Combinatorial screening for enzyme-mediated coupling. Tyrosinase-catalyzed coupling to create protein--chitosan conjugates

Carbon-Centered Radicals in Protein Manipulation

Methods to directly post-translationally modify proteins are perhaps the most straightforward and operationally simple ways to create and study protein post-translational modifications (PTMs). However, precisely altering or constructing the C-C scaffolds pervasive throughout biology is difficult with common two-electron chemical approaches. Recently, there has been a surge of new methods that have utilized single electron/radical chemistry applied to site-specifically "edit" proteins that have started to create this potential-one that in principle could be near free-ranging. This review provides an overview of current methods that install such "edits", including those that generate function and/or PTMs, through radical C-C bond formation (as well as C-X bond formation via C• where illustrative). These exploit selectivity for either native residues, or preinstalled noncanonical protein side-chains with superior radical generating or accepting abilities. Particular focus will be on the radical generation approach (on-protein or off-protein, use of light and photocatalysts), judging the compatibility of conditions with proteins and cells, and novel chemical biology applications afforded by these methods. While there are still many technical hurdles, radical C-C bond formation on proteins is a promising and rapidly growing area in chemical biology with long-term potential for biological editing.

Structural and Functional Characterization of Laccase-Induced β-Lactoglobulin-Ferulic Acid-Chitosan Ternary Conjugates

The purpose of current research is to design and acquire novel biological macromolecule materials with enhanced functional properties. Chitosan-ferulic acid binary conjugate (CFC) was synthesized based on the carbodiimide-mediated coupling reaction, and then β-lactoglobulin-ferulic acid-chitosan ternary conjugate (BFCC) was fabricated by laccase induction. Furthermore, the impact of laccase concentration on the formation mechanism of BFCC was investigated by the analyses of reaction group content, ultraviolet-visible (UV-vis) absorption, circular dichroism (CD), and fluorescence spectroscopy. Results showed that hetero- and homo-conjugates between CFC and β-lactoglobulin (β-LG) were achievable at the low concentration (≤4 U/mL) and high concentration (≥6 U/mL) of laccase, respectively. The CD spectrum indicated that the interaction with CFC made β-LG more disorderly. Functional evaluation results revealed that the antioxidant activity and thermal stability of BFCC were improved compared with β-LG. The knowledge obtained in the present study provided an effective method to acquire innovative biological macromolecule materials with desirable functional characteristics.

Tailoring the Interface of Biomaterials to Design Effective Scaffolds

Tissue engineering (TE) is a multidisciplinary science, which including principles from material science, biology and medicine aims to develop biological substitutes to restore damaged tissues and organs. A major challenge in TE is the choice of suitable biomaterial to fabricate a scaffold that mimics native extracellular matrix guiding resident stem cells to regenerate the functional tissue. Ideally, the biomaterial should be tailored in order that the final scaffold would be (i) biodegradable to be gradually replaced by regenerating new tissue, (ii) mechanically similar to the tissue to regenerate, (iii) porous to allow cell growth as nutrient, oxygen and waste transport and (iv) bioactive to promote cell adhesion and differentiation. With this perspective, this review discusses the options and challenges facing biomaterial selection when a scaffold has to be designed. We highlight the possibilities in the final mold the materials should assume and the most effective techniques for its fabrication depending on the target tissue, including the alternatives to ameliorate its bioactivity. Furthermore, particular attention has been given to the influence that all these aspects have on resident cells considering the frontiers of materiobiology. In addition, a focus on chitosan as a versatile biomaterial for TE scaffold fabrication has been done, highlighting its latest advances in the literature on bone, skin, cartilage and cornea TE.

Aspergillus niger PA2 Tyrosinase Covalently Immobilized on a Novel Eco-Friendly Bio-Composite of Chitosan-Gelatin and Its Evaluation for L-DOPA Production

Tyrosinase (EC 1.14.18.1) a copper-containing monooxygenase, isolated from a fungal isolate Aspergillus niger PA2 was subjected for immobilization onto a composite consisting of chitosan and gelatin biopolymers. The homogeneity of the chitosan-gelatin biocomposite film was characterized by X-ray diffraction analyses. To evaluate immobilization efficiency, chitosan-gelatin-Tyr bio-composite films were analyzed by field emission scanning electron microscopy, atomic force microscopy and UV-spectroscopy. The rough morphology of the film led to a high loading of enzyme and it could retain its bioactivity for a longer period. The enzyme adsorbed onto the film exhibited 72% of its activity after 10 days and exhibited good repeatability for up to nine times, after intermittent storage. Moreover, the immobilized enzyme exhibited broader pH and temperature profile as compared to free counterpart. Immobilized enzyme was further evaluated for the synthesis of L-DOPA (2,4-dihydroxy phenylalanine) which is a precursor of dopamine and a potent drug for the treatment of Parkinson's disease and for myocardium neurogenic injury.

