Preparation and Structure of Oligolides from (R)-3-Hydroxypentanoic Acid and comparison with the hydroxybutanoic-acid derivatives: A small change with large consequences

Recyclable and (Bio)degradable Polyesters in a Circular Plastics Economy

Polyesters carrying polar main-chain ester linkages exhibit distinct material properties for diverse applications and thus play an important role in today's plastics economy. It is anticipated that they will play an even greater role in tomorrow's circular plastics economy that focuses on sustainability, thanks to the abundant availability of their biosourced building blocks and the presence of the main-chain ester bonds that can be chemically or biologically cleaved on demand by multiple methods and thus bring about more desired end-of-life plastic waste management options. Because of this potential and promise, there have been intense research activities directed at addressing recycling, upcycling or biodegradation of existing legacy polyesters, designing their biorenewable alternatives, and redesigning future polyesters with intrinsic chemical recyclability and tailored performance that can rival today's commodity plastics that are either petroleum based and/or hard to recycle. This review captures these exciting recent developments and outlines future challenges and opportunities. Case studies on the legacy polyesters, poly(lactic acid), poly(3-hydroxyalkanoate)s, poly(ethylene terephthalate), poly(butylene succinate), and poly(butylene-adipate terephthalate), are presented, and emerging chemically recyclable polyesters are comprehensively reviewed.

Ring Size as an Independent Variable in Cyclooligomeric Depsipeptide Antiarrhythmic Activity

Hit-to-lead studies employ a variety of strategies to optimize binding to a target of interest. When a structure for the target is available, hypothesis-driven structure-activity relationships (SAR) are a powerful strategy for refining the pharmacophore to achieve robust binding and selectivity characteristics necessary to identify a lead compound. Recrafting the three-dimensional space occupied by a small molecule, optimization of hydrogen bond contacts, and enhancing local attractive interactions are traditional approaches in medicinal chemistry. Ring size, however, is rarely able to be leveraged as an independent variable because most hits lack the symmetry required for such a study. Our discovery that the cyclic oligomeric depsipeptide ent-verticilide inhibits mammalian cardiac ryanodine receptor calcium release channels with submicromolar potency provided an opportunity to explore ring size as a variable, independent of other structural or functional group changes. We report here that ring size can be a critical independent variable, suggesting that modest conformational changes alone can dramatically affect potency.

Synthesis of novel adamantyl and homoadamantyl-substituted β-hydroxybutyric acids

Several new adamantyl and homoadamantyl-substituted [Formula: see text]-hydroxybutyric acids, 2-[2-(1-adamantyl)ethyl]-3-hydroxybutyric acid (2), 2-(3-homoadamantyl)-3-hydroxybutyric acid (3), and 2-(1-homoadamantyl)-3-hydroxybutyric acid (4), analogues of the 2-(1-adamantyl)-3-hydroxybutyric acid (1), have been prepared as mixtures of diastereoisomers using selective reduction of corresponding [Formula: see text]-keto esters or aldol condensation of the corresponding carboxylic acid and acetaldehyde. The rearrangement of adamantylmethyl and 3-homoadamantyl groups provided entry to both 3-homoadamantyl and 1-homoadamantyl-substituted hydroxy acids 3 and 4, respectively. The relative configurations of diastereoisomers 3 and 4 have been determined by NMR spectroscopy comparing the values of coupling constants. Adamantyl-substituted [Formula: see text]-hydroxybutyric acid 2 has also been prepared in enantiomerically pure form by Evan's asymmetric synthesis and the absolute configuration has been determined by X-ray crystallography. Contrary to the long-chain acid 2, the attempt to prepare short-chain hydroxy acids 1 and 4 by the same method failed indicating pronounced sensitivity of the used method to the vicinity of the bulky cage group.

Physiological importance of poly-(R)-3-hydroxybutyrates

Poly-(R)-3-hydroxybutyrates (PHB), linear polymers of (R)-3-hydroxybutyrate, are components of all biological cells in which short polymers (<200 monomer residues) are covalently attached to certain proteins and/or noncovalently associated with polyphosphates - inorganic polyphosphate (polyP), RNA, and DNA. The low concentrations, lack of unusual atoms or functional groups, and flexible backbones of this complexed PHB, referred to as cPHB, make them invisible to many analytical procedures; whereas other physical properties - water-insolubility, high intrinsic viscosity, temperature sensitivity, multiple bonding interactions with other molecules - make them requisite participants in vital physiological processes as well as contributors to the development of certain diseases.

