Guide for Carotenoid Identification in Biological Samples - American

Review pubs.acs.org/jnp

Guide for Carotenoid Identification in Biological Samples Sol Maiam Rivera Vélez* Program in Individualized Medicine, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman, Washington 99164, United States ABSTRACT: In recent years there has been considerable interest in carotenoids with respect to their biological roles in animals, microorganisms, and plants, in addition to their use in the chemical, cosmetics, food, pharmaceutical, poultry, and other industries. However, the structural diversity, the different range of concentration, and the presence of cis/trans-isomers complicate the identification of carotenoids. This review provides updated information on their physical and chemical properties as well as spectroscopic and chromatographic data for the unambiguous determination of carotenoids in biological samples.

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AN INTRODUCTION TO CAROTENOIDS IN BIOLOGICAL SAMPLES: DISTRIBUTION AND STRUCTURAL FEATURES Over 750 carotenoids have been found in natural sources, and more than 20 carotenoids are reported each year.1 These pigments are isoprenoid compounds, biosynthesized by tail-totail linkage of two C20 geranylgeranyl diphosphate molecules.2 They are divided into the following: (a) carotenes, composed of only carbon and hydrogen, and (b) xanthophylls, which are oxygenated and may contain epoxy, carbonyl, hydroxy, methoxy, or carboxylic acid functional groups. Carotenoids are produced commercially mainly by chemical synthesis3 or, less often, by extracting them as “natural” compounds from algae, fungi, bacteria, or plants. Figure 1 shows the structure of most common compounds, such as bixin (1), crocin (2), lutein (3), lycopene (4), and capsanthin (5), that are produced from natural resources, and Table 1 shows their sources.4−6 The great interest in studying these compounds arises from their various functions. In the past, carotenoids were regarded as only secondary metabolites, known to confer colors to many flowers and fruits, and therefore important for plant−animal interactions. More recently, carotenoids have also been demonstrated to play vital roles in metabolism.3 Indeed, carotenoids serve as regulators of plant growth and development, as accessory pigments in photosynthesis, and as photoprotectors, preventing damage by photo-oxidation. In humans, carotenoids help protect against illnesses such as cancer, heart disease, and macular degeneration.7 In industry, carotenoids are used in nutrient supplementation, for pharmaceutical purposes,4 and as feed additives for crustaceans, fish, livestock, and poultry.8 Carotenoid cleavage products are vital not only for the survival of plants and animals9 but also for the flavor and perfume industries.10 For products derived from the oxidation of carotenoids, the formation of epoxides and apocarotenoids11 appears to be the initial step. Subsequent © XXXX American Chemical Society and American Society of Pharmacognosy

fragmentations yield a series of low-molecular-weight compounds.12 These types of molecules include volatile/flavor compounds, vitamins, phytohormones, and apocarotenoid pigments.13 The primary aroma molecules derived from carotenoids are C13, C11, C10, and C9 metabolites formed via photo- and enzymatic oxidation of the various carotenoids found in plants.14 Some important volatile carotenoid derivatives are β-ionone (C13; 6), isophorone-4-acetaldehyde (C11; 7), safranal (C10; 8), and 4-ketoisophorone (C9; 9).14 Plant hormones derived from carotenoids (Figure 1) are abscisic acid (ABA; 10) and strigolactones (11).15,16 Apocarotenoids such as 2 and crocetin glycosides (12) contribute to the red color of saffron,17 one of the world’s most expensive spices and used widely as a natural colorant.8 Similarly, 1 and βcitraurin (13) are important food colorants, found in annatto seeds and citrus fruits, respectively.17 Another product of carotenoid degradation is vitamin A (14), which is important for human health and plays a critical role in vision. The identification of carotenoids in biological samples is far from simple because some compounds exhibit very similar structures, great diversity, and the occurrence of cis/trans isomeric forms. This review is intended to aid researchers working on carotenoid analysis by providing information on the latest enhancements in carotenoid separation and detection. The distribution and the various functions of these pigments in Nature are also described.

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CAROTENOIDS FOUND IN ANIMALS Carotenoids are often responsible for the yellow, red, and orange coloration observed in some birds,26 fish, and reptiles (e.g., Carpodacus mexicanus, Poecilia reticulata, and Anolis distichus, respectively).27 This coloration advertises some Received: August 23, 2015

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DOI: 10.1021/acs.jnatprod.5b00756 J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products

Review

Figure 1. Structures of the main commercial carotenoids and some cleavage products.

from their diets. However, recent studies have demonstrated that some arthropods (pea aphids, spider mites, and gall midges) are capable of biosynthesizing carotenoids.29−31

Table 1. Carotenoids of Commercial Interest and Their Primary Natural Sources carotenoid bixin (1) crocin (2) lutein (3) lycopene (4) capsanthin (5) β-carotene (15) zeaxanthin (16) β-cryptoxanthin (17) astaxanthin (18)

canthaxanthin (19)

organism Bixa orellana (higher plant) Crocus sativus (higher plant) Tagetes erecta (higher plant) Lycopersicon esculentum (higher plant) Capsicum annuum (higher plant) Dunaliella salina (microalga) Blakeslea trispora (fungus) Rhodococcus maris (bacterium) Capsicum annuum (higher plant)