Enzymes as Green Catalysts for Precision Macromolecular Synthesis

The present article comprehensively reviews the macromolecular synthesis using enzymes as catalysts. Among the six main classes of enzymes, the three classes, oxidoreductases, transferases, and hydrolases, have been employed as catalysts for the in vitro macromolecular synthesis and modification reactions. Appropriate design of reaction including monomer and enzyme catalyst produces macromolecules with precisely controlled structure, similarly as in vivo enzymatic reactions. The reaction controls the product structure with respect to substrate selectivity, chemo-selectivity, regio-selectivity, stereoselectivity, and choro-selectivity. Oxidoreductases catalyze various oxidation polymerizations of aromatic compounds as well as vinyl polymerizations. Transferases are effective catalysts for producing polysaccharide having a variety of structure and polyesters. Hydrolases catalyzing the bond-cleaving of macromolecules in vivo, catalyze the reverse reaction for bond forming in vitro to give various polysaccharides and functionalized polyesters. The enzymatic polymerizations allowed the first in vitro synthesis of natural polysaccharides having complicated structures like cellulose, amylose, xylan, chitin, hyaluronan, and chondroitin. These polymerizations are "green" with several respects; nontoxicity of enzyme, high catalyst efficiency, selective reactions under mild conditions using green solvents and renewable starting materials, and producing minimal byproducts. Thus, the enzymatic polymerization is desirable for the environment and contributes to "green polymer chemistry" for maintaining sustainable society.

Bio-inspired adhesive catechol-conjugated chitosan for biomedical applications: A mini review

The development of adhesive materials, such as cyanoacrylate derivatives, fibrin glues, and gelatin-based adhesives, has been an emerging topic in biomaterial science because of the many uses of these materials, including in wound healing patches, tissue sealants, and hemostatic materials. However, most bio-adhesives exhibit poor adhesion to tissue and related surfaces due to the presence of body fluid. For a decade, studies have aimed at addressing this issue by developing wet-resistant adhesives. Mussels demonstrate robust wet-resistant adhesion despite the ceaseless waves at seashores, and mussel adhesive proteins play a key role in this adhesion. Adhesive proteins located at the distal end (i.e., those that directly contact surfaces) are composed of nearly 60% of amino acids called 3,4-dihydroxy-l-phenylalanine (DOPA), lysine, and histidine, which contain side chains of catechol, primary amines, and secondary amines, respectively. Inspired by the abundant catecholamine in mussel adhesive proteins, researchers have developed various types of polymeric mimics, such as polyethylenimine-catechol, chitosan-catechol, and other related catecholic polymers. Among them, chitosan-catechol is a promising adhesive polymer for biomedical applications. The conjugation of catechol onto chitosan dramatically increases its solubility from zero to nearly 60mg/mL (i.e., 6% w/v) in pH 7 aqueous solutions. The enhanced solubility maximizes the ability of catecholamine to behave similar to mussel adhesive proteins. Chitosan-catechol is biocompatible and exhibits excellent hemostatic ability and tissue adhesion, and thus, chitosan-catechol will be widely used in a variety of medical settings in the future. This review focuses on the various aspects of chitosan-catechol, including its (1) preparation methods, (2) physicochemical properties, and (3) current applications.

Enzymatic processing of protein-based fibers

Wool and silk are major protein fiber materials used by the textile industry. Fiber protein structure-function relationships are briefly described here, and the major enzymatic processing routes for textiles and other novel applications are deeply reviewed. Fiber biomodification is described here with various classes of enzymes such as protease, transglutaminase, tyrosinase, and laccase. It is expected that the reader will get a perspective on the research done as a basis for new applications in other areas such as cosmetics and pharma.

