Chemical and Pharmacological Studies on Curcuminoids
P. Venugopalan* and T. V. Deepthi
Department of Chemistry, Sree Neelakanta Government Sanskrit College, Pattambi, Palakkad -679 306. Kerala
*Corresponding Author E-mail: venugpamrita@gmail.com
ABSTRACT:
Curcuminoids, the active ingredients of turmeric, possess innumerous physiological activities. Structural aspects and biological activities and pharmacological studies of curcuminoids have been critically reviewed. The photochemistry and analytical applications of curcuminoids were also included.
KEYWORDS: Turmeric, Curcuminoids, antioxidant, anti-inflammatory agent.
Curcuminoids, a group of structurally related compounds, are the most important active chemical constituents present in the roots and shoots of the herbaceous plant Curcuma longa and several other related Curcuma species of the family Zingiberaceae. Although confusions regarding the nomenclature of various Curcuma species still exist, the plant Curcuma longa Linn., popularly known as turmeric is the principal source of curcuminoids. Chemically they belong to1,7-diarylheptanoids which comprise a distinct class of natural products of wide pharmacological interest[1].
Ethnologically turmeric occupies an important position in the life of Indian peoples as much as it forms an integral part of various food preparations, rituals and ceremonies. Turmeric ( Haridra in Sanskrit ) is ascribed for its aromatic, stimulant and carminative properties in Hindu ethics and several indigenous medicinal preparations make use of it in one or other form. Due to the strong antiseptic properties, turmeric has been reputed as a common remedy for all kinds of poisonous affections. Local application of a paste of turmeric with slaked lime is an household remedy for muscular pains, inflamed joints, sprains and swelling caused by injury. The traditional Indian systems of medicine such as Ayurveda,Unani,Sidha, etc. claims the use of turmeric against biliary disorders, anorexia, coryza, cough, diabetic wounds, hepatic disorders, rheumatism and sinusitis[2,5].
Apart from the religious and medicinal uses, turmeric has also been used for dyeing fabrics and as a coloring agent in food and drugs[6]. It is not the purpose of the review to go into such details as there exist several excellent and authentic reports on these aspects of turmeric[2,3].This account is mainly centered on the isolation of curcuminoids from turmeric, their synthesis, structural investigations, applications and other related chemical and biochemical studies appeared in the general literature
2. TURMERIC PRODUCTS:
India is the largest producer and exporter of the spice turmeric in the world, hence the name Indian Saffron. In general, the quality of turmeric which varies according to its source is judged by the intensity of its yellow colour which in turn depends on the curcuminoid content in the raw turmeric. It has been reported that the ‘Alleppy variety` is the richest in curcuminoids[2].
After harvesting the rhizomes of turmeric are boiled which allows the starch to gelatinize and helps in producing a product of fairly uniform colour due to the diffusion of the yellow pigments (curcuminoids) from individual cells into the surrounding tissues. The rhizomes are then dried in the sun for 10 to 15 days which makes them hard and brittle. The appearance is further improved by smoothening and polishing the outer surface by manual or mechanical rubbing and the material is then sold as such for all its commercial uses[2].
The most important chemical products derived from raw turmeric are turmeric powder, turmeric oleoresin, turmeric oil and curcuminoids.
Turmeric oleoresin[8] is obtained (~15%) by the extraction of the processed turmeric tubers with acetone or dichloromethane and contain both the essential oil and colouring matter along with some resinous materials. Curcuminoids, the colouring matter of turmeric forms about one-third of a good quality oleoresin. For commercial use, turmeric oleoresins is usually mixed with a suitable solubiliser such as propylene glycol, polysorbate or a vegetable oil. The product is priced according to its colour and extensively used as a natural food colourant.
The volatile content of turmeric ranges from 2.5 to7.2 % of the spice and it can be isolated by the steam distillation of turmeric powder[9-12]. Turmeric oil is, however, not as much valued as the colouring matter and its aroma and flavor qualities are due to a mixture of terpenoids. Turmerone (1) and ar-turmerone (2) form about 50 % of the oil. The rest is made up largely of zingiberene, cineole, d-sabinene and d-α-phellandrene12.
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6- methyl-2-(4-methylcyclohexa-1,3-dienyl hept-5-en-3-one |
6- methyl-2-p-tolylhept-5-en-3-one |
The dried rhizomes of Curcuma longa usually contain 1-5% of curcuminioids. They can easily be isolated from the powdered rhizomes by extraction with organic solvents. Although no standard procedures for the extraction is available, chloroform, ethanol and acetone are reported as good extractants[2,13-16]. However, butanol, diethyl ether, benzene, ethylene dichloride and petroleum ether have also been used as solvents.
