The primary source of hydroxyfatty acids is castor oil that contains ˜90% ricinoleic acid (12-hydroxy-cis-9-octadecenoic acid, 18:1 (OH)). Its hydroxyl functional group is highly valued since it provides a site for facile chemical derivatization. Unfortunately, castor plant surfaces harbor allergenic compounds that harm workers harvesting these plants. An additional concern is residual ricin, a toxic byproduct from castor oil production. Ricinoleic acid is used in high-volume products that include coatings, surfactants, polymers and cosmetics. Competitive chemical routes to ricinoleic analogs require multiple steps, use harsh chemical reagents, and generally lack selectivity. Furthermore, there is a need for a broadened spectrum of agro-based hydroxyl fatty acids that are more reactive primary substituents.
Unlike rinoleic acid, the ω-hydroxyfatty acids produced by the novel method described herein can be derived from a wide range of oil sources while also providing hydroxyl functional groups. Furthermore, ω-hydroxyfatty acids have primary instead of secondary hydroxyl groups which increase their reactivity for esterification and urethane synthesis. As such, they can replace ricinoleic acid and hydrostearic acid in certain applications requiring higher performance.
Owing to their unique attributes of new functional ω-hydroxy fatty acids and α,ω-dicarboxylic acids, they can be used in a wide variety of applications including as monomers to prepare next generation polyethylene-like polyhydroxyalkanoates, surfactants, emulsifiers, cosmetic ingredients and lubricants. They also can serve as precursors for vinyl monomers used in a wide-variety of carbon back bone polymers. Direct polymerization of ω-hydroxy fatty acids via condensation polymerization gives next generation polyethylene-like polyhydroxyalkanoates that can be used for a variety of commodity plastic applications. Alternatively, the polymers can be designed for use as novel bioresorbable medical materials. Functional groups along polymers provide sites to bind or chemically link bioactive moieties to regulate the biological properties of these materials. Another use of functional polyesters is in industrial coating formulations, components in drug delivery vehicles and scaffolds that support cell growth during tissue engineering and other regenerative medicine strategies.
2.1 Polymer Properties
Aliphatic polyesters are a group of biodegradable polymers that may be synthesized from readily renewable building blocks such as lactic acid and fatty acid-derived materials. Such polyesters can be synthesized via polycondensation reactions between aliphatic dicarboxylic acids with diols, transesterification of diesters with diols, polymerization of hydroxy acids, and ring-opening polymerization of lactones. Resulting products can be used in industrial and biomedical applications such as for controlled release drug carriers, implants and surgical sutures. Moreover, polyesters with functional groups along chains or in pendant groups are attracting increased interest since these groups can be used to regulate polymeric material properties. Furthermore, functional polymers can be post-modified to attach biologically active groups that allow the preparation of biomaterials for use in drug delivery system and as scaffold materials for tissue engineering. Polymers from ricinoleic acid have proved highly valuable for controlled drug delivery system. However, high purity ricinoleic acid is extremely expensive due to difficulties in its purification from the natural mixture.
2.2 Polymerization Reactions
Both chemical and enzymatic approaches have been explored to synthesize polyesters from diol/diacid and hydroxyacid monomers. Chemical synthetic methods often require harsh reaction conditions and metal catalysts that are difficult to remove subsequent to polymerization. Introduction of functional groups along chains or in pendant groups is difficult to accomplish by chemical methods due to the lack of selectivity of chemical catalysts and associated harsh reaction conditions. Typically, to incorporate functional groups in chains or pendant groups using a chemical catalyst, protection-deprotection steps are required. In other words, prior to polymerization, functional groups are protected and after polymerization a deprotection step is performed to liberate functional groups. Such methods required by chemical polymerization catalysts are tedious, costly, and produce undesirable by-products.
Compared to chemical synthesis, enzyme-catalyzed polymerizations can be performed under mild reaction conditions, using proteins that are metal-free and that have high enantio- and regioselectivities. Regioselectivity of enzyme-catalysts circumvents the need for protection of functional groups and allows the preparation of polymers from multifunctional monomers with control of branching.
