Synthesis and characterization of novel poly(γ‐benzyl‐L‐glutamate) derivatives tailored for the preparation of nanoparticles of pharmaceutical interest

Four poly(γ‐benzyl‐L ‐glutamate) (PBLG) derivatives bearing at one end specific groups were synthesized by ring‐opening polymerization of the corresponding γ‐benzyl‐L ‐glutamate N‐carboxyanhydride using different amine‐terminated initiators. These moieties were chosen to introduce, on demand, specific functionalities in nanoparticles of pharmaceutical interest. The PBLG and PBLG derivatives were characterized by ¹H NMR, viscosimetry, Fourier transform infrared spectroscopy and differential scanning calorimetry. Nanoparticles smaller than 100 nm in diameter could be easily prepared from these PBLG derivatives by slight modification of a known nanoprecipitation technique. Copyright © 2006 Society of Chemical Industry     

Synthesis of poly (γ‐benzyl‐L‐glutamate) with well‐defined terminal structures and its block polypeptides with alanine,leucine and phenylalanine

The ring‐opening polymerization of γ‐benzyl‐L ‐glutamate N‐carboxyanhydride (BLG‐NCA) was initiated by n‐hexylamine in N,N‐dimethyformamide under normal pressure at 0 °C. The products were characterizated by gel permeation chromatography, matrix‐assisted laser desorption/ionization time of flight mass spectroscopy (MALDI‐TOF MS), nuclear magnetic resonance etc. MALDI‐TOF MS gave direct evidence that the side reactions during the polymerization of BLG‐NCA could be greatly reduced by decreasing the reaction temperature, e.g. from room temperature to 0 °C. As a result, over 90% of the products were amino‐terminated poly(γ‐benzyl‐L ‐glutamate) (PBLG) with low polydispersity index when the polymerization was carried out at 0 °C, which could be used to re‐initiate the polymerization of other NCAs. Then several well‐defined PBLG‐containing block copolypeptides were successfully synthesized in a convenient way. Copyright © 2012 Society of Chemical Industry     

Structure of the Ergothioneine‐Biosynthesis Amidohydrolase EgtC

The ubiquitous sulfur metabolite ergothioneine is biosynthesized by oxidative attachment of a sulfur atom to the imidazole ring of Nα‐trimethylhistidine. Most actinobacteria, including Mycobacterium tuberculosis, use γ‐glutamyl cysteine as a sulfur donor. In subsequent steps the carbon scaffold of γ‐glutamyl cysteine is removed by the glutamine amidohydrolase EgtC and the β‐lyase EgtE. We determined the crystal structure of EgtC from Mycobacterium smegmatis in complex with its physiological substrate. The set of active site residues that define substrate specificity in EgtC are highly conserved, even in homologues that are not involved in ergothioneine production. This conservation is compounded by the phylogenetic distribution of EgtC‐like enzymes indicates that their last common ancestor might have emerged for a purpose other than ergothioneine production.     

Multi‐Enzymatic Synthesis of Optically Pure β‐Hydroxy α‐Amino Acids

A novel enzymatic production system of optically pure β‐hydroxy α‐amino acids was developed. Two enzymes were used for the system: an N‐succinyl L ‐amino acid β‐hydroxylase (SadA) belonging to the iron(II)/α‐ketoglutarate‐dependent dioxygenase superfamily and an N‐succinyl L ‐amino acid desuccinylase (LasA). The genes encoding the two enzymes are part of a gene set responsible for the biosynthesis of peptidyl compounds found in the Burkholderia ambifaria AMMD genome. SadA stereoselectively hydroxylated several N‐succinyl aliphatic L ‐amino acids and produced N‐succinyl β‐hydroxy L ‐amino acids, such as N‐succinyl‐L ‐β‐hydroxyvaline, N‐succinyl‐L ‐threonine, (2S,3R)‐N‐succinyl‐L ‐β‐hydroxyisoleucine, and N‐succinyl‐L ‐threo‐β‐hydroxyleucine. LasA catalyzed the desuccinylation of various N‐succinyl‐L ‐amino acids. Surprisingly, LasA is the first amide bond‐forming enzyme belonging to the amidohydrolase superfamily, and has succinylation activity towards the amino group of L ‐leucine. By combining SadA and LasA in a preparative scale production using N‐succinyl‐L ‐leucine as substrate, 2.3 mmol of L ‐threo‐β‐hydroxyleucine were successfully produced with 93% conversion and over 99% of diastereomeric excess. Consequently, the new production system described in this study has advantages in optical purity and reaction efficiency for application in the mass production of several β‐hydroxy α‐amino acids.

