PREPARATION OF PHARMACEUTICAL GRADE PURE ACID CHELATES

Abstract: Pharmaceutical grade amino acid chelates, free of interfering anions, are made by reacting an amino acid ligand with a metal member selected from the group consisting of elemental metals, metal oxides, metal hydroxides and metal carbonates in an aqueous environment wherein the ligand to metal mole ratio is at least 2:l and recovering the amino acid chelate from said aqueous environment by means of spray or drum drying. The reaction may be carried out in the presence of non-interfering anions such as anions from citric acid, ascorbic acid, acetic acid, carbonic acid and ammonium and alkali metal saltes thereof.

Description: This application relates to methods of preparing amino acid chelates which are essentially free of contaminating anions. More particularly, this invention relates to methods of preparing pharmaceutical grade amino acid chelates. This application is related to copending application Serial No. 738,065 filed May 24, l985 entitled "Pure Amino Acid Chelates", which will issue July 8, l986 as U.S. Patent 4,599,l52. Amino acid chelates are products resulting from the reaction of a polypeptide, dipeptide or naturally occurring alpha amino acids with a metal ion having a valence of two or more to form a ring structure wherein the positive electrical charges of the metal ion are neutralized by the electrons available through the carboxylate or free amino groups of the alpha amino acid. For convenience sake, metal ions having a valence of two or more will simply be referred to as divalent metal ions or divalent cations. Chelate formation through neutralization of the positive charges of the divalent metal ions can be through the formation of ionic, covalent or coordinate covalent bonding. In the past, amino acid chelates have generally been made by first dissolving a water soluble divalent metal salt in water. An amino acid ligand is then reacted with the metal ion at a ratio of ligand to metal of at least l:l and preferably at least 2:l. Often, the ligand is a hydrolysis product obtained by acid, base, base-acid, or base-acid-base hydrolysis. In such cases, the by products from hydrolysis, such as anions including chlorides, sulfates, phosphates and nitrates, and cations, including potassium and sodium remain in the hydrolysate. Reaction products of metal salts with proteins or with acid and/or base hydrolyzed proteins are taught in U.S. Patents, 2,960,406 (Carson); 3,396,l04 (Miller); 3,463,858 (Anderson); 3,775,l32 (Richards); 4,020,l58 (Ashmead et al); 4,l03,003 (Ashmead) and 4,l72,072 (Ashmead). In order for the reaction to proceed to completion with the formation of a cyclic chelate ring, the amino acid has had to be at a pH which is preferably above, or more basic than, the isoelectric point of the amino acid. For that reason, a certain amount of an alkali metal hydroxide, carbonate or bicarbonate has usually been added to the reaction mixture. Most water soluble salts used in making amino acid chelates have been either sulfates or chlorides. Using the sulfate ion as exemplary, the reaction has generally proceeded as follows: where M is a bivalent metal cation and R is a radical of a naturally occurring amino acid, dipeptide or polypeptide. It is apparent from the above formula that the sulfate anion is present in the reaction mixture in the form of sodium sulfate. U.S. Patent 2,877,253 teaches a product formed by the reaction of one mole of glycine with one mole of ferrous sulfate. That patent indicates that the sulfate anion becomes tied up in the reaction which allegedly forms a ferrous sulfate-glycine complex. Whether or not the sulfate actually participates in the reaction, or is present as the salt of an alkali metal, it nevertheless is present in the reaction mixture. Such products are difficult, if not impossible, to purify. While sodium sulfate, per se, is water soluble, the reaction between a metal sulfate and an amino acid is never carried to l00% completion and the sulfate ion is always present. The same holds true for the presence of chloride ions when utilizing a metal chloride salt for amino acid chelate preparation. Even if one were to attempt to wash out the excess sulfate or chloride ions with repeated washes, such an attempt could well be counter productive inasmuch as glycine and other amino acid ligands are also soluble to a degree. Hence, depending upon pH, the unreacted ligands or weakly held ligands could also be removed along with the unwanted anions. Other methods teaching the reaction of metal salts with ligands made up of protein, protein hydrolysates or amino acids are found in U.S. Patents 3,l68,54l (Hobbs); 3,969,540 (Jensen); 4,l67,564 (Jensen); 4,2l6,l43 (Ashmead) and 4,2l6,l44 (Ashmead). These salt by-products in amino acid chelates cause a variety of problems, not the least of which is cost. When salts are present, it is impossible to obtain an amino acid chelate with as high a metal content as with the salts absent. It also costs both to prepare and ship the unneeded salt. Moreover, these salts often pick up moisture and make handling of the product difficult. Most importantly, the health of humans, plants and animals receiving amino acid chelates is, in general, affected adversely by the presence of these salts. In U.S. Patent 4,599,152 (Ashmead), electrolytic methods for preparing anion free amino acid chelates are taught. While these methods are successful in preparing such pure forms of chelates, they are relatively expensive and time consuming. In GB-A-1 019 574, there is disclosed a method of isolating α - amino acids from cupric complex salts to obtain free amino acids. In order to demonstrate the disassociation reaction, cupric complex salts are first prepared by reacting either malachite, a natural form of a basic copper carbonate (CuCO₃. Cu(OH)₂), or an ammonium hydroxide extract of malachite, with an α - amino acid to prepare a crude cupric complex salt. This crude salt is then washed and dried to obtain a salt of sufficient purity from which the desired amino acids may be isolated. The patent is not directed to the preparation of chelates. The complex salts are merely prepared to illustrate the manner in which the free amino acids can be isolated. There is no disclosure of pharmaceutical grade, anion free, amino acid chelates or methods of producing such chelates. It is an object of the present invention to provide methods for the preparation of substantially pure amino acid chelates suitable for pharmaceutical use. It is also an object of this invention to provide methods of preparing amino acid chelates which are substantially free of interfering anions. Another object of this invention is to provide methods of preparing pharmaceutical grade amino acid chelates on a commercial scale. These and other objects may be attained by providing a method for the preparation of pharmaceutical grade amino acid chelates, substantially free of interfering anions, having a molecular weight which does not exceed 1500, which comprises reacting an anion free ligand selected from the group consisting of naturally occurring amino acids, or dipeptides, tripeptides or quadrapeptides thereof, in an aqueous environment, with a metal member selected from the group consisting of metal oxides, metal hydroxides and metal carbonates, wherein the metal has a valency of at least two and is a member selected from the group consisting of calcium, copper, iron, magnesium, manganese, zinc, molybdenum, cobalt,selenium and vanadium, and wherein the mole ratio of ligand to metal is between about 2:1 and 4:1 and recovering the amino acid chelate thus formed therefrom. There follows a detailed description of three embodiments of the invention. The basic principle of each method is to have the reactants produce the desired amino acid chelate and a substance such as water, carbon dioxide or hydrogen which can be easily removed from the chelate thus formed. The common feature in each method is also the key to its successful operation. That key is that amino acids function as acids. While that statement may not sound so profound, many researchers in the past have not considered amino acids, including dipeptides or polypeptides, to be sufficiently acid to be a propelling force for a reaction. Even though amino acids are very weak (pKa for glycine - 9.6), their acidity is great enough to cause the desired reactions to occur. In each case, the acidic amino acid proton is removed to form water or hydrogen and the nitrogen lone pair on the alpha amino acid nitrogen atom becomes available, along with the carboxylate ion, for bonding with the chosen metal. The metal will preferably be a member selected from the group consisting of calcium, magnesium, manganese, iron, copper, and zinc. However, other bioessential divalent metal ions such as selenium, cobalt, molybdenum and vanadium could also be utilized. Each embodiment to be described in detail is illustrated by the following general equations: wherein M is a divalent metal cation and R is a radical of a naturally occurring amino acid, or dipeptide, tripeptide or quadrapeptide moiety. The amino acid chelates formed from the methods of this invention will ordinarily contain between two to four ligands for each divalent metal ion regardless of the oxidation state or valence of the metal ion. The following structural formulae, in addition to those shown above