L,L‐Lactide and ε‐Caprolactone Block Copolymers by a ‘Poly(L,L‐lactide) Block First’ Route

L,L ‐lactide (LA) and ε‐caprolactone (CL) block copolymers have been prepared by initiating the poly(ε‐caprolactone) (PCL) block growth with living poly(L,L ‐lactide) (PLA*). In the previous attempts to prepare block copolymers this way only random copolyesters were obtained because the PLA* + CL cross‐propagation rate was lower than that of the PLA–CL* + PLA transesterification. The present paper shows that application of Al‐alkoxide active centers that bear bulky diphenolate ligands results in efficient suppression of the transesterification. Thus, the corresponding well‐defined di‐ and triblock copolymers could be prepared.

     

Cationic polymerization of L,L‐lactide

Cationic bulk polymerization of L ,L‐ lactide (LA) initiated by trifluromethanesulfonic acid [triflic acid (TfA)] has been studied. At temperatures 120–160 °C, polymerization proceeded to high conversion (>90% within ～8 h) giving polymers with M_n ～ 2 × 10⁴ and relatively high dispersity. Thermogravimetric analysis of resulting polylactide (PLA) indicated that its thermal stability was considerably higher than the thermal stability of linear PLA of comparable molecular weight obtained with ROH/Sn(Oct)₂ initiating system. Also hydrolytic stability of cationically prepared PLA was significantly higher than hydrolytic stability of linear PLA. Because thermal or hydrolytic degradation of PLA starting from end‐groups is considerably faster than random chain scission, both thermal and hydrolytic stability depend on molecular weight of the polymer. High thermal and hydrolytic stability, in spite of moderate molecular weight of cationically prepared PLA, indicate that the fraction of end‐groups is considerably lower than in linear PLA of comparable molecular weight. According to proposed mechanism of cationic LA polymerization growing macromolecules are fitted with terminal ? OH and ? C(O)OSO₂CF₃ end‐groups. The presence of those groups allows efficient end‐to‐end cyclization. Cyclic nature of resulting PLA explains its higher thermal and hydrolytic stability as compared with linear PLA. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2650–2658, 2010     

New hydrophobic L‐amino acid salts: maleates of L‐leucine,L‐isoleucine and L‐norvaline

Crystals of maleates of three amino acids with hydrophobic side chains [L‐leucenium hydrogen maleate, C₆H₁₄NO₂⁺·C₄H₃O₄⁻, (I), L‐isoleucenium hydrogen maleate hemihydrate, C₆H₁₄NO₂⁺·C₄H₃O₄⁻·0.5H₂O, (II), and L‐norvalinium hydrogen maleate–L‐norvaline (1/1), C₅H₁₁NO₂⁺·C₄H₃O₄⁻·C₅H₁₂NO₂, (III)], were obtained. The new structures contain C₂²(12) chains, or variants thereof, that are a common feature in the crystal structures of amino acid maleates. The L‐leucenium salt is remarkable due to a large number of symmetrically non‐equivalent units (Z′ = 3). The L‐isoleucenium salt is a hydrate despite the fact that L‐isoleucine is a nonpolar hydrophobic amino acid (previously known amino acid maleates formed hydrates only with lysine and histidine, which are polar and hydrophilic). The L‐norvalinium salt provides the first example where the dimeric cation L‐Nva...L‐NvaH⁺ was observed. All three compounds have layered noncentrosymmetric structures. Preliminary tests have shown the presence of the second harmonic generation (SGH) effect for all three compounds.     

A water wire in L‐prolyl‐L‐serine monohydrate

Despite the extra functional group in the serine side chain, the crystal packing arrangement of the title compound {systematic name: (S)‐3‐hydroxy‐2‐[(S)‐pyrrolidine‐2‐carboxamido]propanoic acid monohydrate}, C₈H₁₄N₂O₄·H₂O, is essentially the same as observed for a series of L‐Pro‐L‐Nop peptide hydrates, where Nop is a strictly nonpolar residue. This is rendered possible by a monoclinic P2₁ packing arrangement with Z′ = 2 that deviates from orthorhombic P2₁2₁2₁ symmetry only for the seryl hydroxy groups, which form infinite O—H...O—H hydrogen‐bonded chains along the 5.3 Åa axis. At the same time, cocrystallized water molecules form parallel water wires.     