5 6 18 19 20 21 21 22 20

Haematococcus pluvialis (alga)

20, 21 23

Xanthophyllomyces dendrorhous/Phaf f ia rhodozyma (yeast) Dietzia natronolimnaea, Corynebacterium michiganense, Micrococcus roseus, Brevibacterium sp. strain KY 4313, Gordonia jacobaea MV-1, Bradyrhizobium sp., and Haloferax alexandrines (bacteria) Chlorella pyrenoidosa and Chlorella zof ingiensis (microalgae)

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ref

CAROTENOID ANALYSIS IN BIOLOGICAL SAMPLES Geometrical Isomers. In general, all-trans-isomers are the most common in Nature. However, various cis-isomers occur naturally in algae, bacteria, plants, and invertebrate animals such as 9′-cis-bixin (1), 9′-cis-neoxanthin (20), and 15-cis-phytoene (21) (found in annatto seeds, spinach, and tomato, respectively).32,33 Another example of a cis-isomer is 9-cisviolaxanthin (22), the main carotenoid found in the blossoms of the yellow pansy and the peels of the Valencia orange.34 Under the influence of heat, light, oxygen, or certain chemical reactions, the double bonds can isomerize from the trans-form to mono- or poly-cis-forms.35 UV−Visible Spectroscopy and Physicochemical Properties. In the past two decades, intense efforts have been made to accurately determine cis/trans-carotenoid profiles of biological tissues to acquire a better understanding of the biological significance of cis-carotenoids. The following constitute important diagnostic features used to identify these isomeric compounds: • Usually, cis-isomers have lower melting points than their all-trans counterparts, due to a decreased tendency to crystallize.36 • A cis-carotenoid absorbs light at lower wavelengths and has a lower absorption coefficient and loss of fine structure compared with the all-trans configuration.36,37 Mono-cis-carotenoids absorb maximally at wavelengths

22, 24, 25 24

specific aspect of their competitive ability. When these pigments bind proteins, they acquire green, purple, or blue colors.7 Astaxanthin (18) is typically red but undergoes a bathochromic shift when forming crustacyanins (the carotenoproteins found in a lobster’s carapace), yielding a purple color in β-crustacyanin and blue in α-crustacyanin.28 Until recently, animals were assumed to be unable to produce their own carotenoids and therefore had to acquire these compounds B



Review

Figure 2. Structures of cis-carotenoids.

methyl-tert-butyl ether (MTBE) or MeOH, MTBE, and water (H2O). Figures 1 and 2 show the structures of 15 and its cisisomers. Similarly, the order of elution of lutein isomers was 13cis- (27) and 13′-cis-isomers (28), trans-isomer (3), 9-cis- (29), and 9′-cis-isomers (30), using the aforementioned mixture of solvents as mobile phase.38 Other studies have reported the same elution order for the mono-cis-isomers of 18, 17, αcarotene (31), 16, and several epoxycarotenoids on C30 columns.39,43,44 Chemical Testing. An iodine-catalyzed isomerization reaction may be used to verify the geometric configuration of carotenoids. The chemical reaction can be carried out as follows: dissolve a few crystals of iodine in petroleum ether, add a drop of iodine solution to the carotenoid solubilized in petroleum ether, and expose the extract to daylight for approximately 1−5 min. The λmax values of trans-carotenoids will shift 3−5 nm to a lower wavelength, whereas the λmax values of cis-carotenoids will shift by the same amount to a longer wavelength.7 Moreover, it is possible to distinguish between 9-cis- and 9′-cis-isomers of unsymmetrical carotenoid 5,6-epoxides by using the well-known 5,6-epoxy → 5,8-epoxy rearrangement. The furanoid-oxide reaction of 9-cis-carotenoid5,6-epoxides results in the corresponding all-trans-5,8-epoxide epimers, but a similar reaction of 9′-cis-carotenoid-5,6-epoxides produces furanoid derivatives without isomerization of the 9′cis double bond.34 Solubility. cis-Isomers tend to be more soluble in organic solvents than all-trans-isomers.37 Epoxycarotenoids and Furanoid Oxides. Epoxycarotenoids are found in dietary sources such as beans, broccoli, brussels sprouts, cabbage, kale, kiwi, lettuce, mango, oranges, papaya, peas, spinach, and squash.45 In photosynthetic organisms, epoxycarotenoids play a key ecological function as energy dissipaters, protecting the photosynthetic apparatus