Bioinspired self-assembly of tyrosinase-modified silicatein and fluorescent core-shell silica spheres

Inspired by the intermolecular cross-linking of mussel foot proteins and their adhesive properties, tyrosinase has been used to modify recombinant silicatein. DOPA/DOPAquinone-mediated cross-linking and interfacial interactions enhanced both self-assembly of silicatein building blocks and templating of core-shell silica spheres, resulting in fluorescent biomimetic silicatein-silica hybrid mesofibers.

Biomimetic materials for medical application through enzymatic modification

Living organisms synthesize functional materials, based on proteins and polysaccharides, using enzyme-catalyzed reactions. According to the biomimetic approach, biomaterial matrices for tissue engineering are designed to be able to mimic the properties and the functions of the extracellular matrix (ECM). In this chapter, the most significant research efforts dedicated to the study and the preparation of biomimetic materials through enzymatic modifications were reviewed. The functionalizations of different polymeric matrices obtained through the catalytic activity of two enzymes (Transglutaminase, TGase and Tyrosinase, TYRase) were discussed. Specifically, the biomimetic applications of TGase and TYRase to confer appropriate biomimetic properties to the biomaterials, such as the possibility to obtain in situ gelling hydrogels and the incorporation of bioactive molecules (growth factors) and cell-binding peptides into the scaffolds, were reviewed.

Catechol-based biomimetic functional materials

Catechols are found in nature taking part in a remarkably broad scope of biochemical processes and functions. Though not exclusively, such versatility may be traced back to several properties uniquely found together in the o-dihydroxyaryl chemical function; namely, its ability to establish reversible equilibria at moderate redox potentials and pHs and to irreversibly cross-link through complex oxidation mechanisms; its excellent chelating properties, greatly exemplified by, but by no means exclusive, to the binding of Fe(3+); and the diverse modes of interaction of the vicinal hydroxyl groups with all kinds of surfaces of remarkably different chemical and physical nature. Thanks to this diversity, catechols can be found either as simple molecular systems, forming part of supramolacular structures, coordinated to different metal ions or as macromolecules mostly arising from polymerization mechanisms through covalent bonds. Such versatility has allowed catechols to participate in several natural processes and functions that range from the adhesive properties of marine organisms to the storage of some transition metal ions. As a result of such an astonishing range of functionalities, catechol-based systems have in recent years been subject to intense research, aimed at mimicking these natural systems in order to develop new functional materials and coatings. A comprehensive review of these studies is discussed in this paper.

A Biopolymer Chitosan and Its Derivatives as Promising Antimicrobial Agents against Plant Pathogens and Their Applications in Crop Protection

Recently, much attention has been paid to chitosan as a potential polysaccharide resource. Although several efforts have been reported to prepare functional derivatives of chitosan by chemical modifications, few attained their antimicrobial activity against plant pathogens. The present paper aims to present an overview of the antimicrobial effects, mechanisms, and applications of a biopolymer chitosan and its derivatives in crop protection. In addition, this paper takes a closer look at the physiochemical properties and chemical modifications of chitosan molecule. The recent growth in this field and the latest research papers published will be introduced and discussed.

Cross-linking and immobilisation of different proteins with recombinant Verrucomicrobium spinosum tyrosinase

This paper reports on the cross-linking and immobilisation of various proteins by the recombinant tyrosinase from Verrucomicrobium spinosum (Vs-tyrosinase). In general it is found that Vs-tyrosinase can readily cross-link proteins with a low degree of complexity, such as casein, but that the enzyme cannot readily cross-link well folded protein substrates such as lysozyme, myoglobin, cytochrome c or Candida antarctica lipase B (CALB). However, the inclusion of phenolic compounds (phenol or caffeic acid) to reaction mixtures of these proteins can greatly enhance the levels of cross-linking. For example it is possible to prepare cross-linked aggregates of industrially applicable enzymes such as CALB by simply incubating it with Vs-tyrosinase and phenol. The resulting aggregates can be collected by centrifugation and retain high levels of activity and may find applications in biocatalysis.