Novel Route to L‐Hexoses from L‐Ascorbic Acid: Asymmetric Synthesis of L‐Galactopyranose and L‐Talopyranose Preliminary Communication

A novel route with L ‐ascorbic acid as a single common starting material to asymmetric synthesis of all eight diastereomers of L ‐hexoses is described. Assessment of this new approach is demonstrated by the expedient synthesis of L ‐galactopyranose and L ‐talopyranose derivatives. Key steps involve stereoselective preparation of chiral (E)‐ and (Z)‐γ‐hydroxy‐α,β‐unsaturated esters and their stereo‐controlled dihydroxylation by OsO₄.     

Density Functional Theory Investigations on Vibrational Spectra,Molecular Structure,and Properties of the L‐Serine,L‐Cysteine,and L‐Aspartic Acid Molecules

In this paper, we present a thorough investigation of the conformational space to characterize all possible gas‐phase structures of the neutral L‐serine, L‐cysteine, and L‐aspartic acid molecules. A total of 120 trial structures were generated for L‐aspartic acid and 96 trial structures for L‐serine and L‐cysteine by combining all internal single‐bond rotamers. Various combinations of the Hartree–Fock and density functional theory/B3LYP methods with different bases were used to optimize all possible trial structures. The theoretical studies on the structure, harmonic vibrational spectra, and molecular properties of these amino acids are presented. The assignments of the calculated wave numbers resulting from potential energy distributions were performed using the VEDA 4 program to allow a good interpretation of the theoretical vibrational spectra of the title compounds. The fundamental harmonic frequencies were found to be in good agreement with data in the literature. A natural bond orbital analysis was performed to investigate the charge delocalization throughout the molecules for the three test compounds. Moreover, an extensive discussion of the highest occupied molecular orbital–lowest unoccupied molecular orbital energy gap as well as other related molecular properties are reported.     

Synthesis of poly(D,L‐lactic acid‐alt‐glycolic acid) from D,L‐3‐methylglycolide

D ,L ‐3‐Methylglycolide (MG) was synthesized via two step reactions with a good yield (42%). It was successfully polymerized in bulk with stannous octoate as a catalyst at 110 °C. The effects of the polymerization time and catalyst concentration on the molecular weight and monomer conversion were studied. Poly(D ,L ‐lactic acid‐co‐glycolic acid) (D ,L ‐PLGA50; 50/50 mol/mol) copolymers were successfully synthesized from the homopolymerization of MG with high polymerization rates and high monomer conversions under moderate polymerization conditions. ¹H NMR spectroscopy indicated that the bulk ring‐opening polymerization of MG conformed to the coordination–insertion mechanism. ¹³C NMR spectra of D ,L ‐PLGA50 copolymers obtained under different experimental conditions revealed that the copolymers had alternating structures of lactyl and glycolyl. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4179–4184, 2000     

Selective transport of D,L‐tryptophan through poly(L‐glutamic acid) membranes

Permeability coefficients of D ‐ and L ‐tryptophan (D ‐, L ‐Trp) were estimated for poly(L ‐glutamic acid) (PLG) membranes immersed in aqueous ethanol. D ‐tryptophan was selectively transported (the maximum permeability ratio was 2.6) depending on the amount and the species of crosslinking agent, and on the composition of immersing solvent. It is suggested that hydrogen bonding between uncharged permeates and carboxyl and/or amide groups of PLG is an essential factor for the selective transport. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1035–1041, 1999     

Short and Sweet: D‐Glucose to L‐Glucose and L‐Glucuronic Acid

The scarcity and expense of access to L ‐sugars and other rare sugars have prevented the exploitation of their biological potential; for example D ‐psicose, only recently available, has been recognized as an important new food. Here we give the definitive and cheap synthesis of 99.4% pure L ‐glucose from D ‐glucose which requires purification of neither intermediates nor final product other than extraction into and removal of solvents; a simple crystallization will raise the purity to >99.8%.     