in equations (l) - (5), are representative of the amino acid chelates which may be prepared by the methods of this invention: wherein M and R have the same meanings previously given. It will be noted in each of these formulae that the metal atom is completely neutralized and is carrying no net electrical charge. Again, it is to be stressed that these formulae are but representative and that other structures formed by the combination of a divalent metal ion with at least two amino acid ligands are also within the scope of the present invention. While, in theory, any suitable amino acid or protein hydrolysate ligand may be utilized as long as it is free of interfering anions, low molecular weight ligands are utilised such that the chelate, when formed, will have a molecular weight not in excess of 1500. Preferably, the molecular weights will not exceed 1000 and most preferably not be in excess of 500. Chelates having molecular weights of 300 and under are especially preferred as they are absorbed into biological tissues much more rapidly. These chelates, when orally ingested, are believed to be absorbed into humans and animals via a dipeptide absorption pathway. The intact absorption of dipeptide like amino acid chelates is documented in the book "Intestinal Absorption of Metal Ions and Chelates", by Ashmead, et al, published by Charles C. Thomas, Springfield, Illinois, 1985. Since the desired molecular weight of the amino acid chelates is limited, relative to the molecular weight of proteins, it is preferable to utilize as the amino acid source, purified amino acids selected from the group consisting of alanine, arginine, asparagine, aspartic acid, cysteine, cystine, glutamine, glutamic acid, glycine, histidine, hydroxyproline, isoleucine, leucine, lysine, methionine, ornithine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine and valine or dipeptides, tripeptides or quadrapeptides formed by any combinations of the above. The metals are preferably selected from the group consisting of calcium, magnesium, manganses, iron, copper and zinc. Other bioessential metal ions such as cobalt, molybdenum, vanadium or selenium may also be utilized. It is anticipated that it will generally be desirable to produce pharmaceutical grade metal chelates in which the metal is the same throughout the product. However, there may be instances when a mixture of metal chelates may be desired in which the chelates have amino acid ligands combined with two or more differing metals. Such mixtures can be produced by mixing of finished products or, if feasible, by utilizing a mixture of metal ions in the reaction with the amino acid ligands. The amino acid chelates prepared according to this invention are of a quality suitable for pharmaceutical use and are considered to be substantially pure. By pure or pharmaceutical grade is meant that they are free of the interfering metal salt anions referred to above, i.e. sulfates, nitrates, chlorides, etc. However, in the formation of these pure chelates, it is sometimes preferable to utilize weak organic acids or their alkali metal or ammonium salts. In addition, buffering agents may also be used. Typical of these are citrates, ascorbates, acetates, carbonates and bicarbonates. These reagents are soluble and may be removed from the precipitated amino acid chelates by washing, if desired. The term "interfering anion" is utilized throughout this disclosure to refer to the aforementioned sulfates, nitrates, chlorides, etc. These anions not only have the disadvantages already mentioned, they are also believed to interfere with metal ion absorption in biological systems. For example, the sulfate ion is believed to interfere with the absorption of amino acid chelates through the intestinal walls of humans and animals. Thus, it is apparent that there are multiple reasons for not wanting these anions present. On the other hand, anions such as citrates, ascorbates, bicarbonates and the like are not "interfering" and in fact may be beneficial. In the gastrointestinal tract, they may act as buffering agents protecting the amino acid chelates from destruction by the acidic media of the stomach thereby allowing them to pass into the small intestine intact where they are absorbed. Thus, for purposes of this disclosure, whenever the term "anion free", "pharmaceutical grade", "pure" or "substantially pure" or the equivalent are utilized in describing the amino acid chelates prepared by the processes of this invention, it is intended to mean free of anions of inorganic acids but is not meant to exclude added amounts of non-interferring anions of weak organic acids, hydroxyl ions or buffering agents. Metal Oxide Method Calcium, magnesium and