     

Characterization of a novel glutamate decarboxylase from Streptococcus salivarius ssp. thermophilus Y2

BACKGROUND: γ‐Aminobutyric acid with several well‐known physiological functions is biosynthesized via the irreversible α‐decarboxylation of L ‐glutamate catalysed by glutamate decarboxylase (GAD). Although Streptococcus salivarius ssp. thermophilus has been widely applied to the dairy, the characterization of its GAD has not been reported. In this paper, the purification and the characterization of S. salivarius ssp. thermophilus GAD were investigated. RESULTS: GAD was purified 22‐fold from crude protein extracts with a yield of 7.8% in five steps. The final preparation gave a single band on SDS‐PAGE. The molecular weight of GAD determined by SDS‐PAGE and gel filtration was 46.9 kDa and 103.6 kDa, respectively, indicating that the enzyme exists as a dimmer of homological subunits. The optimum temperature and pH of GAD was 55 °C and pH 4.0, respectively. The enzyme reacted only with L ‐glutamate among 19 α‐amino acids with apparent K_m at 2.3 mmol L^?1 and did not react with D ‐glutamic acid. Activity of the enzyme could significantly be activated by 5 mmol L^?1 of BaCl₂ and inhibited by FeSO₄, ZnSO₄, CuSO₄, MnSO₄, Na₂SO₄, AgNO₃, CoCl₂, LiCl and KCl, respectively. The N‐terminal amino acid sequence of GAD was NH₂‐MNEKLFREI. CONCLUSION: Both the characterization and the deduced amino sequence (ABI31651) showed the purified enzyme was a novel GAD. Copyright © 2008 Society of Chemical Industry     

Synthesis and characterization of polypeptide containing liver‐targeting group

Glycyrrhetinic acid, glycyrrhizic acid and bile acid were reported to be accumulated in liver. A series of novel liver‐specific targeting polypeptides, poly(γ‐benzyl‐L ‐glutamate), were synthesized by ring‐opening polymerization (ROP) of N‐carboxyanhydride of γ‐benzyl‐L ‐glutamate (BLG‐NCA) with amine‐terminated compounds containing the above‐mentioned liver‐targeting group as an initiator. The molecular weights of the polypeptides were measured by gel permeation chromatography. The structures of these polypeptides and their initiators were confirmed by FTIR and ¹H‐NMR spectroscopy. The results demonstrate that it is an efficient strategy to introduce a liver‐targeting group into polymers via amine‐terminated compounds containing glycyrrhetinic acid, glycyrrhizic acid and bile acid. The novel polymers have potential medical applications in targeted drug delivery. Copyright © 2006 Society of Chemical Industry     

How to Spoil the Taste of Insect Prey? A Novel Feeding Deterrent against Ants Released by Larvae of the Alder Leaf Beetle,Agelastica alni