only 2−6 nm lower than the corresponding transcarotenoids, and di-cis-carotenoids absorb much farther from trans-carotenoids.35 • The UV absorption spectra of cis-carotenoids are characterized by the appearance of a new maximum at approximately 330−350 nm (“cis peak”). The intensity of the cis band is greater when the cis double bond is nearer the center of the molecule (the cis band may be completely absent in di- and poly-cis-isomers).38,39 cisIsomers may be identified by the Q-ratio or DB/DII, which is defined as the ratio of the height at the cis peak to the height at the maximum absorption peak.40 Unlike cyclic carotenoids, little is known about the spectroscopic characteristics of the geometrical isomers of 21 and phytofluene (23). However, Melendez et al.33 studied recently the identification of geometrical isomers of these acyclic carotenoids in biological samples. They identified three geometrical isomers for 21 and six for phytofluene. Most of the phytoene and phytofluene isomers exhibited absorption maxima at the same wavelength, but they showed different fine structures. Chromatographic Properties. Excellent separations of cis/trans isomeric carotenoids have been achieved using C30 stationary phases in chromatographic systems.39 An overview of the systems used for separating these isomers has been provided by Rivera et al.7 The retention times of the monocis-isomers on C30 stationary phases decrease the more centrally the cis double bonds are positioned. For example, Breitenbach et al.,40 Sander et al.,41 and Zepka et al.42 reported the following order of elution for these β-carotene isomers: 15-cis-isomer (24), 13-cis-isomer (25), and 9-cis-isomer (26). The trans-βcarotene (15) eluted between 25 and 26. All mobile phases used were based on mixtures of methanol (MeOH) and C


D

solvents A, B, and C: H2O, MeOH, and MTBE, respectively; gradient elution; column temperature: 25 °C; flow rate: 1 mL/min solvents A and B: MeOH and MTBE, respectively; gradient elution; column temperature: 22 °C; flow rate: 0.9 mL/min solvents A and B: MeOH and MTBE, respectively; both containing BHT and TEA (0.1% and 0.05%, respectively); solvent C: H2O; gradient elution; column temperature: 17 °C; flow rate: 1.0 mL/min solvent A and B: MeOH−MTBE−H2O (81:15:4, v/v) and MeOH−MTBE−H2O (6:90:4, v/v), respectively; gradient elution; column temperature: not controlled; flow rate: 1.0 cm3/min solvent A: MeOH−CH3CN−H2O (79:14:7, v/v); solvent B: CH2Cl2 (100%); gradient elution; column temperature: not controlled; flow rate: 1.0 mL/min solvent A: H2O−20 mM ammonium acetate; solvent B: MeOH−20 mM ammonium acetate; solvent C: MTBE; gradient elution; column temperature: 25 °C; flow rate: 1.0 mL/min

C30 YMC C30 YMC C30 YMC

C30 YMC

C30 YMC

C30 YMC

20, 22, 32, 35, and 48

20, 22, 32, 34, and 35

20, 22, 32, 34, and 35

20, 22, 32, 33, and 38 20, 22, 32, 34, 35, 38, 44, 45, and 48 22, 32, 35, 38, 45, and latochrome (50)

solvents A and B: MeOH and MTBE, respectively; gradient elution; column temperature: 22 °C; flow rate: 0.9 mL/min solvents A and B: MeOH and MTBE, respectively; gradient elution; column temperature: 22 °C; flow rate: 0.9 mL/min

C30 YMC C30 YMC

C30 YMC C30 YMC

20, 22, 32, 33, 34, 35, 38, and 45

20, 22, 34, and 45

20, 22, 32, 34, 35, 38, and 45

20, 22, 32, 35, 38, capsanthin 5,6-epoxide (39), capsanthin 3,6-epoxide (40), curcubitaxanthins A (41) and B (42), and cucurbitachrome (43) 20, 22, 32, 33, 35, 38, and 5,8-epoxy-βcarotene (44) 20, 22, 32, 35, 36, 38, and auroxanthin (45)

20, 22, 32, 34, 38, 45, and trollichrome (46) 3, 15, 16, 20, 22, 33, 34, 35, 36, 45, and cisisomers of 3 20, 22, 32, 34, and 35 20, 22, 32, 34, 35, 38, 5,6-epoxy-β-carotene (47), 5,6-epoxy-β-cryptoxanthin (48), and 5,6:5′,6′-diepoxy-β-carotene (49)

solvent A: MeOH−CH3CN−H2O (84:14:2, v/v); solvent B: CH2Cl2 (100%); gradient elution; column temperature: not controlled; flow rate: 1 mL/min solvents A and B: MeOH and MTBE, respectively; both containing BHT and triethylamine (TEA; 0.1% and 0.05%, respectively); solvent C: H2O; gradient elution; column temperature: 17 °C; flow rate: 1.0 mL/min

solvents A, B, and C: acetonitrile (CH3CN), H2O, and ethyl acetate (EtOAc), respectively; gradient elution; column temperature: 29 °C; flow rate: 1 mL/min solvent A: MeOH−H2O 88:12, v/v; solvent B: MeOH; solvent C: MeOH−dichloromethane (CH2Cl2) 70:30, v/v; gradient elution; column temperature: not controlled; flow rate: not reported solvent A: CH3CN−MeOH 7:3, v/v; solvent B: H2O 100%; gradient elution; column temperature: 32 °C; flow rate: 0.5−0.7 mL/min

carotenoids determined 20, 22, 33, neochrome (34), luteoxanthin (35), and flavoxanthin (36) 22, 32, valenciaxanthin (37), and mutatoxanthin (38)

solvents A, B, and C: MeOH, H2O, and MTBE, respectively; gradient elution; column temperature: not controlled; flow rate: 1 mL/min solvents A, B, and C: MeOH, H2O, and MTBE, respectively; gradient elution; column temperature: not controlled, flow rate: 0.2 mL/min

