Preparation of New α-Hydroxy Acids Derived from Amino Acids and Their Corresponding Polyesters

2-Hydroxyisovalerate production by Klebsiella pneumoniae

2-Hydroxyisovalerate is a valuable chemical that can be used in the production of biodegradable polyesters. In nature, it was only produced at a very low level by Lactococcus lactis. 2-Ketoisovalerate is an intermediate metabolite of the branched-chain amino acid biosynthesis pathway, and Klebsiella pneumoniae ΔbudAΔldhA (Kp ΔbudAΔldhA) was a 2-ketoisovalerate producing strain. In this research, 2-hydroxyisovalerate was identified as a metabolite of Kp ΔbudAΔldhA, and its synthesis pathway was revealed. It was found that 2-ketoisovalerate and 2-hydroxyisovalerate were produced by Kp ΔbudA and Kp ΔbudAΔldhA, but not by Kp ΔbudAΔldhAΔilvD in which the 2-ketoisovalerate synthesis was blocked. budA, ldhA, and ilvD encode α-acetolactate decarboxylase, lactate dehydrogenase, and dihydroxy acid dehydratase, respectively. Thus, it was deduced that 2-hydroxyisovalerate was synthesized from 2-ketoisovalerate. Isoenzymes of ketopantoate reductase PanE, PanE2, and IlvC were suspected of being responsible for this reaction. Kinetic parameters of these enzymes were detected, and they all hold the 2-ketoisovalerate reductase activities. PanE and PanE2 use both NADH and NADPH as co-factors. While IlvC only uses NADH as a co-factor. Over-expression of panE, panE2, or ilvC in Kp ΔbudAΔldhA all enhanced the production of 2-hydroxyisovalerate. Accordingly, 2-hydroxyisovalerate levels were reduced by knocking out panE or panE2. In fed-batch fermentation, 14.41 g/L of 2-hydroxyisovalerate was produced by Kp ΔbudAΔldhA-panE, with a substrate conversion ratio of 0.13 g/g glucose.

A light-controlled biocatalytic system for precise regulation of enzymatic decarboxylation

A light-controlled biocatalytic one-pot system is developed, which enables precise regulation of gene expression and photocatalysis by illumination and yields high conversion and stereoselectivity.

Biobased High-Performance Aromatic-Aliphatic Polyesters with Complete Recyclability

The development of high-performance recyclable polymers represents a circular plastics economy to address the urgent issues of plastic sustainability. Herein, we design a series of biobased seven-membered-ring esters containing aromatic and aliphatic moieties. Ring-opening polymerization studies showed that they readily polymerize with excellent activity (TOF up to 2.1 × 10⁵ h^-1) at room temperature and produce polymers with high molecular weight (M_n up to 438 kg/mol). The variety of functionalities allows us to investigate the substitution effect on polymerizability/recyclability of monomers and properties of polymers (such as T_gs from -1 to 79 °C). Remarkably, a stereocomplexed P(M2) exhibited significantly increased T_m and crystallization rate. More importantly, product P(M)s were capable of depolymerizing into their monomers in solution or bulk with high efficiency, thus establishing their circular life cycle.

A Guideline for the Synthesis of Amino-Acid-Functionalized Monomers and Their Polymerizations

Amino acids have emerged as a sustainable source for the design of functional polymers. Besides their wide availability, especially their high degree of biocompatibility makes them appealing for a broad range of applications in the biomedical research field. In addition to these favorable characteristics, the versatility of reactive functional groups in amino acids (i.e., carboxylic acids, amines, thiols, and hydroxyl groups) makes them suitable starting materials for various polymerization approaches, which include step- and chain-growth reactions. This review aims to provide an overview of strategies to incorporate amino acids into polymers. To this end, it focuses on the preparation of polymerizable monomers from amino acids, which yield main chain or side chain-functionalized polymers. Furthermore, postpolymerization modification approaches for polymer side chain functionalization are discussed. Amino acids are presented as a versatile platform for the development of polymers with tailored properties.

Amino acid derived biopolymers: Recent advances and biomedical applications

Over the past few years, amino acids (AA) have emerged as promising biomaterials for the synthesis of functional polymers. Owing to the diversity of functional groups in amino acids, various polymerization methods may be used to make a wide range of well-defined functional amino-acid/peptide-based optically active polymers with varying polymer lengths, compositions, and designs. When incorporated with chirality and self-assembly, they offer a wide range of applications and are particularly appealing in the field of drug delivery, tissue engineering, and biosensing. There are several classes of these polymers that include polyamides (PA), polyesters (PE), poly(ester-amide)s (PEA)s, polyurethanes (PU)s, poly(depsipeptide)s (PDP)s, etc. They offer the ability to control functionality, conjugation, crosslinking, stimuli responsiveness, and tuneable mechanical/thermal properties. In this review, we present the recent advancements in the synthesis strategies for obtaining these amino acid-derived bio-macromolecules, their self-assembly properties, and the wealth of prevalent applications.