Preparation of low-molecular-weight chitosan derivative zinc complexes and their effect on the growth of liver cancer cells in vitro

Low-molecular-weight (LMW) chitosan salicylaldehyde Schiff-base and its zinc(II) complexes were synthesized and characterized by Fourier transform-infrared (FT-IR) spectra, transmission electron microscopy (TEM), dynamic light scattering (DLS), gel permeation chromatography-multiangle laser light scattering (GPC-MALLS), and elemental analysis. The results of electrophoretic analysis suggest that the Zn complexes bound to DNA by means of electrostatic interactions and intercalation. The effect of the Zn complexes on the growth of SMMC-7721 liver cancer cells was investigated by sulforhodamine B assay in vitro. The results reveal that the growth of liver cancer cells was inhibited by LMW-chitosan and their Zn complexes. The inhibition rate of the Zn complexes was higher than that of LMW-chitosan ligand. The LMW-chitosan Schiff-base Zn complex exhibited higher anticancer activity than the LMW-chitosan Zn complex. Combining LMW-chitosan with Schiff-base and Zn improved its anticancer activity, which we ascribe to the synergistic effect between the chitosan matrix and the planar construction of the Zn complexes.

Electrodeposition of Carbon Nanotubes−Chitosan−Glucose Oxidase Biosensing Composite Films Triggered by Reduction of p-Benzoquinone or H2O2

We report here on the electroreduction of p-benzoquinone (BQ) or H2O2 as a new trigger for simple, fast, uniform, and controllable electrodeposition of chitosan (CS) hydrogels and biosensing nanocomposite films of CS, multiwalled carbon nanotubes (MWCNTs), and glucose oxidase (GOD). The multiparameter electrochemical quartz crystal microbalance (EQCM) based on crystal electroacoustic impedance analysis was used to dynamically monitor the deposition processes. When the EQCM Au electrode was immersed in a weakly acidic solution (here pH 5.1 acetic buffer) containing BQ (or H2O2) and CS, the proton consumption during BQ (or H2O2) electroreduction increased the local solution pH near the electrode surface and led to the deposition of CS hydrogel on the electrode surface at local pH near and above the pKa value of CS. The concentration of BQ (or H2O2) required for CS electrodeposition was theoretically evaluated based on an electrogenerated base-to-acid titration model and supported by experiments. Co-deposition of GOD and MWCNTs with the CS hydrogel was achieved, and the resulting MWCNTs-CS-GOD nanocomposite films were demonstrated for glucose biosensing. The MWCNTs-CS-GOD enzyme electrode prepared by BQ reduction exhibited a current sensitivity of 6.7 microA mM-1 cm-2 to glucose, and the linear range for glucose detection at 0.7 V vs SCE was from 5 microM to 8 mM, with a detection limit of 2 microM and a Michaelis-Menten constant of 6.8 mM. The BQ-electroreduction protocol exhibited the best sensor performance, as compared with H2O2-reduction and previously reported water-reduction values. The present protocol via electroreduction of a deliberately added oxidant that is accompanied by a local pH change is highly recommended for wider applications in pH-dependent deposition of other films.

Smart Polymeric Gels: Redefining the Limits of Biomedical Devices

This review describes recent progresses in the development and applications of smart polymeric gels, especially in the context of biomedical devices. The review has been organized into three separate sections: defining the basis of smart properties in polymeric gels; describing representative stimuli to which these gels respond; and illustrating a sample application area, namely, microfluidics. One of the major limitations in the use of hydrogels in stimuli-responsive applications is the diffusion rate limited transduction of signals. This can be obviated by engineering interconnected pores in the polymer structure to form capillary networks in the matrix and by downscaling the size of hydrogels to significantly decrease diffusion paths. Reducing the lag time in the induction of smart responses can be highly useful in biomedical devices, such as sensors and actuators. This review also describes molecular imprinting techniques to fabricate hydrogels for specific molecular recognition of target analytes. Additionally, it describes the significant advances in bottom-up nanofabrication strategies, involving supramolecular chemistry. Learning to assemble supramolecular structures from nature has led to the rapid prototyping of functional supramolecular devices. In essence, the barriers in the current performance potential of biomedical devices can be lowered or removed by the rapid convergence of interdisciplinary technologies.

Magnetic nanofactories: Localized synthesis and delivery of quorum-sensing signaling molecule autoinducer-2 to bacterial cell surfaces

Magnetic 'nanofactories', for localized manufacture and signal-guided delivery of small molecules to targeted cell surfaces, are demonstrated. They recruit nearby raw materials for synthesis, employ magnetic mobility for capture and localization of target cells, and deliver molecules to cells triggering their native phenotypic response, but with user-specified control. Our nanofactories, which synthesize and deliver the "universal" bacterial quorum-sensing signal molecule, autoinducer AI-2, to the surface of Escherichia coli, are assembled by first co-precipitating nanoparticles of iron salts and the biopolymer chitosan. E. coli AI-2 synthases, Pfs and LuxS, constructed with enzymatically activatable "pro-tags", are then covalently tethered onto the chitosan. These enzymes synthesize AI-2 from metabolite S-adenosylhomocysteine. Chitosan serves as a molecular scaffold and provides cell capture ability; magnetite provides stimuli responsiveness. These magnetic nanofactories are shown to modulate the natural progression of quorum-sensing activity. New prospects for small molecule delivery, based on localized synthesis, are envisioned.