In recent years it has been shown that lipase-catalyzed condensation polymerizations may be performed using non-activated diacids and diols. Resulting products were obtained in high yield and with useful molecular weights. Mahapatro et al., 2004, Macromolecules 37, 35-40, describes catalysis of condensation polymerizations between adipic acid and 1,8-octanediol using immobilized Lipase B from Candida antarctica (CALB) as the catalyst. Furthermore, effects of substrates and solvents on lipase-catalyzed condensation polymerizations of diacids and diols have been documented. See Olsson, et al., 2003, Biomacromolecules 4: 544-551. These publications demonstrate the feasibility of lipase-catalyzed polymerizations between diacids and diols.
Lipase-catalyzed polymerization of monomers containing functional groups including alkenes and epoxy groups to prepare polyesters has also been disclosed. Warwel et al. report the polymerization through transesterification reactions of long-chain unsaturated or epoxidized α,ω-dicarboxylic acid diesters (C18, C20 and C26 α,ω-dicarboxylic acid methyl esters) with diols using Novozym 435 as catalyst. See Warwel, 1995, et al. J. Mol. Catal. B: Enzymatic. 1, 29-35, which is hereby incorporated by reference herein. The α,ω-dicarboxylic acid methyl esters were synthesized by metathetical dimerization of 9-decenoic, 10-undecenioc and 13-tetradecenioc acid methyl esters, and polycondensation with 1,4-butanediol in diphenyl ether yielded the polyesters with molecular weight (Mw) of 7800-9900 g mol−1. Uyama et al. report polymerization of epoxidized fatty acids (in side-chain) with divinyl sebacate and glycerol to prepare epoxide-containing polyesters in good yields. See Uyama, et al., 2003, Biomacromolecules 4, 211-215, which is hereby incorporated by reference herein. Cis-9,10-epoxy-18-hydroxyoctadecanoic acid, isolated from suberin in the outer bark of birch, was used as a monomer to synthesize an epoxy-functionalized polyester by Novozym 435 catalysis (Biomacromolecules 8, 757-760 (2007)). Thus, prior work describes the preparation of functional polyesters using Novozym 435 catalysis. However, in each instance, monomer synthesis was performed either by (i) a chemical method that lacks selectivity, gives undesirable by-products and/or uses a toxic catalyst or (ii) an inefficient extraction of the monomer from a plant source.
2.3 Production of Monomers Prior to Polymerization
Currently, α,ω-dicarboxylic acids are almost exclusively produced by chemical conversion processes. However, the chemical processes for production of α,ω-dicarboxylic acids from non-renewable petrochemical feedstocks usually produces numerous unwanted byproducts, requires extensive purification and gives low yields (Picataggio et al., 1992, Bio/Technology 10, 894-898). Moreover, α,ω-dicarboxylic acids with carbon chain lengths greater than 13 are not readily available by chemical synthesis. While several chemical routes to synthesize long-chain α,ω-dicarboxylic acids are available, their synthesis is difficult, costly and requires toxic reagents. Furthermore, most methods result in mixtures containing shorter chain lengths. Furthermore, other than four-carbon α,ω-unsaturated diacids (e.g. maleic acid and fumaric acid), longer chain unsaturated α,ω-dicarboxylic acids or those with other functional groups are currently unavailable since chemical oxidation cleaves unsaturated bonds or modifies them resulting in cis-trans isomerization and other by-products.