Chemical defense of leaf beetle larvae (Chrysomelidae) against enemies is provided by secretions containing a wide range of deterrent compounds or by unpalatable hemolymph constituents. Here we report a new, very strong feeding deterrent against ants released by larvae of the alder leaf beetle Agelastica alni when attacked. The larvae release a defensive fluid from openings of pairwise, dorsolaterally located tubercles on the first to the eighth abdominal segments. The fluid, consisting of hemolymph and probably a glandular cell secretion, has previously been shown to contain a very stable, non‐volatile feeding deterrent. The major deterrent component was isolated by repeated HPLC separation and analyzed by NMR and MS. The compound proved to be γ‐L ‐glutamyl‐L ‐2‐furylalanine ( 1 ), a novel dipeptide containing the unusual amino acid L ‐2‐furylalanine. This amino acid, although synthetically well known, has not previously been reported from natural sources. The absolute configuration of the natural compound was elucidated by enantioselective gas chromatography after derivatization. The structure of the dipeptide was verified by the synthesis of several isomeric dipeptides. In bioassays a concentration of 1 μg μL^?1 was sufficient to deter polyphagous Myrmica rubra ants from feeding.     

Chiral separation with polyurea membrane consisting of L‐lysinyl residue: Proposal of facile method for prediction of permselectivity

Novel chiral polyureas were prepared from L ‐lysine‐4‐nitroanilide and 2,4‐toluene diisocyanate. The polyurea thus prepared gave a durable self‐standing membrane that can be directly converted into molecular recognition membranes by applying an alternative molecular imprinting. The Z‐D ‐glutamic acid (D ‐Glu) molecularly imprinted membrane adsorbed D ‐Glu in preference to L ‐glutamic acid (L ‐Glu) from racemic mixture of Glu and vice versa. Those two types of molecularly imprinted membrane showed optical resolution ability, adopting a concentration gradient as a driving force for membrane transport. A facile method to predict permselectivity of racemic mixture of charged permeant was proposed. From the observed membrane resistance, the permselectivity was predicted. The predicted permselectivity coincided with the observed one. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Biodegradable polymers based on renewable resources. IV. Enzymatic degradation of polyesters composed of 1,4:3.6‐dianhydro‐D‐glucitol and aliphatic dicarboxylic acid moieties

Enzymatic degradation of a series of polyesters prepared from 1,4:3.6‐dianhydro‐D ‐glucitol (1) and aliphatic dicarboxylic acids of the methylene chain length ranging from 2 to 10 were examined using seven different enzymes. Enzymatic degradability of these polyesters as estimated by water‐soluble total organic carbon (TOC) measurement is dependent on the methylene chain length (m) of the dicarboxylic acid component for most of the enzymes examined. The most remarkable substrate specificity was observed for Rhizopus delemar lipase, which degraded polyester derived from 1 and suberic acid (m = 6) most readily. In contrast, degradation by Porcine liver esterase was nearly independent of the structure of the polyesters. Enzymatic degradability of the polyesters based on three isomeric 1,4:3.6‐dianhydrohexitols and sebacic acid was found to decrease in the order of 1, 1,4:3.6‐dianhydro‐D ‐mannitol (2), and 1,4:3.6‐dianhydro‐L ‐iditol (3). Structural analysis of water‐soluble degradation products formed during the enzymatic hydrolysis of polyester 5g derived from 1 and sebacic acid has shown that the preferential ester cleavage occurs at the O(5) position of 1,4:3.6‐dianhydro‐D ‐glucitol moiety in the polymer chain by enzymes including Porcine pancreas lipase, Rhizopus delemar lipase, and Pseudomonas sp. lipase. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 338–346, 2000     

One‐Pot Conversion of L‐Threonine into L‐Homoalanine: Biocatalytic Production of an Unnatural Amino Acid from a Natural One