C30 YMC

C18 NovaPak ODS Chromsyl C18 Acquity UPLC C18 BEH C30 YMC

solvent A: MeOH−H2O 88:12, v/v; solvent B: MeOH; solvent C: acetone−MeOH 50:50, v/v; gradient elution; column temperature: not controlled; flow rate: 1.5 mL/min

solvents A, B, and C: 0.1 M ammonium acetate in MeOH, H2O, and MTBE, respectively; gradient elution; column temperature: 20−30 °C; flow rate: not reported

Vydac 201TP54 C18 Chromsyl C18

chromatographic conditions

solvents A and B: acetone and H2O, respectively; gradient elution; column temperature: not controlled; flow rate: 1 mL/min

C18

column type

Table 2. Analytical Conditions for the Separation of Epoxycarotenoids and Furanoid Oxides in Different Matrices by Liquid Chromatography matrix

63 64

Taraxacum formosanum Solanum betaceum

60 42 61 62

58 59

56 57

43

55

Artocarpus heterophyllus dovyalis fruit (Dovyalis abyssinica × D. hebecarpa) and Cyphomandra betacea fruit of citrus varieties Anacardium occidentale orange juices from concentrate Dunaliella salina

isolation and semisynthesis of epoxycarotenoids orange juice mature grapes

Chlorella pyrenoidosa

49

54

Calendula of f icinalis apple, pear, and peach juices and purees

53

52

Capsicum annuum

Myrciaria dubia

50

51

ref

natural orange and orange−carrot juices

Vitis vinifera

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Figure 3. Structures of carotenoids containing 5,6-epoxide and 5,8-furanoid groups.

Figure 4. Structures of carotenoids containing 5,6-epoxide and 5,8-furanoid groups.

consisting of MeOH, MTBE, and H2O in gradient elution mode are used, carotenoids containing 5,8-furanoid groups elute later than their 5,6-epoxide counterparts.43,47,48 This chromatographic behavior has also been observed on C18 columns using different mobile phases.49,50 Furthermore, the C30 column allows the separation of the usually coeluting antheraxanthin (32) and lutein epoxide (33).43 Table 2 shows a summary of current methods for epoxycarotenoid and furanoid oxide separations in different matrices. Figures 3 and 4 display the chemical structures of the targeted compounds described in Table 2. Mass Spectrometry. Fragmentation patterns are strongly dependent on the chemical and physical properties of the analytes and the ionization technique used.65 In addition, when

when light conditions favorable to photosynthesis switch to stressful high-irradiation conditions.46 The analysis of these pigments is gaining importance because they can provide information about the processing and/or storage of foods43 and may have interesting pharmacological applications.46 UV−Vis Spectroscopy and Chromatographic Properties. Carotenoids containing 5,8-furanoid groups absorb light at lower wavelengths (ca. 15−20 nm for each 5,8-furanoid group) than carotenoids containing the corresponding 5,6epoxycarotenoid isomer. This is due to a shortening of the chromophore by one conjugated double bond.47,48 C 30 chromatographic columns are particularly useful for the separation of structural, geometrical, and optical isomers of these molecules. When this column and a mobile phase E



Review

Figure 5. Epoxy-furanoxy rearrangement (according to De Abreu et al., 2013).66

Figure 6. Chemical structures of ketokarotenoids and other xanthophylls.

oxygen-containing heterocyclic ring fused to the hydroxy-βring.10,47 These ions are observed using electron ionization (EI) and FAB. The ion at m/z 181 has also been confirmed by ESI.10 However, none of these ions can be used to distinguish between 5,6-epoxy and 5,8-furanoid carotenoids. Therefore, spectroscopic and chromatographic analyses must be employed to confirm the identification of these compounds. De Rosso et al.48 reported that the fragment ion at m/z 393 (ESI) observed in the MS/MS of 20 can be used to distinguish this pigment from its structural isomer 22. This fragment is produced by the cleavage of the double bond allylic to the allenic carbon.48 Similarly, these molecules can also be differentiated by comparing the intensity of the transition m/z 601 → 167,

using soft ionization [e.g., electrospray (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), or fast-atom bombardment (FAB)], factors such as the mobile phase, the modifiers added (acids, bases, dopants, metals, and salts), the experimental conditions used (collision energy, flow collision gas, temperatures, etc.), and the instrument design also affect the ionization of the analytes.10 Therefore, the mass spectrum of a given compound should not be expected to exactly match a published mass spectrum. Characteristic diagnostic ions used to identify epoxycarotenoids and furanoid oxides are found at m/z [M − 80]+•, corresponding to the loss of one dimethylcyclobutadiene unit, and at m/z 352, 221, and 181, corresponding to the F



