalcohol surfaces using SPR coupled with light scattering, Haemers et al. (2001)
reported that a pH-dependent protein aggregation preceded adsorption. For example, average mefp-1 aggregate diameters increased in steps from 50 nm (monomer) to 140 nm and then to 200 nm at pH values 3.5, 4.5 and 6.5. Aggregation was attributed to charge transfer interactions between DOPA and DOPA-quinone, but diDOPA formation can not be ruled out. FTIR analyses of mefp-1 films oxidized by periodate or tyrosinase generally concurred with the absence of quinone-alkylamine adducts, but it also exposed a possible new complication: amino groups in mefp-1 and polylysine appear to be unstable to periodate treatment (Suci and Geesey, 2000). Both polymers show NH2 losses apparently associated with periodate treatment. The chemistry involved and significance, if any, to cross-linking in byssal adhesive plaques remain unknown.
Biomimetic attempts
Availability of the complete primary sequence of several plaque-derived proteins has inspired attempts to produce complete or partial biomimetic analogs. All efforts to date pertain to mefp-1 and its tandemly repeated decapeptide (Fig. 2). The field abounds with reports of the synthesis of "approximate" DOPA- and hydroxyproline-containing decapeptides (see Olivieri et al., 1989; Swerdloff et al., 1989), however, Yamamoto (1987) significantly advanced the field by using a fragment condensation strategy to make a polymer with ten decapeptide repeats (10mer). A more fastidious synthesis of the decapeptide replete with DOPA and hydroxyproline and dihydroxyproline has been reported (Taylor and Weir, 2000). A comparison of the conformation and surface binding properties of this decapeptide with native and more approximate synthetic versions should prove interesting as two conflicting conformations have been proposed for synthetic decapeptides without dihydroxyproline: a bent right-handed 
-helix (Olivieri et al., 1997) and a left-handed type II polyproline helix (Kanyalkar et al., 2002). Sufficient quantity of the 10mer was produced to attempt some tests of tensile strength. On steel coupons, however, this analog showed bonding strengths (
2.8 MPa) that were less than half those of poly-L-lysine and gelatin on steel at 60% humidity (Yamamoto, 1987). It is not clear from the report whether a cross-linking catalyst was included or what the pH of the polymer solution was. Furthermore, expectations of "heroic" adhesive tensile strength may not be realistic under any conditions given that native mefp-1 functions as a protective sealant in byssus.
Simpler adhesive analogs of the mefps have been prepared as random copolymers of DOPA and lysine or glutamic acid (Yu and Deming, 1998; Yu et al., 1999). These have provided some penetrating insights into the possible chemical role of DOPA in adhesion. Regarding involvement of lysine in cross-link formation, random copolymers of DOPA (20 mol %) and lysine were compared with those of DOPA (20 mol %) and glutamic acid in lap shear adhesion on aluminum substrates. There was no significant difference in the performance of these as adhesives (5 MPa). Regarding the proportion of DOPA, random copolymers were prepared with three different DOPA concentrations (0, 10 and 20 mol %): adhesive bonding strength increased directly with DOPA concentration. Finally, with respect to chemical maturation, it was demonstrated using the random copolymers (DOPA:Lys ratio 1:4) that optimal adhesion depended on limited polymer oxidation. Using hydrogen peroxide as the DOPA oxidant, Yu et al. (1999) showed that absence of oxidant led to cohesive failure in the adhesive, while high oxidant levels (molar ratio of DOPA: H2O2 = 1:0.25) resulted in interfacial failure. Highest lap shear bond strengths (5 MPa) were achieved at intermediate levels of oxidation (1:0.05). These data support what has already been suggested: that the DOPA side-chain in proteins serves two important roles in adhesion—the formation of cross-links and interfacial complexes in chemisorption (Waite et al., 1992). Exactly how mussels balance these mutually competing needs remains to be further investigated. It should be pointed out that adhesive bonding tests using biomimetic polymers have yet to assemble and test their bonded materials underwater.
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CONCLUSIONS |
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SYNOPSISINTRODUCTIONADHESIVE STRENGTH OF MUSSEL...PROTEIN DIVERSITY IN ADHESIVE...PROTEIN ADSORPTION AS A...CONCLUSIONSReferences |
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The permanent and opportunistic byssal attachment of mussels is derived from an assortment of proteins that show extensive posttranslational modifications. Chief among these is DOPA, which occurs at highest levels in those proteins distributed near the plaque-substratum interface. In an attempt to understand the role of DOPA-proteins in events leading up to adhesion, many researchers have studied the adsorption of DOPA-peptides and proteins to surfaces. Those features of DOPA protein adsorption with which most researchers concur are itemized as follows: 1) mefp-1 and -2 adsorb rapidly and irreversibly to various surfaces; 2) the morphologies of adsorbed mefps are surface dependent; 3) the mechanical properties of adsorbed mefps are surface dependent; 3) the irreversibility of adsorption may be related to two features: on polar surfaces mefps are likely to be chemisorbed by reactive functionalities, e.g., phospho-esters and DOPA. On all surfaces including nonpolar types, irreversibility of adsorption is enhanced by protein aggregation. Because aggregation is driven by increasing pH or periodate oxidation, it is believed to involve charge transfer between DOPA and DOPA-quinone or aryl coupling (i.e., diDOPA); 4) Given their strong net positive charge mefps may be predisposed to coadsorption with polyanions (Höök et al., 2001
; Suci and Geesey, 2000).
DOPA-based adhesive strategies are probably fairly rare in nature having been positively detected only in marine and freshwater mussels, tunicates, reef-building polychaetes, and some trematodes (Waite, 1990a). Notwithstanding this, it is proposed that a deeper understanding of the molecular mechanism of byssal adhesion would lead to fundamental insights with regard to adhesion in nature.
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ACKNOWLEDGMENTS |
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I thank Peter Suci, Manoj Chaudhuri, Fredrik Höök and Hans Elwing, for sharing insightful speculations about adhesion and protein adsorption.
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FOOTNOTES |
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1 From the Symposium Biomechanics of Adhesion presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 2–6 January 2002, at Anaheim, California.
2 E-mail: waite{at}lifesci.ucsb.edu
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References |
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SYNOPSISINTRODUCTIONADHESIVE STRENGTH OF MUSSEL...PROTEIN DIVERSITY IN ADHESIVE...PROTEIN ADSORPTION AS A...CONCLUSIONSReferences |
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