A novel biocatalytic process for production of L ‐homoalanine from L ‐threonine has been developed using coupled enzyme reactions consisting of a threonine deaminase (TD) and an ω‐transaminase (ω‐TA). TD catalyzes the dehydration/deamination of L ‐threonine, leading to the generation of 2‐oxobutyrate which is asymmetrically converted to L ‐homoalanine via transamination with benzylamine executed by ω‐TA. To make up the coupled reaction system, we cloned and overexpressed a TD from Escherichia coli and an (S)‐specific ω‐TA from Paracoccus denitrificans. In the coupled reactions, L ‐threonine serves as a precursor of 2‐oxobutyrate for the ω‐TA reaction, eliminating the need for employing the expensive oxo acid as a starting reactant. In contrast to α‐transaminase reactions in which use of amino acids as an exclusive amino donor limits complete conversion, amines are exploited in the ω‐TA reaction and thus maximum conversion could reach 100%. The ω‐TA‐only reaction with 10 mM 2‐oxobutyrate and 20 mM benzylamine resulted in 94% yield of optically pure L ‐homoalanine (ee>99%). However, the ω‐TA‐only reaction did not produce any detectable amount of L ‐homoalanine from 10 mM L ‐threonine and 20 mM benzylamine, whereas the ω‐TA reaction coupled with TD led to 91% conversion of L ‐threonine to L ‐homoalanine.     

Synthesis of novel thermo‐ and redox‐sensitive polypeptide hydrogels

Multi‐responsive hydrogels have recently received considerable attention for bioapplications. Here, novel temperature‐ and redox‐responsive polypetide hydrogels have been developed. Thermo‐sensitive hydrogels based on poly(ethyleneglycol)‐block ‐poly(γ‐propargyl‐l ‐glutamate) (PEG‐PPLG ) were first synthesized by the ring opening polymerization of γ‐propargyl‐l ‐glutamate N ‐carboxyanhydride (PLG‐NCA ) with amino group terminated PEG monomethyl ether (mPEG‐NH₂ ) as macroinitiator and were then functionalized via the ‘thiol‐yne’ click reaction between the propargyl pendents and the thiol‐containing 1‐propanethiol. The sol ? gel phase transition of the obtained copolymer aqueous solution in response to temperature change was studied. The mass loss of the hydrogel in vitro was accelerated in the presence of H₂O₂ , exhibiting a redox‐responsive property. Further, the methyl thiazolyl tetrazolium viability results revealed that this polypetide hydrogel has excellent biocompatibility, presenting potential applications in the biomedical field. © 2016 Society of Chemical Industry     

(R)‐α‐Lipoyl‐Glycyl‐L‐Prolyl‐L‐Glutamyl Dimethyl Ester Codrug as a Multifunctional Agent with Potential Neuroprotective Activities

The (R)‐α‐lipoyl‐glycyl‐L ‐prolyl‐L ‐glutamyl dimethyl ester codrug (LA‐GPE, 1 ) was synthesized as a new multifunctional drug candidate with antioxidant and neuroprotective properties for the treatment of neurodegenerative diseases. Physicochemical properties, chemical and enzymatic stabilities were evaluated, along with the capacity of LA‐GPE to penetrate the blood–brain barrier (BBB) according to an in vitro parallel artificial membrane permeability assay for the BBB. We also investigated the potential effectiveness of LA‐GPE against the cytotoxicity induced by 6‐hydroxydopamine (6‐OHDA) and H₂O₂ on the human neuroblastoma cell line SH‐SY5Y by using the 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT) reduction assay. Our results show that codrug 1 is stable at both pH 1.3 and 7.4, exhibits good lipophilicity (log P=1.51) and a pH‐dependent permeability profile. Furthermore, LA‐GPE was demonstrated to be significantly neuroprotective and to act as an antioxidant against H₂O₂‐ and 6‐OHDA‐induced neurotoxicity in SH‐SY5Y cells.     