L‐arabinitol‐based functional polyurethanes

Three new polymerizable diols, based on mono‐, di‐, and tri‐O‐allyl‐L ‐arabinitol derivatives, were prepared from L ‐arabinitol as versatile materials for the preparation of tailor‐made polyurethanes with varied degrees of functionalization. Their allyl functional groups can take part in thiol‐ene reactions, to obtain greatly diverse materials. This “click” reaction with 2‐mercaptoethanol was firstly studied on the highly hindered sugar precursor 2,3,4‐tri‐O‐allyl‐1,5‐di‐O‐trityl‐L ‐arabinitol, to apply it later to macromolecules. A polyurethane with multiple pendant allyl groups was synthesized by polyaddition reaction of 2,3,4‐tri‐O‐allyl‐L ‐arabinitol with 1,6‐hexamethylene diisocyanate, and then functionalized by thiol‐ene reaction. The coupling reaction took place in every allyl group, as confirmed by standard techniques. The thermal stability of the novel polyurethanes was investigated by thermogravimetric analysis and differential scanning calorimetry (DSC). This strategy provides a simple and versatile platform for the design of new materials whose functionality can be easily modified. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Electrocatalytic Oxidation of Sulfur Containing Amino Acids at Renewable Ni‐Powder Doped Carbon Ceramic Electrode: Application to Amperometric Detection L‐Cystine,L‐Cysteine and L‐Methionine

A sol‐gel technique was used here to prepare a renewable carbon ceramic electrode modified with nickel powder. Cyclic voltammograms of the resulting modified electrode show stable and a well defined redox couple due to Ni(II)/Ni(III) system with surface confined characteristics. The modified electrode shows excellent catalytic activity toward L ‐cystine, L ‐cysteine and L ‐methionine oxidation at reduced overpotential in alkaline solutions. In addition the antifouling properties at the modified electrode toward the above analytes and their oxidation products increases the reproducibility of results. L ‐cystine, L ‐cysteine and L ‐methionine were determined chronoamperometricaly at the surface of this modified electrode at pH range 9–13. Under the optimized conditions the calibration curves are linear in the concentration range 1–450 μM, 2–90 μM and 0.2–75 μM for L ‐cystine, L ‐methionine and L ‐cysteine determination, respectively. The detection limit and sensitivity were 0.64 μM, 3.8 nA/ μM for L ‐cystine, 2 μM, 5.6 nA/ μM for L ‐methionine and 0.2 μM and 8.1 nA/μM for L ‐cysteine. The advantageous of this modified electrode is high response, good stability and reproducibility, excellent catalytic activity for oxidation inert molecules at reduced overpotential and possibility of regeneration of the electrode surface by potential cycling for 5 minutes. Furthermore, the modified electrode has been prepared without using specific reagents. This sensor can be used as an amperometric detector for disulfides detection in chromatographic or flow systems.     

Sonication‐Induced Coiled Fibrous Architectures of Boc‐L‐Phe‐L‐Lys(Z)‐OMe

An ultra‐short peptide Boc‐L ‐Phe‐L ‐Lys(Z)‐OMe (Z=carbobenzyloxy) was shown to act as a highly efficient and versatile low molecular weight gelator (LMWG) for a variety of aliphatic and aromatic solvents under sonication. Remarkably, this simple dipeptide is not only able to form coiled fibres but also demonstrates self‐healing and thermal chiroptical switching behaviour. The formation of coiled assemblies was found to be influenced by the nature of the solvent and the presence of an additive. By exploiting these properties it was possible to modulate the macroscopic and microscopic properties of the organogels of this ultra‐short peptide, allowing the formation of highly ordered single‐domain networks of helical fibres with dimeric or alternatively fibre‐bundle morphology. The organogels were characterized by using FTIR, SEM, NMR and circular dichroism (CD) spectroscopy. Interestingly, CD experiments showed that the organogels of Boc‐L ‐Phe‐L ‐Lys(Z)‐OMe in aromatic solvents exhibit thermal chiroptical switching. This behaviour was hypothesized to stem from changes in the morphology of the gel accompanied by conformational transformation of the gelling agent. The fact that such a small peptide can demonstrate hierarchical assemblies and the possibility of controlling the self‐association is rather intriguing. The self‐healing ability, chiroptical switching and more importantly the formation of helical assemblies by Boc‐L ‐Phe‐L ‐Lys(Z)‐OMe under sonication, make this dipeptide an interesting example of the self‐assembly ability of ultra‐short peptides.     