3. CONSTITUTION OF CURCUMINOIDS:
The earliest report on the constituents of turmeric yellow is by Vogel and Pelletier[17] in 1815, who reported the isolation of the colouring matter in a crude form by the partial evaporation of turmeric extracts and named the pigment as curcumin. In 1870, Daube[18] isolated curcumin in the crystalline form. Later Jackson[19] and Perkins and Phillips[20] obtained curcumin by precipitating through its lead salt from an alcoholic extract of turmeric. The molecular formula C21H20O6 was first suggested by Ciamicin and Silber[21] and the structure was elucidated by Lampe and co-workers[22] in 1910, who later completed a synthesis[23].
In 1953, Srinivasan[13] (using column chromatography over silica gel) showed the presence of three well defined yellow compounds along with other resinous materials in the crude pigment extracted from turmeric. These three compounds were tentatively identified on the basis of elemental analysis and methoxy values as curcumin I (3) ,diferuloylmethane or simply as curcumin in the early reports, curcumin II (4) or feruloyl-p-hydroxycinnamoylmethane and curcumin III (5) or bis-(p-hydroxycinnamoyl)methane. In 1973, the structure of curcuminoids were re-examined by Roughley and Whiting[24] who confirmed the above structures ( 3,4,5 ) on the basis of spectral evidence and by synthesis
There are several reports on the separation of the three curcumoinoids from the crude extract using different chromatographic techniques[1]. They can easily and effectively be separated by column chromatography using different mobile phases including chloroform, acetone, diethyl ether, benzene, etc.Curcumin I is the major component ( ~40 % ) and eluted first from the column followed by curcumin II ( ~ 16 % ) and curcumin III ( ~ 10 %).
4. SYNTHESIS OF CURCUMINOIDS:
Isolation and separation of pure curcuminods from the plant material is time consuming and laborious. However, the ingenious work of synthetic organic chemists developed easy methods for the synthesis of these compounds.
The first reported synthesis of curcumin was in 1913 by Lampe and Milobedzka[23] through the condensation of carbomethoxyferuloyl chloride and acetoacetic ester as illustrated in Scheme 1.
An improved method of synthesis was reported by Povoloni, et al [25] which involved condensation of vanillin with acetylacetone in presence of boric oxide at elevated temperatures (Scheme 2). The curcumin obtained was purified through its lead salt. This method was later modified by Pabon who obtained curcumin in ~80 % yield by condensing vanillin with acetylacetone at room temperatures using n-butyl amine as the condensing agent. The procedure of Pabon[26] has been developed as a general procedure to synthesise curcuminoids and other 1,7-diarylheptanoids from acetylacetone and suitable aromatic aldehydes[24]. However, it is to be pointed out that this method also gave appreciable quantities (5-10 %) of mono condensation product (6) along with the major condensation product, 1,7- diarylheptanoids (7). The relative yield of two products can be reversed by chilling the reaction mixture.
5. STRUCTURE OF CURCUMINOIDS:
Chemical degradation studies
The structure of curcumin as diferuloylmethane was established only in 1910 although isolated as early as 1815[17].
On boiling with aqueous alkali, curcumin yielded vanillin and ferulic acid (8) while fusion with alkali gave protocatechuic acid (9). On oxidation with KMnO4, vanillin is formed. It also formed an isoxazole derivative (10) with hydroxylamine indicating the presence of a β-diketone system. These reactions are depicted in Scheme 3.
Spectral characterization
Electronic, Infra red, NMR and mass spectral data of curcuminoids have been reported[2,24]. The existence of the compounds entirely in the intramolecularly hydrogen bonded enol form with trans orientation of the alkenyl groups was established from the spectral analysis.
Crystallographic data
The three dimensional molecular structure of curcumin was determined by X-ray crystallographic methods. The results obtained were quite unexpected[5,27]. In contrast to other β-diketones where the enolic hydrogen preferentially bonded to one oxygen, the enolic hydrogen in curcumin is equally associated with the two oxygens. There are no significant difference in the C-C or the C-O bonds in the enol ring giving a pseudo-aromatic character to the chelate ring system.
Conjugation between one of the aromatic enol groups in curcumin and the pseudo-aromatic chelate ring system is observed. These two ring systems are essentially coplanar. The second aromatic ring in the molecule has less conjugation with the enol ring system and the angle between these two ring planes is about 450. The oxygens of the ring are also engaged in intermolecular hydrogen bonding to phenolic hydrogen(s) in the crystal lattice[27].