Tyrosinase-catalyzed grafting of sericin peptides onto chitosan and production of protein–polysaccharide bioconjugates

The capability of Agaricus bisporus tyrosinase to catalyze the oxidation of tyrosine residues of silk sericin was studied under homogeneous reaction conditions, by using sericin peptides purified from industrial wastewater as the substrate. Tyrosinase was able to oxidize about 57% of sericin-bound tyrosine residues. The reaction rate was higher than with silk fibroin, but lower than with other silk-derived model peptides, i.e. tryptic and chymotryptic soluble peptide fractions of silk fibroin, suggesting that the size and the molecular conformation of the substrate influenced the kinetics of the reaction. The concentration of tyrosine in oxidized sericin samples decreased gradually with increasing the enzyme-to-substrate ratio. The average molecular weight of sericin peptides significantly increased by oxidation, indicating that cross-linking occurred via self-condensation of o-quinones and/or coupling with the free amine groups of lysine and, probably, with sulfhydryl groups of cysteine. The high temperature shift of the main thermal transitions observed in the differential scanning calorimetry curves confirmed the formation of peptide species with higher molecular weight and higher thermal stability. Fourier transform-infrared spectra of oxidized sericin samples showed slight changes related to the loss of tyrosine and formation of oxidation products. Oxidized sericin peptides were able to undergo non-enzymatic coupling with chitosan. Infrared spectra provided clear evidence of the formation of sericin-chitosan bioconjugates under homogeneous reaction conditions. Spectral changes in the NH stretching region seem to support the formation of bioconjugates via the Michael addition mechanism.

Tyrosinase-catalyzed modification of Bombyx mori silk fibroin: Grafting of chitosan under heterogeneous reaction conditions

The capability of mushroom tyrosinase to catalyze the oxidation of tyrosine residues of Bombyx mori silk fibroin was studied under heterogeneous reaction conditions, by using a series of silk substrates differing in surface and bulk morphology and structure, i.e. hydrated and insoluble gels, mechanically generated powder and fibre. Tyrosinase was able to oxidize 10-11% of the tyrosine residues of silk gels. The yield of the reaction was very low for the powder and undetectable for fibres. FT-Raman spectroscopy gave evidence of the oxidation reaction. New bands attributable to vibrations of oxidized tyrosine species (o-quinone) appeared, and the value of the I853/I829 intensity ratio of the tyrosine doublet changed following oxidation of tyrosine. The thermal behaviour of SF substrates was not affected by enzymatic oxidation. o-Quinones formed by tyrosinase onto gels and powder were able to undergo non-enzymatic coupling with chitosan. FT-IR and FT-Raman spectroscopy provided clear evidence of the formation of silk-chitosan bioconjugates under heterogeneous reaction conditions. Chitosan grafting caused a beta-sheet --> random coil conformational transition of silk fibroin and significant changes in the thermal behaviour. Chitosan grafting did not occur, or occurred at an undetectable level on silk fibres. The results reported in this study show the potential of the enzymatically initiated protein-polysaccharide grafting for the production of a new range of bio-based, environmentally friendly polymers.

Tyrosine-based “Activatable Pro-Tag”: Enzyme-catalyzed protein capture and release