zinc oxides react most readily with amino acids in aqueous solutions to form amino acid chelates. There is some evidence that other transition metals such as manganese, copper and iron also react to slowly form amino acid chelates. In the case of magnesium and calcium, the heat of reaction and change of pH indicate that the reaction has taken place. For example, in the case of magnesium, reacting with two moles of glycine, the pH of the glycine prior to magnesium oxide addition is about 6.0 and after the magnesium oxide is added the pH is raised to about 10.55 accompanied by the release of heat at about 6.5 kcal/mole. The reaction between metal oxides and amino acids occurs because the metal oxides are basic and the amino acids are weak acids. When they react, water and the chelate are the only products produced. There are no interfering anions entering into the reaction as impurities. In the case of alkaline amino acids, such as arginine, it may be necessary to add a reaction aid, such as citric acid, to enable the reaction to proceed. The desired amount of ligand is preferably first at least partially dissolved in an aqueous solution along with any reaction aid, buffering or stabilizing agent. To this solution is then added the desired amount of the metal oxide and the solution is allowed to stand until the reaction is completed. Because metal oxides are relatively insoluble in water, relatively large amounts of water are required to get the metal oxide into solution and the reaction can proceed slowly. Therefore, sufficient amounts of water and time should be provided to allow the reaction to proceed to completion and at least until there is no more any evolution of heat. Upon completion, the solution is preferably filtered to remove any unreacted metal oxide before spray drying. Because large amounts of water are required in each of the embodiments of this invention, spray or drum drying is required to remove water and obtain the pharmaceutical grade amino acid chelate product. EXAMPLE I A solution was prepared consisting of 12.4 parts by weight of glycine dissolved in 82.2 parts by weight water containing 1.0 part by weight sodium carbonate. To this solution was added 4.4 parts by weight zinc oxide. The molar ratio of glycine to zinc was 3:1. The reaction mixture was allowed to stand for about 14 hours and turned an opalescent color. After standing, the mixture was heated to about 70° C. and spray dried to obtain a zinc triglycine amino acid chelate powder having a melting point of about 209° C which turned red upon melting. The zinc content of the chelate was about 20% w. The dried product had a moisture content of about 7% w. and when reconstituted in water had a pH of about 8.0. Metal Hydroxide Method Almost all amino acid chelates can be prepared from the corresponding metal hydroxides; however, not all metal hydroxides are readily available or convenient to prepare. Calcium hydroxide can be prepared by the addition of calcium oxide to water, but hydroxides must be prepared from aqueous solutions containing metal salts, such as chlorides or nitrates, by being precipitated as metal hydroxides by the addition of sodium or potassium hydroxide. With magnesium, manganese and copper there is no problem in adding excess base. However, care must be taken with zinc and iron which react with excess hydroxide and are not effective in the production of chelates. The driving force for the production of amino acid chelates from corresponding metal hydroxides is the production of the weak electrolyte, water, as shown in equation (3) above. The basicity of the metal hydroxides varies greatly with the stronger bases reacting more readily with the weakly acidic amino acids. The basicity and solubility of the principal metal hydroxides is Ca>Mg>Mn>Fe>Zn>Cu. This is better illustrated by the following table: Metal Hydroxide Ksp pH Ca(OH)₂ 5.5 x 10⁻⁶ 12.35 Mg(OH)₂ 1.8 x 10¹¹ 10.52 Mn(OH)₂ 1.9 x 10⁻¹³ 9.86 Fe(OH)₂ 8.0 x 10⁻¹⁶ 9.07 Zn(OH)₂ 1.2 x 10⁻¹⁷ 8.68 Cu(OH)₂ 2.2 x 10⁻²⁰ 7.55 Because of the relative insolubility of metal hydroxides in water, sufficient water must be provided to bring the metal hydroxide into solution and it may be advantageous to allow a reaction mixture of metal hydroxides and amino acids to stand for a period of time to allow the reaction to proceed to completion. EXAMPLE II Calcium oxide is added to water forming a saturated calcium hydroxide solution having an initial pH of about 12.75. Glycine is added to this solution in an amount to provide two moles of glycine per mole of calcium. There is a noticable evolution of heat and the pH decreases, to about 10.55 in a matter of a few minutes after adding glycine. It is calculated that the