Design And Adhesive Properties Of Short Peptide Underwater Adhesives

Adhesion proteins secreted by aquatic attached organisms have attracted a great deal of attention from researchers and have provided a template and idea for the study of bionic underwater adhesives.By analysing the key sequences of adhesion proteins,researchers have developed adhesive materials such as polymers and polypeptides containing critical adhesion sequence components or 3,4-dihydroxy-L-phenylalanine(DOPA)grafting,as well as recombinant proteins,natural proteins and peptides.However,the mechanisms and principles of adhesive properties of these materials are still understood at the level of adhesive proteins secreted by aquatic attached organisms.Because of the complexity of the adhesion protein system and the limitations of the research,we need to find a more simplifying model system to explore and understand the adhesion mechanism and further optimise the material design of underwater adhesives.Short peptide molecules generally consist of 3-9 amino acids and the sequences are easy to design,synthesise and characterise.In particular,it is possible to simulate the essential sequences of adhesion proteins of natural aquatic attached organisms,which can be used as a simpler model to study the adhesion mechanism of underwater adhesives,thus offering more ideas and inspiration for the design of short peptide underwater adhesives.In addition,short peptides and their derivatives,which can mimic the key sequences of functional proteins in living organisms,can be selfassembled or co-assembled to construct a variety of bioactive adhesive materials.Researches on short peptide-based adhesive materials are currently focused on hydrogels formed by self-assembly for biomedical applications such as tissue engineering.However,there are poor mechanical properties and insufficient adhesive capacities to meet the needs of practical applications.Non-covalent interaction-driven constructs of short peptide underwater adhesives,nevertheless,are still limited to polyvalent crosslinkers and have a single type of system.The bulk strength and adhesive performance of small molecule short peptide underwater adhesives based on multiple non-covalent interactions are relatively poor.Therefore,there are still problems and challenges of limited material systems,lack of design rules and poor underwater adhesive performance of short peptide underwater adhesives.In this thesis,the cationic short peptides carrying arginine residues were used as the building blocks,and by changing the cross-linker system,the multivalent molecular cross-linkers were replaced by monovalent molecular cross-linkers,which were further assembled to form supramolecular cross-linkers.Subsequently,a category of short peptides underwater adhesives were successfully constructed by multiple non-covalent interactions between them.On this basis,the curing of the short peptide underwater adhesives was adjusted by the optimization of the short peptide sequence design using a covalent cross-linking strategy to further enhance the adhesive performance of the short peptide adhesives.The main studies are as follows.Firstly,we successfully prepared a category of short peptide underwater adhesives by using a cationic short peptide carrying two arginine residues at each N-terminal and C-terminal ends and the supramolecular cross-linker cholesterate as the building blocks,driven by electrostatic interactions,hydrophobic effects and hydrogen bonding synergy.The aggregation behaviours of NaDC were investigated by transmission electron microscopy(TEM)and X-ray diffraction spectroscopy(XRD)using a representative submerged adhesive(R1-8/NaDC)constructed from sodium deoxycholate(NaDC)and a cationic short peptide(R1-8)carrying an arginine residue,respectively.NaDC is a biosurfactant with a rigid hydrophobic steroid structure,which differs from conventional surfactants in that it consists of a non-polar steroid structure that first aggregates to form primary micelles through hydrophobic effects,and then aggregates to secondary micelles driven by hydrogen bonding.Adjusting the concentration,temperature,pH and salt ion concentration can transform the aggregation morphology of NaDC,including vesicles,sponge phases and layered stacks of nanofibres,making it an outstanding candidate for the construction of supramolecular crosslinkers.The secondary structure and morphology of R1-8 was characterised by means of circular dichroism(CD),thioflavin T fluorescence titration experiments,TEM and Fourier transform infrared spectroscopy(FTIR)techniques,and the short cationic sequence peptides were in a random conformation.Subsequently,we verified the formation of the adhesive by dynamic rheological characterisation,SEM and sample vial inversion and observed that the adhesives have a highly cross-linked three-dimensional network structure in the bulk phase and are universal for the bonding of different substrates.We further characterised the underwater adhesive strength by quantitative lap shear adhesion testing.Substrate-assisted laser-resolved ionisation time-of-flight mass spectrometry(MALDI-TOF MS)and FTIR showed that the two building blocks were structurally integrated during adhesive formation without covalent reactions.zeta potential titration experiments and X-ray photoelectron spectroscopy(XPS)showed that the anionic supramolecular cross-linker NaDC and the protonated cationic sequence of short peptides carrying arginine residues formed the adhesives mainly through electrostatic interactions.Furthermore,we have analysed and explained the adhesive formation mechanism from a thermodynamic point of view.The R1-8/NaDC system is an enthalpy and entropy compensated driven process,driven by hydrogen bonding,hydrophobic interactions,and electrostatic interactions.The influence of cationic short peptides on the construction of adhesives was also explored in terms of the number and type of residues carried by them and their hydrophobicity,as well as the type of structure of cholestyramine,extending the range of uses of underwater adhesives.In summary,this class of materials extend the material system for constructing short peptide underwater adhesives.Secondly,based on the work in the previous chapter,we have refined the sequence of cationic short peptides carrying arginine residues,and introduced lysine residues into the sequence to prepare a viscous soft adhesive by non-covalent interaction with the polyvalent cross-linking agent glycyrrhetinic acid,which further covalently crosslinked with genipin to obtain short peptide underwater adhesives with significantly improved adhesive properties.The formation and morphology of the non-covalent and covalent strategy-modulated adhesives were characterized by sample bottle inversion,dynamic rheological behaviour and SEM.Electrospray mass spectrometry(ESI-MS)and FTIR were used to verify the component integrity of the non-covalently constructed glycyrrhetinic acid/cationic short peptide adhesives,and zeta potential titration tests and XPS demonstrated that the main driving force was the electrostatic interaction.For the adhesives after modulation using a covalent strategy,the influences of genipin introduction,pH,reaction time and temperature on the modulation of adhesive curing were investigated.In addition,the reactions of genipin with short peptide primary amine groups were specifically demonstrated by ultraviolet-visible spectroscopy(UVVis),MALDI-TOF MS,FTIR and XPS.Finally,the enhancement of the adhesive properties were quantitatively characterised by lap shear adhesion testing.The results showed that its underwater adhesive strength bonding on PC substrates reached 252.45 kPa.In summary,we have extended the material system by constructing the short peptide underwater adhesives based on the co-assembly of cationic short peptides and cholesterates.Moreover,the adhesive performance of the short peptide underwater adhesives has been significantly improved and enhanced by the covalent curing strategy.