Substituted glycolides from natural sources: preparation, alcoholysis and polymerization

Herein we present a comparative study of substituted glycolides MeGL, iPrGL, iBuGL, BnGL, PhGL and MePhGL, synthesized from natural sources and polymers therefrom.

Symmetrical substituted glycolides: methodology and polymerization

Poly(substituted glycolide)s have emerged during the past decades to create extraordinary breakthroughs in a wide range of therapeutic applications due to superior properties as an alternative to PLA and PLGA systems.

Understanding the ring-opening polymerisation of dioxolanones

Eliminating small molecules from dioxolane rings affords isotactic poly(mandelic acid), with competing chain transfer overcome through dynamic vacuum polymerisation.

A synthetic pathway for the production of 2-hydroxyisovaleric acid in Escherichia coli

Synthetic biology, encompassing the design and construction of novel artificial biological pathways and organisms and the redesign of existing natural biological systems, is rapidly expanding the number of applications for which biological systems can play an integral role. In the context of chemical production, the combination of synthetic biology and metabolic engineering approaches continues to unlock the ability to biologically produce novel and complex molecules from a variety of feedstocks. Here, we utilize a synthetic approach to design and build a pathway to produce 2-hydroxyisovaleric acid in Escherichia coli and demonstrate how pathway design can be supplemented with metabolic engineering approaches to improve pathway performance from various carbon sources. Drawing inspiration from the native pathway for the synthesis of the 5-carbon amino acid l-valine, we exploit the decarboxylative condensation of two molecules of pyruvate, with subsequent reduction and dehydration reactions enabling the synthesis of 2-hydroxyisovaleric acid. Key to our approach was the utilization of an acetolactate synthase which minimized kinetic and regulatory constraints to ensure sufficient flux entering the pathway. Critical host modifications enabling maximum product synthesis from either glycerol or glucose were then examined, with the varying degree of reduction of these carbons sources playing a major role in the required host background. Through these engineering efforts, the designed pathway produced 6.2 g/L 2-hydroxyisovaleric acid from glycerol at 58% of maximum theoretical yield and 7.8 g/L 2-hydroxyisovaleric acid from glucose at 73% of maximum theoretical yield. These results demonstrate how the combination of synthetic biology and metabolic engineering approaches can facilitate bio-based chemical production.

A simple ternary ion-pair complexation protocol for testing the enantiopurity and the absolute configurational analysis of acid and ester derivatives

The ternary ion-pair complexation protocol for rapid testing of the enantiopurity and the assignment of absolute configurations of various acid and ester derivatives.

Poly(α-hydroxy acid)s and poly(α-hydroxy acid-co-α-amino acid)s derived from amino acid

Polyesters derived from the α-hydroxy acids, lactic acid, and glycolic acid, are the most common biodegradable polymers in clinical use. These polymers have been tailored for a range of applications that require a physical material possessing. The physical and mechanical properties of these polymers fit the specific application and also safely biodegrade. These polymers are hydrophobic and do not possess functional side groups. This does not allow hydrophilic or hydrophobic manipulation, conjugation of active agents along the polymer chain, etc. These manipulations have partly been achieved by block copolymerization with, for example, poly(ethylene glycol), to obtain an amphiphilic copolymer. The objective of this review is to survey PLA functional copolymers in which functional α-hydroxy acids derived from amino acids are introduced along the polymer chain, allowing endless manipulation of PLA. Biodegradable functional polyesters are one of the most versatile biomaterials available to biomedical scientists. Amino acids with their variable side chains are ideal candidates for synthesizing such structural as well as stereochemically diverse polymers. They render control over functionalization, conjugation, crosslinking, stimulus responsiveness, and tunable mechanical/thermal properties. Functionalized amino acid derived polyesters are widely used, mainly due to advancement in ring opening polymerization (primarily O-carboxyanhydride mediated). The reaction proceeds under milder conditions and yields high molecular weight polymers. We reviewed on advances in the synthetic methodologies for poly-α-hydroxy esters derived from amino acids with appropriate recent examples.

A Functionalized Cyclic Lactide Monomer for Synthesis of Water-Soluble Poly(Lactic Acid) and Amphiphilic Diblock Poly(Lactic Acid)

Biodegradable and bioabsorbable poly(lactic acid)s are one of the most important biomedical materials. However, it is difficult to introduce the functional groups into poly(lactic acid)s in order to improve their hydrophilicity and degradation rate. Here the authors describe the synthesis of functionalized cyclic lactide monomer 3,6-bis(benzyloxymethyl)-1,4-dioxane-2,5-dione (BnLA) using an advanced synthetic route. Water-soluble hydroxyl-functionalized homopoly(lactic acid) (P(OH)LA) is synthesized via ring-opening polymerization (ROP) of BnLA, followed by a hydrogenolytic deprotection reaction. Amphiphilic diblock poly(lactic acid) (P(OH)LA-PLA) is synthesized via ROP of DL-lactide using PBnLA as an initiator, followed by a hydrogenolytic deprotection reaction. P(OH)LA-PLA is able to form polymeric micelles with the diameter of sub-100 nm.