Synthesis of biodegradable and biocompatible ABC triblock copolymers

A facile approach is offered to synthesize well‐defined amphiphilic ABC triblock copolymers composed of poly(ethylene glycol) monomethyl ether (MPEO) as A block, poly(L ‐lysine) (PLLys) as B block, and poly(ε‐caprolactone) (PCL) as C block by a combination of ring‐opening polymerization (ROP) and click reactions. The propargyl‐terminated poly(Z‐L ‐lysine)‐block‐poly(ε‐caprolactone) (MPEO‐PzLLys‐PCL) diblock copolymers were synthesized via the ring‐opening polymerization of N^ε‐carbobenzoxy‐L ‐lysine N‐carboxyanhydride (Z‐L ‐Lys NCA) in DMF at room temperature using propargyl amine as an initiator and the resulting amino‐terminated poly(Z‐L ‐lysine) then used in situ as a macroinitiator for the polymerization of ε‐caprolactone in the presence of stannous octoate as a catalyst. The triblock copolymers poly(ethylene glycol) monomethyl ether –block‐poly(Z‐L ‐lysine)‐block‐poly(ε‐caprolactone) (MPEO‐PzLLys‐PCL) were synthesized via the click reaction of the propargyl‐terminated PzLLys‐PCL and azido‐terminated poly(ethylene glycol) monomethyl ether (PEO‐N₃) in the presence of CuBr and 1,1,4,7,7‐pentamethyldiethylenetriamine (PMDETA) catalyst system. After the removal of Z groups of L ‐lysine units, amphiphilic and biocompatible ABC triblock copolymers MPEO‐PLLys‐PCL were obtained. The structural characteristics of these ABC triblock copolymers and corresponding precursors were characterized by NMR, IR, and GPC. These results showed the click reaction was highly effective. Therefore, a facile approach is offered to synthesize amphiphilic and biocompatible ABC triblock copolymers consisting of polyether, polypeptide and polyester. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Lipid production by oleaginous Mucorales cultivated on renewable carbon sources

Mortierella isabellina and Cunninghamella echinulata were cultivated on glucose‐, pectin‐, starch‐ and lactose‐based media. Culture on glucose at two initial C/N ratios favored lipid synthesis in the media with increased C/N. Starch was an adequate substrate for both molds, but lipid (in g/g of biomass) was produced in lower quantities compared with the glucose trial. Pectin was inadequate for C. echinulata whereas growth of M. isabellina was satisfactory (8.4 g/L), followed by moderate lipid production. Growth of C. echinulata on lactose was negligible, while that of M. isabellina was notable (9.5 g/L) although lipid in biomass was only 0.36 g/g. Hydrolytic enzymes (α‐amylase, polygalacturonase and β‐galactosidase) activities of both strains seemed to be low enough to saturate their metabolic capabilities. This seemed a major cause for the lower amount of lipid accumulated during growth on complex media compared with that on glucose. Cellular fatty acids of M. isabellina were oleic, linoleic and palmitic acid, while γ‐linolenic acid (GLA) was produced in low quantities. In C. echinulata grown on glucose, lactose or starch, GLA concentration was notable at the beginning and end of culture. Growth on pectin at the first growth steps was accompanied by the production of saturated fatty acids, the amount of which decreased thereafter.     

A Simple Procedure for Selective Hydroxylation of L‐Proline and L‐Pipecolic Acid with Recombinantly Expressed Proline Hydroxylases

Due to their diverse regio‐ and stereoselectivities, proline hydroxylases provide a straightforward access to hydroxprolines and other hydroxylated cylic amino acids, valuable chiral building blocks for chemical synthesis, which are often not available at reasonable expense by classical chemical synthesis. As yet, the application of proline hydroxylases is limited to a sophisticated industrial process for the production of two hydroxyproline isomers. This is mainly due to difficulties in their heterologues expression, their limited in vitro stability and complex product purification procedures. Here we describe a facile method for the production of cis‐3‐, cis‐4‐ and trans‐4‐proline hydroxylase, and their application for the regio‐ and stereoselective hydroxylation of L ‐proline and its six‐membered ring homologue l‐ pipecolic acid. Since in vitro catalysis with these enzymes is not very efficient and conversions are restricted to the milligram scale, an in vivo procedure was established, which allowed a quantitative conversion of 6 mM l‐ proline in shake flask cultures. After facile product purification via ion exchange chromatography, hydroxyprolines were isolated in yields of 35–61% (175–305 mg per flask). L ‐Pipecolic acid was converted with the isolated enzymes to prove the selectivities of the reactions. In transformations with optimized iron(II) concentration, conversions of 17–68% to hydroxylated products were achieved. The regio‐ and stereochemistry of the products was determined by NMR techniques. To demonstrate the applicability of the preparative in vivo approach for non‐physiological substrates, L ‐pipecolic acid was converted with an E. coli strain producing trans‐4‐proline hydroxylase to trans‐5‐hydroxy‐L ‐pipecolic acid in 61% yield. Thus, a synthetically valuable group of biocatalysts was made readily accessible for application in the laboratory without a need for special equipment or considerable development effort.     