Many microorganisms have the ability to produce α,ω-dicarboxylic acids when cultured in n-alkanes and fatty acids, including Candida tropicalis, Candida cloacae, Cryptococcus neoforman and Corynebacterium sp. (Shiio et al., 1971, Agr. Biol. Chem. 35, 2033-2042; Hill et al., 1986, Appl. Microbiol. Biotech. 24: 168-174; and Broadway et al., 1993, J. Gen. Microbiol. 139, 1337-1344). Candida tropicalis and similar yeasts are known to produce α,ω-dicarboxylic acids with carbon lengths from C12 to C22 via an ω-oxidation pathway. The terminal methyl group of n-alkanes or fatty acids is first hydroxylated by a membrane-bound enzyme complex consisting of cytochrome P450 monooxygenase and associated NADPH cytochrome reductase that is the rate-limiting step in the ω-oxidation pathway. Two additional enzymes, the fatty alcohol oxidase and fatty aldehyde dehydrogenase, further oxidize the alcohol to create ω-aldehyde acid and then the corresponding α,ω-dicarboxylic acid (Eschenfeldt et al., 2003, Appl. Environ. Microbiol. 69, 5992-5999). However, there is also a β-oxidation pathway for fatty acid oxidation that exists within Candida tropicalis. Both fatty acids and α,ω-dicarboxylic acids in wild type Candida tropicalis are efficiently degraded after activation to the corresponding acyl-CoA ester through the β-oxidation pathway, leading to carbon-chain length shortening, which results in the low yields of α,ω-dicarboxylic acids and numerous by-products.
Mutants of C. tropicalis in which the β-oxidation of fatty acids is impaired may be used to improve the production of α,ω-dicarboxylic acids (Uemura et al., 1988, J. Am. Oil. Chem. Soc. 64, 1254-1257; and Yi et al., 1989, Appl. Microbiol. Biotech. 30, 327-331). Recently, genetically modified strains of the yeast Candida tropicalis have been developed to increase the production of α,ω-dicarboxylic acids. An engineered Candida tropicalis (Strain H5343, ATCC No. 20962) with the POX4 and POX5 genes that code for enzymes in the first step of fatty acid β-oxidation disrupted was generated so that it can prevent the strain from metabolizing fatty acids, which directs the metabolic flux toward ω-oxidation and results in the accumulation of α,ω-dicarboxylic acids (FIG. 3). See U.S. Pat. No. 5,254,466 and Picataggio et al., 1992, Bio/Technology 10: 894-898, each of which is hereby incorporated by reference herein. Furthermore, by introduction of multiple copies of cytochrome P450 and reductase genes into C. tropicalis in which the β-oxidation pathway is blocked, the C. tropicalis strain AR40 was generated with increased ω-hydroxylase activity and higher specific productivity of diacids from long-chain fatty acids. See, Picataggio et al., 1992, Bio/Technology 10: 894-898 (1992); and U.S. Pat. No. 5,620,878, each of which is hereby incorporated by reference herein. Although the mutants or genetically modified C. tropicalis strains have been used for the biotransformation of saturated fatty acids (C12-C18) and unsaturated fatty acids with one or two double bonds to their corresponding diacids, the range of substrates needs to be expanded to produce more valuable diacids that are currently unavailable commercially, especially for those with internal functional groups that can be used for the potential application in biomaterials. The production of dicarboxylic acids by fermentation of saturated or unsaturated n-alkanes, n-alkenes, fatty acids or their esters with carbon number of 12 to 18 using a strain of the species C. tropicalis or other special microorganisms has been disclosed in U.S. Pat. Nos. 3,975,234; 4,339,536; 4,474,882; 5,254,466; and 5,620,878. However, all of the known processes for the preparation of dicarboxylic acids by means of yeast only give straight-chain saturated or unsaturated (containing one double bond) dicarboxylic acids with carbon number of 12 to 18. Furthermore, the resulting dicarboxylic acids are not readily purified and used for polymer synthesis. Thus, no process is known for the preparation of ricinoleic acid analogs containing internal functionality that may consist of double bonds, triple bonds, epoxide, secondary hydroxyl, Si—O—Si and other moieties, in which the functional groups are transferred into the resulting dicarboxylic acids without change, especially in large scale, and also no processes are known for the preparation of an ω-hydroxy fatty acids with double bond and secondary hydroxyl group.
In some instances it may be advantageous to polymerize long-chain ω-hydroxy fatty acids. These cannot be prepared using any described strain of Candida because the ω-hydroxy fatty acid is oxidized to form an α,ω-dicarboxylic acid. Furthermore, neither the general classes nor the specific sequences of the Candida enzymes responsible for the oxidation from ω-hydroxy fatty acids to α,ω-dicarboxylic acids have been identified. There is therefore a need in the art for methods to produce ω-hydroxy fatty acids from fatty acids by fermentation.