6. PHYSICO-CHEMICAL PROPERTIES OF CURCUMINOIDS:
The curcuminoids are substantially insoluble in water. However, the compounds are freely soluble in alkali and glacial acetic acid. Since most food and drug formulation contain a significant amount of water, these pigments will not colour such products without the aid of a chemical emulsifier of some kind or without a change towards alkaline pH.
The behavior of major pigment curcumin in solution has extensively been studied[5,28,29]. The colour of its solution in aqueous media or in organic solvents is not constant due to degradation of curcumin. The colour also changes with the pH of the solution.
In the pH range 1-7 , the curcumin solution has an yellow colour due to the existence of majority of molecules in the neutral form H3C (Scheme 4). At higher pH values ( pH > 7), the colour is brownish red / deep red and it is believed to be in equilibrium between the three possible anionic forms H2C-, HC2- and C3- as represented in Scheme (4). Above pH 11, the molecules completely assume C3-form.
The hydrolytic degradation of curcumin studied over the pH range 1- 11 showed a second order kinetics[29]. Below pH 7, the degradation rate is low compared to that observed at higher pH values. Alkali degradation of curcumin was irreversibly found to give ferulic acid, feruloylmethane and acetone (Scheme 5 ). However, formation of these products cannot explain the deep red colour 0f decomposed curcumin solution. The colour is attributed to various condensation products rapidly formed in alkali medium from the feruloyl part of the curcumin molecule[29]
7. COORDINATION COMPOUNDS OF CURCUMINOIDS:
The natural yellow colour of turmeric turns to deep red when mixed with slaked lime is one of the earliest known interaction of curcuminoids with metal ions which associated with certain Hindu religious ceremonies. The colour change can be attributed to some kind of interaction between Ca2+ ions and curcuminoids. The Ca2+ ions may replace either enolic/ phenolic protons and changes the chromophoric group.
Most of the reported reactions of curcuminoids and metal ions were associated with various biological investigations. The alkali metal complexes of curcumin were isolated in solid state[30] and their anti-inflammatory activity was found to be greater than the free curcuminoids possibly due to their solubility in aqueous media. While studying the antiarthiritic activity of gold (I) complex of curcumin, Sharma and Chandra[31] isolated and characterized a 2:1 curcumin – gold (I) complex (11)in which curcumin acting as a neutral bidentate ligand
A mercury(II) complex of curcumin was characterized on the basis of electronic and infra red spectral data by Angulo[32] in which curcumin function as a bidentate monobasic ligand through the enolised 1,3-dicarbonyl function.
The colour reaction of boron compounds with curcuminoids has been studied extensively and form the basis of sensitive and specific methods for the qualitative and quantitative determination of both boron and curcumin[5,28]. Formation of different type of complexes between boron and curcumin are postulated, depending on the reaction medium. Knowledge of the structure and chemical properties of these complexes are still incomplete and statements in the literature are often somewhat contradictory. All boron-curcumin chelates are formed from the quinonoid protonised form of of curcumin, H4C+ shown in Scheme 4. The boron-curcumin chelates most frequently referred to are rosocyanin and rubrocurcumin.
Rosocyanin is the compound formed when curcumin reacts with boric acid in the presence of mineral acids (sulfuric / hydrochloric acids ) in combination with glacial acetic acid . Reported structure of this complex is as in 12. Formation of a 1:1 boron – curcumin chelate of structure 13 has also been reported.
Rubrocurcumin is the red complex formed when curcumin reacts with boric acid in the presence of oxalic acid . Spicer and Strickland[34] concluded that it is 2:2:2 complex of curcumin, boron and oxalate, on the basis of absorption spectra and elemental analysis. Whereas, on the basis of elemental analysis, electronic and infra red spectra, a 1:1:1 composition (14) was suggested for the complex by Roth and Miller[35].
The acid dissociation constants (pK) of curcumin and its structural analogues, as well as the stoichiometric stability constants of the copper (II) and nickel (II) complexes of these compounds were determined by potentiometric titrations in aqueous-dioxane medium[36]. The influence of different aryl substituents on the pK values and stability constants were discussed in terms of electronic effects. These results showed that the complexing ability of the structural unit –CO–CH2–CO– of the compounds are not significantly influenced by the aryl substituents.
Transition metal complexes of curcuminoids and structurally related 1,7-diarylheptanoids were synthesized and characterized by electronic, infra red, 1H NMR ,13C NMR and mass spectral studies. In such ML2 complexes, curcumin behaves like a bidentate monobasic ligand[37-42].