Enzymatic preparation of D-phenyllactic acid at high space-time yield with a novel phenylpyruvate reductase identified from Lactobacillus sp. CGMCC 9967

An NADH-dependent phenylpyruvate reductase (LaPPR) was identified through screening the shotgun library of Lactobacillus sp. CGMCC 9967. It belongs to D-3-phosphoglycerate dehydrogenase (PGDH) subfamily of 2-hydroxy acid dehydrogenase superfamily. LaPPR was stable at pH 6.5 and 30 °C, with a half-life of 152 h. LaPPR has a substrate preference towards aromatic to aliphatic keto acids, and various keto acids could be reduced into D-hydroxy acids with excellent enantioselectivity (>99%). By construction the coexpression system with glucose dehydrogenase, as much as 100 g L(-1) phenylpyruvic acid was asymmetrically reduced into D-phenyllactic acid with 91.3% isolation yield and 243 g L(-1) d(-1) productivity. The results suggest that LaPPR is a promising biocatalyst for the efficient synthesis of optically pure D-phenyllactic acid.

Biocontrolled formal inversion or retention of L-α-amino acids to enantiopure (R)- or (S)-hydroxyacids

Natural L-α-amino acids and L-norleucine were transformed to the corresponding α-hydroxy acids by formal biocatalytic inversion or retention of absolute configuration. The one-pot transformation was achieved by a concurrent oxidation reduction cascade in aqueous media. A representative panel of enantiopure (R)- and (S)-2-hydroxy acids possessing aliphatic, aromatic and heteroaromatic moieties were isolated in high yield (67-85 %) and enantiopure form (>99 % ee) without requiring chromatographic purification.

Cytocompatibility of novel extracellular matrix protein analogs of biodegradable polyester polymers derived from α-hydroxy amino acids

One of the challenges in regenerative medicine is the development of novel biodegradable materials to build scaffolds that will support multiple cell types for tissue engineering. Here we describe the preparation, characterization, and cytocompatibility of homo- and hetero-polyesters of α-hydroxy amino acid derivatives with or without lactic acid conjugation. The polymers were prepared by a direct condensation method and characterized using gel permeation chromatography, (1)H-nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, optical activity, and solubility. The surface charge of the polymers was evaluated using zeta potential measurements. The polymers were coated onto glass cover slips followed by characterization using nano-surface profiler, thin film reflectometry, and atomic force microscopy (AFM). Their interaction with endothelial and neuronal cells was assessed using adhesion, proliferation, and differentiation assays. Of the characterized polymers, Poly-HOVal-LA, but not Poly-(D)HOPhe, significantly augmented nerve growth factor (NGF)-induced neuronal differentiation of the PC12 pheochromcytoma cells. In contrast, Poly-HOLeu increased by 20% the adhesion of endothelial cells, but did not affect PC12 cell differentiation. NGF-induced Erk1/2 phosphorylation in PC12 cells grown on the different polymers was similar to the effect observed for cells cultured on collagen type I. While no significant association could be established between charge and the differentiative/proliferative properties of the polymers, AFM analysis indicated augmentation of NGF-induced neuronal differentiation on smooth polymer surfaces. We conclude that overall selective cytocompatibility and bioactivity might render α-hydroxy amino acid polymers useful as extracellular matrix-mimicking materials for tissue engineering.

Well-defined Poly(lactic acid)s Containing Poly(ethylene glycol) Side-chains

Poly(ethylene glycol) (PEG) side-chain functionalized lactide analogues have been synthesized in four steps from commercially available L-lactide. The key step in the synthesis is the 1,3-dipolar cycloaddition between PEG-azides and a highly strained spirolactide-heptene monomer, which proceeds in high conversions. The PEG-grafted lactides analogues were polymerized via ring-opening polymerization using triazacyclodecene as organocatalyst to give well-defined tri- and hepta-(ethylene glycol)-poly(lactide)s (PLA) with molecular weights above 10 kDa and polydispersity indices between 1.6 and 2.1. PEG-poly(lactide) (PLA) with PEG chain M(n) 2000 was also prepared but GPC analysis showed a bimodal profile indicating the presence of starting macromonomer. Cell adhesion assays were performed using MC3T3 E-1 osteoblast-like cells demonstrating that PEG-containing PLA reduces cell adhesion significantly when compared to unfunctionalized PLA.