Counter‐ion‐specific helix formation of poly(γ‐methyl L‐glutamate) derivatives containing sulfonate group in aqueous alcohols

Charged polypeptides containing sulfonate groups were prepared by transesterification of poly(γ‐methyl L ‐glutamate) with isethionic acid. The coil–helix transition of the sulfonated polypeptides was investigated in aqueous alcohols. Marked counter‐ion specificity was observed for helix formation: Li⁺ < Na⁺ < Cs⁺ ≦ Rb⁺ ≦ K⁺; this was different to that for poly(L ‐glutamate) (PLG): Cs⁺ ? K⁺ < Li⁺ < Na⁺. Specific helix stabilization by counter‐ion mixing, which has been found for the PLG system, was not observed for the sulfonated polypeptides. The counter‐ion‐ and solvent‐specific helix formation is discussed and compared with that in PLG. © 2001 Society of Chemical Industry     

The ring‐opening polymerization of D,L‐lactide catalyzed by new complexes of Cu,Zn, Co,and Ni Schiff base derived from salicylidene and L‐aspartic acid

D ,L ‐lactide (LA) was first successfully ring‐opening polymerized in melt by Schiff base complexes K[ML]nH₂O [M = Cu(II), Zn(II), Co(II), Ni(II); n = 2, 2, 3, 3.5; H₃L = L‐aspartic acid‐salicylidene Schiff base], which were prepared by Schiff base ligand derived from salicylidene and L‐aspartic acid and corresponding acetates. The effects of various complexes, the molar ratio of K[ML]nH₂O/LA, the polymerization temperature, and time were studied in detail. The results show that all complexes studied have the ability to initiate the ring‐opening polymerization of D ,L ‐lactide in melt. More than 90% high polymerization conversion and narrow molecular weight distribution (MWD) can be obtained very easily. However, the Ni(II) complex shows better catalytic property than other complexes on the polymerization and the molecular weight (MW) of poly(D ,L ‐lactide) (PLA) produced. With a rise in temperature and a prolongation of time, the MW of PLA decreased remarkably. The MW of PLA prepared by all complexes is not very high, which might be related to the crystalline water of complexes. X‐ray study indicated that PLA produced by Ni(II) complex is an amorphous polymer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3312–3315, 2002     

Biodegradations of statistical copolymers composed of D,L‐lactide and cyclic carbonates

Syntheses and biodegradation of statistical copolymers of D ,L ‐lactide (D ,L ‐LA) with trimethylene carbonate (TMC), rac‐1‐methyltrimethylene carbonate (1‐MTMC) and 2,2‐dimethyltrimethylene carbonate (2,2‐DTMC) were investigated at various monomer ratios using SmMe(C₅Me₅)₂THF as an initiator at 80 °C for 24 h in toluene. Biodegradations of poly(D ,L ‐LA‐co‐racemo‐1‐MTMC) (95/5) and poly(D ,L ‐LA‐co‐2,2‐DTMC) (98/2) with a compost at 60 °C proceed rapidly. Enzymatic degradations of these polymers were also performed using cholesterol esterase, lipoprotein lipase and proteinase K. Only poly(D ,L ‐LA‐co‐TMC) was biodegraded with cholesterol esterase, while poly(TMC), poly(1‐MTMC), poly(2,2‐DTMC) and poly(D ,L ‐LA) were barely degraded with these enzymes. Biodegradations of poly(D ,L ‐LA‐co‐TMC) (87/13) and poly(D ,L ‐LA‐co‐racemo‐1‐MTMC) (95/5) are rapid using proteinase K. Physical properties of these copolymers were also described. © 2003 Society of Chemical Industry     