8. ANALYTICAL APPLICATIONS OF CURCUMINOIDS:
Like many natural dyestuffs, the colour change of turmeric extract with pH of the media has been commerrcialised in ‘ curcuma paper`, a common pH indicator[43]. The red colouration when turmeric extract or its active principle curcumin reacts with boric acid is the basis of the most sensitive spot test for boron. Therefore, this reaction is of considerable importance in agriculture, waste water and metallurgical analysis [27.32-34].
Both rubrocurcumin and rosocyanin are used for boron determinations and both procedures are referred to as ‘the curcumin method` in the literature[5]. The formation of rosocyanin is for various reasons superior method in analytical procedures, because the molar absorptivity is about twice that for rubrocurcumin and have higher stability towards hydrolysis. A method based on the formation of rosocyanin has been developed for quantitative determination of boron with a detection limit in the pictogram area. It is apparent that most of the analysis of boron based on the curcumin method are performed with ‘natural curcumin’ which essentially contain the three curcuminoids.
Rubrocurcumin reaction initiated investigations on the possible similar complex formation with different α- hydroxycarboxylic and dicarboxylic acids instead of oxalic acid [44]. These studies showed that complex formation occurs preferentially with cis-isomers among the unsaturated dicarboxylic acids. This has been exploited to estimate the cis- acids in a mixture of cis- and trans forms2. The complexes formed by dicarboxylic acids with the carboxyl group separated by more than one carbon atom are often unstable. All the complexes evaluated showed a 1:1:1 ratio between boron, curcumin and the carboxylic acid. The curcuminoids exhibite a strong fluorescence in organic solvents which not only enabled to estimate them quantitatively, but also to estimate several elements including boron, beryllium and silicon fluorometrically [5,45].
9. PHOTOCHEMISTRY OF CURCUMINOIDS:
Curcuminoids exhibited poor stability to light. Therefore, turmeric coloured products fades noticeably when no precautions are taken[46]. The reaction mechanism and kinetics of the overall photochemical degradation of curcumin were investigated. It has been showed that curcumin decomposes when exposed to uv/visible light both in the solution and in the solid state. The main degradation product after exposing to visible light has been identified as an yellow coloured cyclisation product(15). This suggest that the apparent colour of curcumin will not change severely when exposed to long wave light. However, when exposed to uv light curcumin rapidly decolourises[5,47].
Curcumin act as a photosensitiser of singlet oxygen and undergoes a self-sensitised decomposition, but the fading of curcumin also involves other mechanisms independent of oxygen. Thus it will not be possible to stabilize curcumin photochemically by excluding oxygen or by adding quenchers to the medium[5].
Curcumin can be protected from light by use of brown glass if other singlet oxygen sources are excluded. In combination with sensitisers like methylene blue, curcumin showed catalytic fading. The possibility that curcumin itself can act as a photosensitising agent has an interesting aspect in many drug formulations[5].
10. BIOSYNTHESIS OF CURCUMINOIDS:
The biosynyhetic pathway of curcumin has been expected to involve two cinnamate coupled to a central carbon atom provided by malonate; e.g., phenylalanine-cinnamate pathway[24,48] shown in Scheme 6
However, experiments by Roughley and Whiting[24] have recorded rather unexpected results, which indicate that the biosynthesis would be unsymmetrical with different origins for the two –C9– parts of the molecule. This would involve a cinnamate starter, extended by five acetate or malonate units. Cyclisation of the chain gives the second aromatic ring, and biosynthesis (acetate pathway ) shown in Scheme 7 has also been postulated for a wide range of natural compounds including piperine and gingerol[49]. However, none of the biosynyhetic pathways mentioned above are not fully accepted and further experimental evidence for the biosynthetic pathways should be required.
11. METABOLISM OF CURCUMINOIDS:
The metabolic fate of curcuminoids has not been examined in detail though it is of relevance in view of their use as food ingredient as a medicine. Reports on the uptake, distribution and excretion of curcumin in rat have appeared only recently[50-54]. However, it is to be pointed out that the results obtained are contradictory. Wahlstrom and Blennow[50] observed that 65- 85 % of the oral administered dose of curcumin was excreted in the faeces, while negligible amounts were recovered in the urine.
Measurements of plasma levels and biliary excretion in anethetised animals given curcumin revealed a very low a very low absorption of curcumin into the blood. The concentration of curcumin in the bile, liver, kidneys and body fat was negligible and the major part of the administered curcumin was found in the intestine. After intravenous injection, curcumin disappeared rapidly from the blood and excreted in the bile. Addition of curcumin to liver perfusion systems and isolated hepatocytes and liver microsomes showed that the sample was quickly metabolized and do not retain in the body over a prolonged period. This led to the conclusion that the liver was the major site of curcumin metabolism[53,54].