Review

which is much higher for 20 than 22 when using APCI in the positive-ion mode.49 Chemical Testing. Epoxy groups in the 5,6 or 5,6, 5′,6′ positions are detected by their conversion to the furanoid derivatives in the presence of an acid catalyst, reflected by a hypsochromic shift of 20−25 or 50 nm, respectively.12 The mechanism of the epoxide-furanoid rearrangement was proposed by De Abreu et al. (Figure 5).66 Since this reaction may occur in naturally acidic matrices, it is advisible to use bases such as sodium bicarbonate to prevent artifact formation during extraction and analysis.47,67 Solubility. Ethanol is well known to dissolve several epoxycarotenoids, including 20, 22, 32, 45, and fucoxanthin (51).32,68 Acetone has also been reported to dissolve 20, 22, 45, and 51,32,55 and MeOH has been reported to dissolve 20 and 51.69 Ketocarotenoids. These compounds constitute a group of carotenoids containing at least one carbonyl group, either in the linear chain or in the β-ring (Figure 6). They are biosynthesized by certain bacteria, several fungi, and some green algae, but rarely occur in plants.70 The ornamental flowering plant Adonis aestivalis accumulates ketocarotenoids, such as 18, while ketocarotenoids such as 5 and capsorubin (52) are found in Capsicum annuum.70,71 Of the highly valued ketocarotenoids employed for food coloration, 18 (3,3′-dihydroxy-β,β-carotene4,4′-dione) and 19 (β,β-carotene-4,4′-dione; Figure 1) are probably the most important. These compounds are commonly found in marine animal tissue.72 They have been reported to be effective antioxidants and to play a valuable role in protection against a broad range of human diseases.73 UV−Vis Spectroscopy and Chromatographic Properties. The spectroscopic characteristics of ketocarotenoids distinguish them from other carotenoids. Most carotenoids exhibit three absorption maxima between 400 and 500 nm, resulting in three-peak spectra. However, ketocarotenoids have a spectrum that consists of a rounded, almost symmetrical single maximum peak.32 Moreover, when a carbonyl group is in conjugation with the main polyene chain, the chromophore is extended, resulting in a bathochromic shift. Figure 7 shows a comparison of the absorption spectra for a hydroxycarotenoid (3), a carotene (15), and a ketocarotenoid (18). In many cases, these compounds are found as complex mixtures, which might include their geometric isomers, esters, glycosides,44,74,75 other carotenoids, and chlorophylls.76 Additionally, 18 can be found in association with proteins or lipoproteins.77 Most separations of ketocarotenoids reported in the literature involve C18 and C30 columns, usually with gradient elution.2,7 Nonetheless, enantiomeric isomers cannot be separated by these conventional reversed phases unless chiral phases are used.44 For most HPLC systems using C18 and C30 columns, carotenoids containing hydroxy groups elute earlier than carotenoids with carbonyl groups when comparing xanthophylls with the same backbone structure. Table 3 summarizes a number of HPLC methods used for the determination of several ketocarotenoids, including 18, 19, 4keto-lutein (53), adonirubin (54), adonixanthin (55), 3- and 3′-hydroxyechinenone (56 and 57, respectively), echinenone (58), and geometrical and optical isomers, esters, and glycosides of 18 in different matrices (Figure 6). Qin et al.78 reported an excellent resolution between the usually coeluting 18 and 32 using a reversed-phase C18 column and a mobile phase consisting of solvent A (CH2Cl2−MeOH−CH3CN− H2O, 5.0:85.0:5.5:4.5, v/v) and solvent B (CH2Cl2−MeOH−

Figure 7. Visible absorption spectra of 3, 15, and 18.

CH3CN−H2O, 22.0:28.0:45.5:4.5, v/v) in gradient elution mode. Mass Spectrometry. Regarding the mass spectrometric profiles of the ketocarotenoids, the ions at m/z 147 and 203 (using APCI) have been associated with these pigments. The ion at m/z 203 is characteristic of carotenoids containing a carbonyl group as the only substituent on the β-ring (e.g., 19), whereas the ion at m/z 147 is typical of molecules containing a hydroxy group at carbon 3 (3′) and a carbonyl group at carbon 4 (4′) in the same β-ring (e.g., 18).10 The ion at m/z 147 may also be confirmed by ESIMS.10 Chemical Testing. Ketocarotenoids undergo reduction with LiAlH4 or NaBH4. This reaction leads to the single broad band of the oxocarotenoid being transformed into the three-peak spectrum of the resulting hydroxycarotenoid.12 Solubility. One liter of CH2Cl2 can dissolve 30 g of 18 at room temperature, and the solubility of 18 in CH2Cl2 is higher than its solubility in trichloromethane (CHCl3), acetone, or dimethyl sulfoxide.76 The solubility of 19 is especially high in 1,2-dichloroethane (C2H4Cl2) and CHCl3.96 Hydroxycarotenoids and Carotenes. Within this class of compounds are the major dietary carotenoids and principal carotenoids found in human blood and tissues: 3, 4, 15, 16, 17, and 31. Hydroxycarotenoids and carotenes can be found in green and yellow/orange fruits and vegetables, while yellow/ red fruits and vegetables contain mostly carotenoid hydrocarbons (Table 4).45 The medicinal properties of these pigments include provitamin A activity, photoprotection of skin against UV light, and prevention of cataracts, retinitis, macular degeneration, atherosclerosis, prostatic hyperplasia, prostate cancer, and acute and chronic coronary syndromes.97,98 Figure 8 shows some examples of the chemical structures of these types of compounds. G