Protein recovery is often achieved by a series of capture and release steps that often involve chromatographic binding and elution. We report an alternative, non-chromatographic, capture and release approach that employs enzymes and the stimuli-responsive polysaccharide chitosan. We capture our protein using the enzyme tyrosinase that oxidizes accessible tyrosine residues of the protein and "activates" these residues for covalent capture (i.e., conjugation) onto chitosan. Using fusions of green fluorescent protein (GFP) we observed that: (i) enzymatic activation is required for protein capture to chitosan; and (ii) capture is enhanced (approximately five-fold) by engineering the protein to have a penta-tyrosine fusion tag that provides additional accessible tyrosine residues for enzymatic activation. Because the fusion tag appears to be the primary site for capture, and capture requires activation, we designate penta-tyrosine as a "pro-tag." The captured GFP-chitosan conjugate possesses the pH-responsive solubility that is characteristic of chitosan. We exploit this pH-responsive solubility to facilitate purification of the captured protein. Two enzymatic methods were explored to release the captured GFP from the chitosan conjugate. The first method employs enterokinase (EK) to cleave the protein at an engineered EK-cleavage site. The second method employs chitosanase to hydrolyze the chitosan backbone. Using GFP as a model protein, we demonstrated that enzymatic capture and release provides a simple, non-chromatographic means to recover proteins directly from cell lysates.

Activity and stability of laccase in conjugation with chitosan

Laccase is one of a few enzymes that can directly reduce oxygen into water under ambient conditions, while oxidizing a variety of aromatic compounds. Its conjugation with chitosan generates a pH-sensitive functional biomaterial that changes its solubility in response to pH variation. The molecular conjugation between laccase and chitosan of different molecular mass was investigated with a carbodiimide reaction to understand the mechanism of the enzyme's activity loss during conjugation. With 81-93% laccase being conjugated, a moderate activity loss (16-28% less than the initial activity) was observed in conjugation solution. A second severe activity loss (63-78% less than the conjugated activity) occurred during a cycle of phase change consisting of precipitation, centrifugation and re-dissolution of the enzyme-chitosan conjugates. The chitosan molecular size has little effect on the first moderate activity loss in the conjugation reaction, but visible effect on the substantial activity loss associated with phase change. Small chitosan molecules gave high residual activity. The conjugated laccase exhibited a high stability in the following repeated phase changes and had the same temperature and pH profile as those of free laccase. Compared to free laccase, the conjugated laccase had a similar affinity (Km), but reduced turnover (kcat) that was adversely affected with increase of molecular mass of chitosan.

Synthesis and Characterization of a Novel Chitosan−Gelatin Bioconjugate with Fluorescence Emission

Polysaccharide-protein conjugations have generated increasing interests for biomedical applications in recent years. A naturally occurring cross-linking reagent, genipin, which has been used in herbal medicine, was employed to cross-link chitosan and gelatin for the preparation of a novel chitosan-gelatin conjugate. The primary amine groups on chitosan and gelatin were covalently linked with genipin, leading to the formation of a chitosan-gelatin conjugate with nitrogen-containing heterocycle units, the pyrindine-like derivatives. The FT-IR and UV-vis studies revealed that chitosan could react with genipin via a nucleophilic ring-opening reaction to construct more sufficient and extensive cross-link networks, as compared with its gelatin counterpart. The UV-vis absorption properties of the chitosan-gelatin conjugates were strongly related to the chitosan-to-gelatin weight ratio in the compositions. It is worth noting that the conjugation process endows the special emission properties of the chitosan-gelatin conjugates, which depends on the cross-linking reaction and the formation of hydrogen bonding involved chitosan-gelatin complex. Fluorescence quenching or enhancement was observed from the chitosan-gelatin conjugates upon coordinated with a wide variety of heavy metal ions (Ag+, Cu2+, Fe2+, and Co2+). This study also examined the possibility of covalent coupling the capture chelator (chitosan) with bioactive protein (e.g., albumin, alpha-globulin, and fibrinogen) to create fluorescence emission. These findings may provide a novel way to deliver therapeutic radionuclides for immuno-targeting purposes in the future.

Enzymatic grafting of chitosan onto Bombyx mori silk fibroin: kinetic and IR vibrational studies

The potential for using tyrosinase to graft the polysaccharide chitosan (Ch) onto Bombyx mori silk fibroin (SF) was examined. FT-IR spectroscopy coupled to HPLC amino acid analysis showed that mushroom tyrosinase (MT) catalyses the oxidation of tyrosine (Tyr) of SF to electrophilic o-quinones. Kinetic studies showed that only a fraction of the Tyr residues available on the SF chain were oxidized. This result was interpreted in the light of the structure assumed by SF in aqueous solution: Tyr aromatic side chain groups buried into the folded hydrophobic portions of the chain were probably less accessible to MT for steric reasons. Using slightly acidic conditions (pH 6), it was possible to modify SF under homogeneous conditions. FT-IR spectroscopy provided evidence that Ch was grafted onto MT-oxidized SF: the o-quinones were found to undergo a subsequent non-enzymatic reaction with nucleophilic amino groups of Ch via Schiff-base and Michael addition mechanisms. Various factors, i.e. reaction time, pH, MT/SF ratio, were found to influence the grafting yield. The highest grafting yield was achieved at pH 7, i.e. more favorable to MT activity rather than to Ch solubility, suggesting that the determining step of the grafting reaction is the formation of o-quinones. The FT-IR spectroscopy revealed that grafting induced a beta-sheet --> random coil conformational transition.

