This invention relates to a process for purifying a highly unsaturated fatty acid or its ester. More particularly, the invention relates to a process for purifying eicosapentaenoic acid or its ester from a mixture containing the highly unsaturated fatty acid or its ester, by a silica gel column chromatography.
Highly unsaturated fatty acids or their esters such as eicosapentaenoic acid (hereafter called xe2x80x9cEPAxe2x80x9d) and docosahexaenoic acid (hereafter called xe2x80x9cDHAxe2x80x9d) contained in fish oil such as sardine oil have been used in the field of drugs and health foods, since they have a pharmacological effect in vivo and are effective against diseases such as arteriosclerosis obliterans and hyperlipemia.
Known processes for the purification of these unsaturated fatty acids include a urea-addition process, a precision distillation process, a column chromatography process and a supercritical fluid extraction process. These processes have been used singly or in combination for purifying highly unsaturated fatty acids.
However, the urea-addition process is not satisfactory as a purification process for removing eicosatetraenoic acid (hereafter called xe2x80x9cETAxe2x80x9d) or its ester, because of its being poor in urea-adduct forming ability. The precision distillation process needs an operation at high temperature, and may easily cause denaturation by polymerization or isomerization. In the purification of EPA or its ester, the process has a great difficulty in removing ETA or its ester, because ETA and EPA have the same carbon numbers and make a small difference in boiling point. Further, since the supercritical fluid extraction process deals with high-pressure fluid, it has a difficulty in installation of equipment and it is unsuitable for mass separation and purification on an industrial scale.
As a process for the purification of highly unsaturated fatty acids using the column chromatography method, Japanese Patent Kokai 5-222392 discloses a combination of precision distillation and reverse phase partition chromatography using a silica gel on which an octadecyl group is held. Further, Japanese Patent Kokai 9-151390 discloses a purification process using a carrier having a silver salt carried on silica gel. These purification processes can separate and purify EPA and its ester from mixtures of highly unsaturated fatty acids. The former process is excellent in the separation and purification, but it has the problem that the cost of filler becomes high because the octadecyl group is held on the silica gel surface. The latter process has the problem that the cost is high due to use of the silver salt, that the method for the preparation of the carrier carrying the silver salt is complicated, that the extraction operation is complicated with repeated agitation and filtration, and that the product may be contaminated with a silver compound by using a particular solvent.
Usually, ETA and its ester are contained in the raw material of EPA and its ester. However, ETA and its ester are impurities very difficult to separate and remove in the purification of EPA and its ester, as mentioned above.
Thus, there has been a demand for a process for the purification of eicosapentaenoic acid or its ester, by which the problems encountered in the prior art can be overcome and the contents of ETA and its ester can be remarkably reduced by selectively removing impurities.
Accordingly, an object of the present invention is to provide a process for selectively separating and purifying eicosapentaenoic acid or its ester effectively at low cost.
A further object of the present invention is to provide a process for selectively separating and removing ETA or its ester by a normal phase column chromatography using the silica gel under a specific condition.
The present invention provides a process for separating and purifying eicosapentaenoic acid or its ester from a mixture containing a highly unsaturated fatty acid or its ester, which comprises developing the mixture containing the highly unsaturated fatty acid or its ester with a mixed solvent of an ether solvent and a hydrocarbon solvent, using a column filled with a silica gel having a particle diameter of 1 to 100 micrometers.
In this process, the silica gel used as a filler of the column is required to have a particle diameter in the range of 1-100 micrometers. If the silica gel of small particle diameter is used, pressure loss will become large, but the amount of raw material loaded and the purifying capability will be increased due to its increased surface area. On the other hand, if the silica gel of large particle diameter is used, pressure loss will become small, but the amount of raw material loaded and the purifying capability will be reduced. Thus, it is required to choose the silica gel of suitable particle diameter in view of the amount of raw material loaded and purifying capability. For the selective separation of ETA or its ester, it is preferable to use the silica gel providing small pressure loss and having as large a specific surface area as possible. In the present invention, the form of silica gel may be spherical or fragment as far as the silica gel has the above-defined particle diameter, but spherical form is preferable. In addition, it is preferable to use porous silica gel having a pore size of not less than 4xc3x9710xe2x88x923 micrometers.