Holder and co-workers[51] reported that following an oral dose, more than 90% of the dose was excreted in faeces as glucuronide conjugates of tetrahydrocurcumin (50%), hexahydrocurcumin (42 %) and dihydroferulic acid. The recovery in the urine was only 6%. This again indicated that curcumin and its metabolites were undergoing biliary excretion.
Ravindranath and Chandrasekhara[52] studied the in vitro absorption of curcumin using everted intestinal sacs and found that 30 to 80% of the added sample disappeared from the mucosal side of the sacs where as the in vivo studies indicated that nearly 40% of the curcumin dose was excreted unchanged in the faeces and curcumin could not be detected in the urine, blood, liver or kidney.
Based on the reported works[50-54], it is difficult to draw any conclusion about the fate of curcumin in vivo. After oral administration in rats it seems likely that curcumin to a certain extent is metabolized in the liver and its metabolites are mainly excreted via the bile and the faeces. The amount of curcumin dose that is excreted unchanged and th exact metabolites of curcumin are not fully established[2,5].
12. TOXICOLOGICAL STUDIES IN CURCUMINODS:
Turmeric and curcuminoids are present in most habitual Indian dietsas a part of the spices used in the traditional cooking and no side effects have been observed. However, the FAO/WHO expert group did recommend that turmeric and curcuminoids should be properly evaluated when listed as a permitted food colourant. A temporary average daily intake (ADI) of 2.5 mg/kg body weight is set for turmeric.
Acute toxicity studies on turmeric in animals indicated no side effects of the drug even at high doses[55-57]. Further, long- and short- term studies in dogs, mice and rats did not reveal any adverse cytogenic and mutagenic effects compared with controls when curcumin was incorporated into the diets in amounts normally consumed by man. Some attention has been given to mutagenecity studies recently and curcumin itself exhibited no mutagenic effects in the salmonella/mammalian microsome test[54]. The above studies thus indicate that both turmeric and curcuminoids are toxicologically safe even in dises far beyond the ADI given by FAO/WHO.
13. PHARMACOLOGICAL ACTIVITIES OF CURCUMINOIDS:
In the Indigenous systems of medicineof the orient, Curcuma Longa L. has been employed since time immemorial. Turmeric enjoys the reputation as an anti-inflammatory agent, as a carminative, diuretic and blood purifier as well as a remedy against jaundice. It is also recommended for use against common cold, cough, leprosy, affections of the liver and among other indications in the treatment of ulcers. These effects are ascribed to the yellow pigment, curcumin isolated from the plant.
Table 1 Important pharmacological activities of Curcuminoids
|
Sl. No |
Pharmacological activity |
References |
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 |
Antiinflammatory Antiarthritic Antispasmodic Antihepatotoxic Antiulcerogenic Anticoagulant Antiprotozoal Antifertility Antitumour Antioxidant Antimutagenic Wound healing Hypotensive Hypolipemic Fungicidal Bactericidal Antimalarial |
[30],[58-67] [31],[63] [61],[68-76] [69],[70] [77-81] [81],[82] [71],[85[,[84] [85-87] [88-95] [66],[67],[84-104] [90-93],[104] [96] [97] [98],[99],[105] [106-109],[114-116] [66-68],[85],[106],[110-113] [117-119] |
Turmeric is shown to have a broad spectrum of beneficial physiological activities; the important among them are brought out in table 1. Amongst the various properties attributed to turmeric in herbal medicine, an important one is its therapeutic value in the treatment of liver and digestive disorders.
The interest in curcuminoids particularly as a choleretic, hypocholesterimic and antihepatotoxic agent is increasing and they now being regarded as a potential drug for several reasons: it is easily obtained, has a low price and gives no toxic effects even in large amounts[4,5].
14. CONCLUSION:
The fast growing research on curcumin, curcuminoids,and natural and synthetic curcumin analogues clearly confirms the versatility and flexibility of curcumin for structural modifications. However the actual role of different functionalities in curcumin that influencing its special physico-chemical properties and pharmacological effects is not clear. Such structure-activity studies are still rewarding and would definitely provide a proper basis for unraveling the wide variety of biological actions of the age old spice. This review describes various chemical and biochemical aspects of curcuminoids.
15. ACKNOWLEDGEMENTS:
The authors are grateful to Dr. K. Krishnankutty, Emeritus Scientist (KSCSTE), Department of Chemistry, University of Calicut, Kerala-673635 for having fruitful discussions.
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Received on 20.03.2014 Modified on 25.03.2014
Accepted on 31.03.2014 © AJRC All right reserved
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