H

Chiralcel(R) OD-RH Chiralpak IC CSP Chiralcel ODRH

HIRPB-250AM

Brownlee Spheri-5 silica 5 μm Spherisorb ODS-2

Spherisorb ODS-2 Luna 5 μm silica

Beckman Ultrasphere C18 Hypersil HyPurity Elite C18 Beckman Ultrasphere C18 Supelco Discovery C18 Beckman Ultrasphere C18 ReproSil 120 C18-AQ Acquity UPLC C18 BEH Cosmosil 5C18AR-II Suplex pKb-100

C30 YMC

C30 YMC

C30 YMC

C30

C30 YMC

C30

ProntoSil C30

column type

solvents A, B, and C: CH3CN, CH2Cl2, and MeOH, respectively; gradient elution; column temperature: 25 °C; flow rate: 0.4 mL/min solvent: CH3CN−3.5 mM phosphoric acid (ratio not reported); isocratic elution; column temperature: 25 °C; flow rate: 0.5 mL/min solvent: MTBE−CH3CN (1:1, v/v); isocratic elution; column temperature: 22 °C; flow rate: 1.0 mL/min solvent: CH3CN−3.5 mM phosphoric acid (85:15, v/v); isocratic elution; column temperature: 25 °C; flow rate: 0.5 mL/min

solvent: hexane−CH2Cl2−isopropyl alcohol−TEA (88.5:10:1.5:0.1, v/v); isocratic elution; column temperature: not controlled; flow rate: 1.5 mL/min

solvents A and B: MeOH−MTBE−H2O (83:15:2, v/v) and (8:90:2, v/v), respectively; gradient elution; column temperature: 30 °C; flow rate: 1 mL/min solvent A: MeOH−H2O, (75:25, v/v); solvent B: EtOAc; gradient elution; column temperature: 25 °C; flow rate: 0.6 mL/min solvent A: MeOH; solvent B: H2O−MeOH (20:80, v/v) containing 0.2% ammonium acetate; solvent C: MTBE; gradient elution; column temperature: 25 °C; flow rate: 1.0 mL/min solvents A, B, and C: MeOH, MTBE, and H2O, respectively; gradient elution; column temperature: 20 °C; flow rate: 1.0 mL/min solvents A and B: MeOH and MTBE, respectively; gradient elution; column temperature: not controlled; flow rate: 0.4 mL/min solvent: MeOH−MTBE−H2O−CH2Cl2 (77:13:8:2, v/v); column temperature: 30 °C; flow rate: 0.8 mL/min solvents A and B: MeOH and MTBE, respectively; gradient elution; column temperature: 29 °C; flow rate: 0.9 mL/min solvents A and B: CH2Cl2−MeOH−CH3CN−H2O (5.0:85.0:5.5:4.5, v/v) and (22:28:45.5:4.5, v/v), respectively; gradient elution; column temperature: not controlled; flow rate: 1.0 mL/min solvent: CH3CN−MeOH−2-propanol (85:10:5, v/v); isocratic elution; column temperature: 32 °C; flow rate: 1.0 mL/min solvents A and B: CH2Cl2−MeOH−CH3CN−H2O (5.0:85.0:5.5:4.5, v/v) and (25:28:42.5:4.5, v/v), respectively; gradient elution; column temperature: 25 °C; flow rate: 1.0 mL/min solvents A and B: CH2Cl2−MeOH−CH3CN−H2O (5.0:85.0:5.5:4.5, v/v) and (22.0:28.0:45.5:4.5, v/ v), respectively; gradient elution; column temperature: not controlled; flow rate: 1 mL/min solvent: MeOH−CH2Cl2−CH3CN−H2O (69:17:11.5:2.5, v/v); isocratic elution; column temperature: not controlled; flow rate: 1.0 mL/min solvents A and B: MeOH−H2O−EtOAc (82:8:10, v/v) and (20:1:79, v/v), respectively; gradient elution; column temperature: 20 °C; flow rate: 1.2 mL/min solvent: CH3CN−MeOH (7:3, v/v); solvent B: H2O; gradient elution; column temperature: 32 °C; flow rate: 0.4 mL/min solvent A: MeOH−CH2Cl2−CH3CN (90:5:5, v/v); solvent B: H2O; gradient elution; column temperature: not controlled; flow rate: 2.0 mL/min solvent A: CH3CN−MTBE−H2O (69.6:20.0:10.4; v/v); solvent B: CH3CN−MTBE (70:30; v/v); gradient elution; column temperature: 20 °C; flow rate: 0.5 mL/min solvent A: CH3CN−MeOH−0.1 M Tris−HCl (pH 8.0) (84:2:14, v/v); solvent B: MeOH−EtOAc (68:32, v/v); gradient elution; column temperature: not controlled; flow rate: 0.4 mL/min solvents A and B: hexane and acetone, respectively; gradient elution; column temperature: not controlled; flow rate: 1.5 mL/min solvents A and B: acetone and hexane, respectively; gradient elution; column temperature: not controlled; flow rate: 1.25 mL/min

chromatographic conditions

79

Haematococcus pluvialis

85 78 86 87

Chlorococcum sp. Scenedesmus obliquus Chlorococcum sp. transgenic Arabidopsis thaliana transgenic maize