evolution of heat amounts to about 7 kcal/mole for the reaction of one mole of calcium hydroxide with two moles of glycine. The solution is filtered and the filtrate is spray dried to produce a calcium diglycine amino acid chelate having a calcium content of about 14% w. In a different embodiment of this mode of preparation, it is not necessary to use excessive amounts of water. It has been found that a moist paste may be formed from a mixture of most metal hydroxides and amino acid ligands in the desired ratio. The paste is allowed to stand for a period of a few hours or even days. Again, there is a noticable evolution of heat and decrease in pH. The completed reaction is then diluted with water, filtered to remove unreacted insolubles and spray dried. Metal Carbonate Method Like their metal oxide and hydroxide counterparts, metal carbonates are relatively insoluble in aqueous solutions. Therefore, sufficient water must be utilized. However, even with small amounts of water there is generally sufficient solubility that an acid-base like reaction occurs between the metal carbonate and the amino acid ligands to cause the reaction to proceed according to equation (5) with the release of carbon dioxide and water. Therefore, the paste method described for the metal hydroxide embodiment is also applicable to the metal carbonate technique. The solubility of metal carbonates may be increased by the addition of carbon dioxide or a soluble carbonate, such as sodium carbonate, to the solution. Care must be taken not to lower the pH to the point that the reaction between the metal from the metal carbonate and amino acid does not take place. The following example illustrates this mode of preparation. EXAMPLE III A copper carbonate solution was prepared by adding 6.1 parts by weight of cupric carbonate to 80.9 parts by weight water. The copper carbonate was rendered more soluble by the addition of 4.9 parts by weight sodium carbonate. This solution was allowed to stand without agitation for about two hours. To this solution was added 8.2 parts by weight of glycine and the mixture was slowly stirred for about two more hours. A hazy blue solution was observed. To this solution was added 65 parts by weight of a 15% citric acid solution and the mixture was stirred until a clear blue solution was observed. This solution was spray dried resulting in a copper diglycine amino acid powder having a copper content of about 14% w. and which melted at about 14°C. Upon being reconstituted in water, the pH of the resulting solution was about 7.5. The above examples and descriptions of the various embodiments are sufficient to allow one skilled in the art to practice the invention with considerable latitude. Since virtually all chemical reactions are in equilibrium, it is desirable to choose those reaction conditions most suited to amino acid chelate preparation, i.e. which will drive the reaction to the right in the above equations. Thus, providing sufficient water to enable the metal oxides, metal hydroxides or metal carbonates to come into solution as the reaction proceeds in the formation of the amino acid chelates is an important aspect. If insufficient water is present to allow all of the metal compound to come into solution at once, the reaction may proceed more slowly. As a metal ion is reacted with amino acid ligands to form a chelate, additional metal ions from the metal source will come into solution. Hence, in the case of a paste, it may take considerable time for the reaction to proceed to equilibrium. As previously mentioned, the use of considerable amounts of water is not a disadvantage in the present invention. Many spray or drum drying techniques require that the solids content of a mixture to be dried be under 30% w. The suitability of any particular metal oxide, metal hydroxide or metal carbonate and the oxidation state of the metal ion may be determined without undue experimentation. In some instances, the higher oxidation state of a transition metal such as copper is required since the ion must be in at least a divalent state. However, in the case of iron, either ferric or ferrous ions may prove acceptable and the same holds true for manganese. Hence, no attempt has been made to limit the metal ions to any particular oxidation state as long as they are at least divalent. While glycine has been used in the above examples for purposes of illustration, the choice of the particular amino acid ligands may be tailored to the particular needs of a human, animal or plant. Certain amino acid chelates or combinations of amino acid chelates may be targeted to specific tissue sites as taught in copending patent application Serial No. 826,786 filed February 6, 1986.