Synthesis of Amino Acid Conjugates and Further Derivatives of 3α‐Hydroxylup‐20(;29)ene‐23,28‐dioic acid

Triterpenes of betulinic acid type exhibit many interesting biological activities. Therefore a series of new 3α‐hydroxy‐lup‐20(29)‐ene‐23,28‐dioic acid derivatives 2a—22 with putative pharmacological activities were synthesized. As starting compounds 3α‐hydroxy‐lup‐20(29)‐ene‐23,28‐dioic acid ( 1a ), isolated from Schefflera octophylla, or its 3‐O‐acetyl derivative 1b were used. Mono‐ and diesters ( 2a—b from 1a , and 4d from 4c ) were prepared with CH₂N₂. Oxidation of the isopropenyl side chain with OsO₄ yielded the 20,29‐diols ( 4a—b from 1b , and 19 from 17 ), which were in the case of 4b further transformed to the 29‐norketones 8a/mdash;b . Oxidation of the isopropenyl side chain with m‐chloroperbenzoic acid afforded the 20,29‐epoxide 12 (from 1b ) and the 29‐aldehydes and a‐hydroxy aldehydes ( 13a—c from 2a, 14a—c from 2b , and 16a—c from 15a ). Ring A was modified by a tosylation—elimination sequence using p‐TsCl/NaOAc, which afforded diolefin 15a (from 2a ) with Δ^2,20(29) double bonds or 23‐nor‐Δ^3,20(29)diolefin 17 (from 1a ). Compounds 4b, 4c , and 8a were coupled with L ‐methionin, L ‐phenylalanin, L ‐alanin, L ‐serin, and L ‐glutaminic acid via amide bonds at positions 23 and 28 to afford the amino acid conjugates 5a—7b and 9a—11 .     

A new approach for the preparation of chitosan from γ‐irradiation of prawn shell: effects of radiation on the characteristics of chitosan

Chitosan is a biodegradable polymer composed of randomly distributed β‐(1,4)‐linked D ‐glucosamine (deacetylated unit) and N‐acetyl‐D ‐glucosamine (acetylated unit). It is produced commercially by deacetylation of chitin, which is the structural element in the exoskeleton of crustaceans (such as crabs and shrimps) and the cell walls of fungi. In the work reported, we developed a facile technique for the preparation of chitosan by irradiating prawn shell at various intensities from 2 to 50 kGy. It was observed that γ‐irradiation of prawn shell increased the degree of deacetylation (DD) of chitin at a relatively low alkali concentration during the deacetylation process. Among the various irradiation doses applied to prawn shell, a dose of 50 kGy and 4 h heating in 50% NaOH solution yielded 84.56% DD while the chitosan obtained from non‐irradiated prawn shell with the same reaction conditions had only 74.70% DD. In order to evaluate the effect of γ‐irradiation on the various physicochemical, thermomechanical and morphological properties, the chitosan samples were again irradiated (2–100 kGy) with γ‐radiation. Molecular weight, DD, thermal properties with differential scanning calorimetry and thermogravimetric analysis, particle morphology by scanning electron microscopy, water binding capacity (WBC), fat binding capacity (FBC) and antimicrobial activity were determined and the effects of various γ‐radiation doses were assessed. The DD, WBC, FBC and antimicrobial activity of the chitosan were found to improve on irradiation. It was obvious that irradiation caused a decrease of molecular weight from 187 128 to 64 972 g mol^?1 after applying a radiation dose of 100 kGy which occurred due to the chain scission of chitosan molecules at glycosidic linkages. The decrease of molecular weight increased the water solubility of the chitosan, the extent of which was explored for biomedical applications. Copyright © 2012 Society of Chemical Industry