N-acylated chitosan: hydrophobic matrices for controlled drug release

N-acylation of chitosan with various fatty acid (C(6)-C(16)) chlorides increased its hydrophobic character and made important changes in its structural features. Unmodified chitosan exhibited a low degree of order (DO) and a weak tablet crushing strength. Chitosan acylated with a short chain length (C(6)) possessed similar properties, but exhibited significant swelling. Acylation with longer side chains (C(8)-C(16)) resulted in a higher DO and crushing strength but lower swelling. The best mechanical characteristics and drug release properties were found for palmitoyl chitosan (substitution degree 40-50%) tablets with 20% acetaminophen as a tracer. The high stability of these monolithic tablets appears to be due to hydrophobic interactions between side chains, as shown by a more organized structure. Infrared spectroscopy, X-ray diffractometry and proton nuclear magnetic resonance analyses of palmitoyl chitosan were consistent with a hydrophobic self-assembling model. Drug dissolution kinetics showed longer release times for higher degrees of functionalization, i.e. 30 h (for 47% substitution) and 90 h (for 69% substitution), suggesting palmitoyl chitosan excipients as interesting candidates for oral and subdermal pharmaceutical applications.

Chitosan scaffolds for biomolecular assembly: coupling nucleic acid probes for detecting hybridization

Chitosan, a naturally occurring biopolymer, was used as a scaffold for the covalent binding of single-stranded DNA oligonucleotide probes in a fluorescence-based nucleic acid hybridization assay. Chitosan's pH dependent chemical and electrostatic properties enable its deposition on electrodes and metal surfaces, as well as on the bottom of microtiter plates. A combinatorial 96-well microtiter plate format was used to optimize chemistries and reaction conditions leading to hybridization experiments. We found the coupling of oligonucleotides using relatively common glutaraldehyde chemistry was quite robust. Our hybridization results for complementary ssDNA oligonucleotides (E. coli dnaK sequences) demonstrated linear fluorescence intensity with concentration of E. coli dnaK-specific oligonucleotide from 0.73 microM to 6.6 microM. Moreover, hybridization assays were specific as there was minimal fluorescence associated with noncomplementary groEL oligonucleotide. Finally, these results demonstrate the portability of a DNA hybridization assay based on covalent coupling to chitosan, which, in turn, can be deposited onto various surfaces. More arduous surface preparation techniques involving silanizing agents and hazardous washing reagents are eliminated using this technique.

In vitro protein-polysaccharide conjugation: tyrosinase-catalyzed conjugation of gelatin and chitosan

The enzyme tyrosinase was used for the in vitro conjugation of the protein gelatin to the polysaccharide chitosan. Tyrosinases are oxidative enzymes that convert accessible tyrosine residues of proteins into reactive o-quinone moieties. Spectrophotometric and dissolved oxygen studies indicate that tyrosinase can oxidize gelatin and we estimate that 1 in 5 gelatin chains undergo reaction. Oxidized tyrosyl residues (i.e., quinone residues) can undergo nonenzymatic reactions with available nucleophiles such as the nucleophilic amino groups of chitosan. Ultraviolet/visible, (1)H-NMR, and ir provided chemical evidence for the conjugation of oxidized gelatin with chitosan. Physical evidence for conjugation was provided by dynamic viscometry, which indicated that tyrosinase catalyzes the sol-to-gel conversion of gelatin/chitosan mixtures. The gels formed from tyrosinase-catalyzed reactions were observed to differ from gels formed by cooling gelatin. In contrast to gelatin gels, tyrosinase-generated gels had different thermal behavior and were broken by the chitosan-hydrolyzing enzyme chitosanase. These results demonstrate that tyrosinase can be exploited for the in vitro formation of protein-polysaccharide conjugates that offer interesting mechanical properties.