Enzymatic hydrolysis of oligomeric models of poly-3-hydroxybutyrate

The mechanism of the enzymatic degradation of poly([R]-3-hydroxybutyrate) (PHB) was investigated by using well-defined model substrates, including both linear and cyclic [R]-3-hydroxybutyrate (3HB) and [R]-3-hydroxyvalerate (3HV) oligomers, with two different PHB depolymerases. The linear and cyclic oligomers containing from 2 to 10 repeating units were hydrolyzed in solutions of the depolymerase isolated from Aspergillus fumigatus and Alcaligenes faecalis, and the rates of hydrolysis and types of products formed were characterized. Both of the depolymerases catalyzed the hydrolysis of the cyclic oligomers (macrolides) which contained more than three 3HB and 3HV repeating units. The degradation reactions of the linear and cyclic 3HB oligomers with the A. fumigatus depolymerase gave similar ratios of monomer-to-dimer products, but PHB itself formed mostly monomer on hydrolysis, indicating that the enzymatic hydrolysis reactions occurred by different mechanisms for these different types of substrates. The results of this study conclusively show that at least the endo mode of polymer hydrolysis occurs with the two enzymes studied, while the A. fumigatus depolymerase was found to utilize both endo and exo modes of hydrolysis to efficiently degrade PHB and 3HB oligomers.

Detection, synthesis, structure, and function of oligo(3-hydroxyalkanoates): contributions by synthetic organic chemists

Two types of the biological macromolecules poly(R-3-hydroxyalkanoates) have been identified: the high-molecular-weight microbial storage material (sPHA) and a short-chain variety, consisting of butyrate and valerate residues, complexed with other biomacromolecules such as calcium polyphosphate or proteins (cPHB/PHV). While sPHA has attracted, and still enjoys, a lot of attention from numerous scientists around the world, research on cPHB and the structurally and functionally related polymalate (PMA) is still in its infancy. In this article, we present a review on the chemical synthesis, structure, function and interactions of monodisperse cPHAs, the oligo(3-hydroxyalkanoates), with emphasis on the butyrates (OHB); we report hitherto unpublished results on the enzymatic degradation of cPHB and PMA, on a new analytical method for HB/HV detection in biological samples, and on OHB-mediated Ca2+ transport through phospholipid bilayers of artificial vesicles; finally, we discuss possible mechanisms of ion transport through cell membranes, as caused by cPHB. The speculative--and provocative--question is asked whether the structurally simple PHAs may have evolved as storage materials and amphiphilic macromolecules before poly-peptides, -saccharides, and -nucleic acids, in the history of life, or under prebiotic conditions.

Bacterial and other biological systems for polyester production

Poly(3-hydroxybutyric acid) and other structurally related aliphatic polyesters from bacteria, referred to as polyhydroxyalkanoic acids, form biodegradable thermoplastics and elastomers that are currently in use, or being considered for use, in industry, medicine, pharmacy and agriculture. At present, they are produced by microbial fermentations; in the future, production will also be possible by in vitro methods or by agriculture using transgenic plants. Representatives from this highly diverse class of polyesters might be produced as commodity chemicals for bulk applications, and others as fine chemicals for special applications.

Low molecular weight complexed poly(3-hydroxybutyrate): a dynamic and versatile molecule in vivo

It is increasingly clear that poly(3-hydroxybutyrate) (PHB) is not just an inert storage polymer, confined to certain bacteria, but a ubiquitous, interactive, solvating biopolymer involved in important physiological functions. Low molecular weight PHB, complexed to other macromolecules (c-PHB), is widely distributed in biological cells, being found in representative organisms of nearly all phyla. Complexation modifies the physical and chemical properties of c-PHB, allowing it to pervade aqueous as well as hydrophobic regions of the cell, and as a result c-PHB can be found in cytoplasm and intracellular fluids as well as in membranes and lipoproteins. The lipidic homopolymer associates with other macromolecules primarily via its ester carbonyl oxygens, which can act as hydrogen-bond acceptors or as ligands for coordinate bonds to cations. The spacing of the electron-donating groups along the flexible backbone allows for multiple bonding interactions, and forms the basis for the ability of c-PHB to bind to proteins, or to form ion-conducting complexes with salts. The singular ability of c-PHB to dissolve salts and facilitate their transfer across hydrophobic barriers defines a potential physiological niche for c-PHB in cell metabolism.

Biodegradation of cyclic and substituted linear oligomers of poly(3-hydroxybutyrate)

Cyclic oligo(3-hydroxybutyrate), oligo(3-HB), was synthesized and purified, resulting in oligolides that contained three to seven (R)-3-hydroxybutyrate units (triolides up to heptolides). In addition, linear 3-HB octamers obtained as either tert-butyl or methyl esters were substituted with different end groups at the hydroxy end. The hydroxy terminus was replaced by either a benzyloxy, trifluoroacetoxy, crotonyloxy (S)-3-hydroxybutyryloxy, or fluorenylmethylcarbonyloxy (FMOC) group. P(3-HB) hairpin loops occurred on the surface of certain regions of the polymer, especially of lamellar crystallites. Cyclic 3-HB oligomers provide a model system for these loops. It is assumed that they provide attachment points for the depolymerizing enzymes. All of the (R)-oligolides tested were degraded except the (R)-triolide. Triolides were not degraded, suggesting that enzymatic attack was prevented presumably by steric hindrance on the rigid ring system. Unsubstituted linear octamers were degraded. Biodegradation was prevented when the hydroxy terminus was protected by the FMOC group, but was not dependent on a free hydroxy terminal group; all other protecting groups did not prevent degradation. Substitution of the carboxy end of a methyl or tert-butyl ester group did not influence biodegradation.