An unexpected tetragonal unit cell for N‐(L‐2‐aminobutyryl)‐L‐serine

The title dipeptide {systematic name: (S)‐2‐[(S)‐2‐azaniumylbutanamido]‐3‐hydroxypropanoate}, C₇H₁₄N₂O₄, was synthesized in the anticipation that it would form nanoporous crystals with hexagonal symmetry. Single‐crystal X‐ray diffraction analysis showed that it had instead adopted a unit cell in the space group I4, similar to L‐alanyl‐L‐alanine [Fletterick, Tsai & Hughes (1970). J. Phys. Chem. 75 , 918–922]. The resulting packing arrangement has a high density for a peptide (1.462 Mg m⁻³), which is rendered possible by extensive disorder over two positions for the ethyl side chain of the 2‐aminobutyric acid fragment and over three positions for the serine side chain.<!?tpb=17.5pt>     

Cationic copolymerization of ε‐caprolactone and L,L‐lactide by an activated monomer mechanism

The cationic homopolymerization and copolymerization of L,L ‐lactide and ε‐caprolactone in the presence of alcohol have been studied. The rate of homopolymerization of ε‐caprolactone is slightly higher than that of L,L ‐lactide. In the copolymerization, the reverse order of reactivities has been observed, and L,L ‐lactide is preferentially incorporated into the copolymer. Both the homopolymerization and copolymerization proceed by an activated monomer mechanism, and the molecular weights and dispersities are controlled {number‐average degree of polymerization = ([M]₀ ? [M]_t)/[I]₀, where [M]₀ is the initial monomer concentration, [M]_t is the monomer concentration at time t, and [I]₀ is the initial initiator concentration; weight‐average molecular weight/number‐average molecular weight ～1.1–1.3}. An analysis of ¹³C NMR spectra of the copolymers indicates that transesterification is slow in comparison with propagation, and the microstructure of the copolymers is governed by the relative reactivity of the comonomers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 7071–7081, 2006     

Novel L‐DOPA‐Derived Poly(ester amide)s: Monomers,Polymers, and the First L‐DOPA‐Functionalized Biobased Adhesive Tape

The synthesis, characterization, and testing of a range of novel bio‐inspired L‐DOPA‐derived poly(ester amide)s is presented, using a widely applicable, straightforward chemistry. A model system is used to study and establish the monomer and polymer synthetic protocols, and to provide a set of optimum reaction conditions. It is further shown that fully biobased L‐DOPA‐containing adhesive tapes can be fabricated, which are positively evaluated in terms of their adhesive properties. The newly developed synthetic protocol constitutes a versatile platform for accessing and tailoring a plethora of relevant structures, including a variety of potentially biocompatible poly(ethylene glycol)‐based materials.

     

Block and random copolymerization of ε‐caprolactone,L‐, and rac‐lactide using titanium complex derived from aminodiol ligand

A series of di‐ and triblock copolymers [poly(L ‐lactide‐b‐ε‐caprolactone), poly(D,L ‐lactide‐b‐ε‐caprolactone), poly(ε‐caprolactone‐b‐L ‐lactide), and poly(ε‐caprolactone‐b‐L ‐lactide‐b‐ε‐caprolactone)] have been synthesized successfully by sequential ring‐opening polymerization of ε‐caprolactone (ε‐CL) and lactide (LA) either by initiating PCL block growth with living PLA chain end or vice versa using titanium complexes supported by aminodiol ligands as initiators. Poly(trimethylene carbonate‐b‐ε‐caprolactone) was also prepared. A series of random copolymers with different comonomer composition were also synthesized in solution and bulk of ε‐CL and D,L ‐lactide. The chemical composition and microstructure of the copolymers suggest a random distribution with short average sequence length of both the LA and ε‐CL. Transesterification reactions played a key role in the redistribution of monomer sequence and the chain microstructures. Differential scanning calorimetry analysis of the copolymer also evidenced the random structure of the copolymer with a unique T_g. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Enzyme‐catalyzed polymerization of L‐aspartate

Diethyl L ‐aspartate was polymerized by proteinase at a temperature between 4 and 50°C to yield poly(ethyl α,β‐L ‐aspartate). Bulk polymerization of diethyl L ‐aspartate, especially at temperatures between 40 and 50°C, and preferably using alkalophilic proteinase with the addition of a small amount of water gave poly(ethyl L ‐aspartate) as a white powder with a molecular weight of up to 3 600 and having about 88% α‐linkages.