3, 15, 18, 19, 55, esters of 18 and 55 3, 15, 18, 19, 20, 22, 32, 54, 55, 57, 58, and mono- and diesters of 18 3, 15, 18, 19, 54, 55, 56, 58, and esters of 18 3, 4, 15, 16, 19, 22, 53, and 54

92

Botryococcus brauni

74

Chlamydomonas nivalis astaxanthin standard synthetic astaxanthin

3S,3′S, 3R,3′R, and 3R,3′S-astaxanthin (18, 69, and 70, respectively) 18, 69, and 70 18, 69, and 70

95

94

74

esters and glycosides of 18

Synechococcus sp. PCC7942 transformed strains Chlamydomonas nivalis 15, 16, 17, 18, 19, 54, 55, 57, and 58

93

91

Oriolus cruentus 18, 19, 53, 54, and papilioerythrinone (68) 3, 16, 18, 19, 55, 56, and 58

90

transgenic tomato

89

88

3, 4, 5, 15, 16, 18, 19, 54, 55, 58, astaxanthin dimethyl disuccinate (64), β-apo-8′carotenal (65), β-apo-8′-carotenoic acid ethyl ester (66), and citranaxanthin (67) 4, 15, 18, 19, 54, 58, and esters of 18

cis-isomers and mono- and diesters of 18

3, 4, 15, 16, 17, 18, 19, 21, 22, 54, 55, 56, 58, and β-zeacarotene (63)

Parapenaeopsis hardwickii carotenoid standards

84

transgenic maize

3, 4, 15, 16, 17, 18, 31, 55, 56, 58, α-cryptoxanthin (61), and γ-carotene (62)

2

76

Haematococcus pluvialis

3, 16, 15, 17, 18, 19, 48, and crustaxanthin (60) 3, 15, 18, 19, 54, 58, and esters of 18

82

81

83

transgenic carrots

41

80

44

transgenic tobacco and tomato carotenoid standards

ref

matrix Haematococcus pluvialis

thermally treated astaxanthin Marthasterias glacialis

four epoxides and di- and mono-cis-isomers of 18

3, 15, 16, 17, 18, 19, 31, 54, 55, and 58

3, 15, 16, 17, 18, 19, 20, 22, 54, 55, 56, 57, 58, mono- and diesters of 18, and esters of 3 and 16 3, 4, 5, 15, 16, 17, 18, 19, 31, 58, and δ-carotene (59)

3, 18, 19, 54, 55, and esters of 3, 18, and 54

3, 18, 19, and mono- and diesters of 18

carotenoid determined

Table 3. Analytical Conditions for the Separation of Ketocarotenoids in Different Matrices by Liquid Chromatography

Journal of Natural Products Review



Review

a lower wavelength than their corresponding isomer containing a β-ring end group (e.g., 15 and 16) due to the earlier fragmentation of the conjugated double bonds in their structure. Thus, in ethanol, 61 has λmax values at 423, 446, and 473 nm, while 17 exhibits λmax values at 428, 450, and 478 nm.32 Cyclization of the molecule results in a hypochromic effect, a hypsochromic shift, and a loss of fine structure.102 Figure 6 shows examples of the absorption spectra for hydroxycarotenoids and carotenes. Rivera et al. described a wide range of HPLC methods to separate these pigments.7 In general, reversed phases are normally used for the separation of carotenes, whereas hydroxycarotenoids (and other xanthophylls) can be separated on both normal-phase and reversed-phase stationary phases.7 The order of elution for carotenes with the same carbon skeletons but different degrees of unsaturation differs between C18 and C30 columns. On a C18 column, the more saturated compounds are eluted later, whereas on a C30 column, the elution behavior is the opposite. For example, when Rivera et al.103 used an Acquity UPLC BEH and Harada et al.103,104 used a Nocapak C18 column, they found the order of elution for acyclic carotenoids was 4, neurospene (71), ζ-carotene (72), 23, and 21. De Rosso et al.48 and Fraser et al.48,105 employed a YMC C30 column with a different mobile phase composition and discovered the order of elution for the same acyclic carotenoids was 21, 23, 74, 71, and 4. The order of elution for a carotenoid with one ε-ring and its corresponding isomer with a β-ring is the same for most C18 and C30 columns.40,42,43,106