Claims: A method for the preparation of pharmaceutical grade amino acid chelates, substantially free of interfering anions, having a molecular weight which does not exceed 1500, which comprises reacting an anion free ligand selected from the group consisting of naturally occurring amino acids, or dipeptides, tripeptides or quadrapeptides thereof, in an aqueous environment, with a metal member selected from the group consisting of metal oxides, metal hydroxides and metal carbonates, wherein the metal has a valency of at least two and is a member selected from the group consisting of calcium, copper, iron, magnesium, manganese, zinc, molybdenum, cobalt,selenium and vanadium, and wherein the mole ratio of ligand to metal is between about 2:1 and 4:1 and recovering the amino acid chelate thus formed therefrom. A method according to Claim 1 wherein the metal member is a metal oxide selected from the group consisting of calcium oxide, copper oxide, iron oxide, magnesium oxide, manganese oxide and zinc oxide. A method according to Claim 2 wherein the metal oxide is a member selected from the group consisting of calcium oxide, magnesium oxide and zinc oxide. A method according to claim 2 or 3 wherein said aqueous environment consists of sufficient water to bring said metal oxide at least partially into solution. A method according to Claim 4 wherein said reaction is carried out in the presence of an agent selected from the group consisting of citric acid, ascorbic acid, acetic acid, carbonic acid and ammonium and alkali metal salts thereof. A method according to Claim 4 wherein said amino acid chelate is recovered from said aqueous environment by means of spray drying or drum drying. A method according to Claim 1, wherein the metal member is a metal hydroxide selected from the group consisting of calcium hydroxide, copper hydroxide, iron hydroxide, magnesium hydroxide, manganese hydroxide and zinc hydroxide. A method according to Claim 7, wherein said aqueous environment consists of sufficient water to bring said metal hydroxide at least partially into solution. A method according to Claim 8 wherein said reaction is carried out in the presence of an agent selected from the group consisting of citric acid, ascorbic acid, acetic acid, carbonic acid and ammonium and alkali metals salts thereof. A method according to Claim 8 wherein said amino acid chelate is recovered from said aqueous environment by means of spray drying or drum drying. A method according to Claim 7, wherein said aqueous environment consists of only sufficient water to form a moist paste of said metal hydroxide and amino acid chelating ligand. A method according to Claim 11 wherein said moist paste is allowed to stand under ambient conditions for a period of time to allow the reaction to proceed to completion, after which it is diluted with water, filtered and the filtrate is spray dried or drum dried. A method according to Claim 1 wherein the metal member is a metal carbonate selected from the group consisting of calcium carbonate, copper carbonate, iron carbonate, magnesium carbonate, manganese carbonate and zinc carbonate. A method according to Claim 13 wherein said aqueous environment consists of sufficient water to bring said metal carbonate at least partially into solution. A method according to Claim 14 wherein said reaction is carried out in the presence of an agent selected from the group consisting of citric acid, ascorbic acid, acetic acid, carbonic acid and ammonium and alkali metal salts thereof. A method according to Claim 15 wherein said agent is a member selected from the group consisting of carbonic acid and ammonium and alkali metal salts thereof. A method according to Claim 13 wherein said metal carbonate is copper carbonate. A method according to Claim 14 wherein said amino acid chelate is recovered from said aqueous environment by means of spray drying or drum drying. A method according to Claim 13 wherein said aqueous environment consists of only sufficient water to form a moist paste of said metal carbonate and amino acid chelating ligand. A method according to Claim 19 wherein said moist paste is allowed to stand under ambient conditions for a period of time to allow the reaction to proceed to completion, after which it is diluted with water, filtered and the filtrate is spray dried or drum dried.

Inventors: ASHMEAD, HARVEY HAROLD
Applicants: ALBION INTERNATIONAL, INC.
Application year: 1987
Main classification: C07C227
IPC codes: C07C227, C07F9, C07C229, C07F3, C07C325, C07F11, C07F13, C07F1, C07F15, C07K14, C07K1, C07C51, C07C391, C07C67, C25B3