The amount of this silica gel used varies depending on the quantity of raw material to be processed and/or size of a column, etc., but it is usually within the range of 5-50 grams, preferably 10-40 grams per gram of raw material, i.e., the mixture comprising the highly unsaturated fatty acid or its ester.
In the present process, the mixture containing the highly unsaturated fatty acid or its ester is charged in the column filled with silica gel, and then developed with an organic solvent serving as a developing solvent. As the developing solvent, a mixed solvent with the highest purifying capability can be selected by varying the polarity of solvent. According to the present invention, the mixed solvent of a hydrocarbon solvent and an ether solvent is advantageous for the separation and purification of eicosapentaenoic acid or its ester.
The hydrocarbon solvents in the mixed solvent can include, but are not limited to, hydrocarbons of 5-8 carbons, preferably alkane hydrocarbons, for example, n-pentane, n-hexane, n-heptane and n-octane. The ether solvents can include, but are not limited to, dialkyl ethers, for example, diisopropyl ether, diethyl ether and methyl tert-butyl ether. In the present process, the developing solvent can be used in any combination of the hydrocarbon solvent and the ether solvent, but a mixed solvent of n-hexane (hereafter called xe2x80x9cn-Hexxe2x80x9d) and diisopropyl ether (hereafter called xe2x80x9cIPExe2x80x9d) is especially preferable. A mixing ratio of these solvents can be suitably decided so as to provide optimum conditions for the purification of the desired EPA or its ester. For a mixed solvent of n-Hex and IPE, for example, the mixing ratio is a volume ratio of n-Hex:IPE=90-99:10-1, preferably 95-99:5-1.
In the preferred embodiment of the present invention, eicosapentaenoic acid or its ester can be separated from the mixture of the highly unsaturated fatty acid or its ester by a column chromatography, which comprises developing said mixture with a mixed solvent of n-Hex and IPE as a developing solvent, using a column filled with silica gel having a particle diameter of 1-100 micrometers, at a linear velocity of 1-10 cm/min.
From another viewpoint, the present invention relates to a process for selectively removing ETA or its ester from a mixture comprising a highly unsaturated fatty acid or its ester containing eicosatetraenoic acid (ETA) or its ester, by the above-described column chromatography.
The column used in the process of the present invention is prepared by suspending silica gel in an organic solvent to slurry it, filling the slurried silica gel in a medium pressure column and flowing an organic solvent about 1-10 times the volume of the column to stabilize the column. A preferable organic solvent used when filling silica gel in slurry is acetone. It is preferable that an organic solvent used in the stabilization of column is the same as the developing solvent used in the purification.
The optimum amount of the mixture containing the highly unsaturated fatty acid or its ester which should be charged in the column can be varied depending on the composition ratio of each fatty acid in the raw material, but it is in the range of 2 to 20% by weight, preferably 3 to 15% by weight, based on the weight of silica gel in the column. Preferably, the raw material, i.e., the mixture containing the highly unsaturated fatty acid or its ester is dissolved in a developing solvent at most 5 times, e.g., twice the weight of the mixture, and thereafter this solution is flowed through the column.
For the column used in the process of the invention, preferable is a medium pressure column with the column length diameter ratio of 2 or more and the theoretical plate number of 15000-20000 plates/m. The pressure resistance of column may be in the range of 1-6 MPa, depending on the length and the linear velocity of column. The column is preferably operated at 10-30xc2x0 C., e.g., at room temperature. In the separation of ETA or its ester, it is preferable that the mixture of raw material is developed while applying the pressure so as to provide a linear velocity of 1-10 cm/min in the column.
The mixture containing the highly unsaturated fatty acid or its ester used as a raw material in the present process can be obtained from fish oils extracted from mackerels, sardines, codfishes, etc., but is not limited especially. This raw material mixture contains ETA, EPA, DHA and other components such as fatty acids of less than 20 carbons.
The invention is further illustrated by the following non-limiting examples and comparative examples.