Table 4. Distribution of the Main Hydroxycarotenoids and Carotenes in Some Foods carotenoid

sources

ref

3

apricot, broad bean, broccoli, brussels sprouts, carrot, green bean, kale, lettuce, parsley, pea, pepper, spinach, and sweet corn papaya, persimmon, pink grapefruit, pink guava, tomato and related tomato products, and watermelon apricot, broccoli, carrots, grapefruit, guava, kale, mango, papaya, parsley, persimmon, pumpkin, spinach, sweet potato, tomato, and watermelon honeydew, lettuce, mango, orange, orange pepper, papaya, peach, spinach, and yellow corn

38, 97, 98 98, 99 97, 100

4 15 16 17 31

avocado, chili, corn, orange, papaya, peach, pepper, persimmon, and starfruit apricot, banana, beans, carrot, corn, pepper, pineapple, pumpkin, and tomato

98, 99, 101 97, 99 98, 99

UV−Vis Spectroscopy and Chromatographic Properties. The values of λmax for these compounds range from the ultraviolet to the visible region. Carotenoids containing fewer than seven conjugated double bonds will absorb light in the UV region; therefore, these carotenoids will be colorless. In general, the fewer the number of conjugated double bonds, the lower the wavelength of maximum absorption. For instance, 4, 23, and 21 (which have 11, 5, and 3 conjugated double bonds, respectively) show their longest λmax at 502, 367, and 297 nm, respectively, when dissolved in petroleum ether.32 Carotenoids bearing an ε-ring end group (e.g., 3 and 31) will absorb light at

Figure 8. Structures of major carotenes and α-crytoxanthin. I



Review

Notes

Mass Spectrometry. In general, the fragment ions at m/z 133, 119, 93, and 69 have been observed for several carotenoids exhibiting distinct end groups (e.g., 4, 15, 16, 32, 61, 62, and 65) and using a number of ionization techniques (APCI, ESI, APPI, FAB, EI).10 These ions are generated by the elimination of part of the central acyclic chain of the carotenoid skeleton.10,103 The fragment ion at m/z [M − 137]+ and/or [M + H − 137]+• is observed in compounds containing a β-ring end group, whereas the ion at m/z [M − 56]+• and/or [M + H − 56]+ confirms the presence of an ε-ring end group in the corresponding carotenoid. These fragments have been detected using EI, FAB, and APCI. The m/z 135.1 (APCI) product ion is characteristic of the hydroxycarotenoids and corresponds to the dehydrated terminal ring with cleavage between carbons C7 and C-8.107 Mass fragments at [M − 153]+ and/or [M + H − 153]+• (EI, ESI, FAB, and APCI) indicate pigments containing a β-ring with a hydroxy group (cleavage between carbons C-7 and C-8 of the polyene chain).108 Chemical Testing. The following reactions can be performed to verify the type and position of the hydroxy group in the carotenoid structure: primary and secondary hydroxy groups are acetylated by acetic anhydride in pyridine,12 and allylic hydroxy groups (isolated or allylic to the chromophore) are methylated with acidic methanol.7 In both reactions, a positive response is shown by an increase in the retention time in reversed-phase HPLC systems, and the extent of the increase depends on the number of hydroxy group substituents. In addition, both the acetylated and methylated products have unchanged UV−vis spectra.7,12 Solubility. Compound 4 is soluble in organic solvents such as benzene, CHCl3, or CH2Cl2,68 while compound 15 is insoluble in water and ethanol and barely soluble in vegetable fats.109 The solubility of both 3 and 15 in THF has been shown to be satisfactory.12 Excellent solubility of 3 has also been observed with CH2Cl2 and boiling MeOH (0.67 g/L).100 However, the pigment is less to almost insoluble in ether, CHCl3, carbon disulfide, pyridine, petroleum ether, and hexane.101 Compound 17 is freely soluble in CHCl3, benzene, and pyridine.32

■

CONCLUSIONS

■

AUTHOR INFORMATION

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The author thanks Drs. P. Christou (PVCF and Institucio Catalana de Recerca i Estudis Avancats, Barcelona, Spain) and R. Canela (Lleida University, Lleida, Spain) for introducing her to the analysis of carotenoids, B. Merchant for editing, and D. H. Barragán for assistance in revising figures.

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REFERENCES

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Carotenoids and their active cleavage products have a wide spectrum of natural functions in organisms. These pigments have been gaining interest in different sectors such as the chemical, cosmetics, food, pharmaceutical, and poultry industries. The identification of carotenoids is complex, due mainly to their high structural diversity. In addition, several new carotenoids may be expected to be discovered in the coming years. To achieve a reliable and accurate carotenoid identification, it is necessary to understand their physical and chemical properties. Therefore, a combination of different approaches involving the use of spectroscopic and spectrometric methods (NMR, MS, and UV−vis), chromatographic analysis, determination of solubility, and chemical tests to verify the presence/absence of a specific functional group is helpful in the unambiguous determination of carotenoids in biological samples.

Corresponding Author

*Tel: +1 509-335-9831. E-mail: [email protected]. J



Review

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Guide for Carotenoid Identification in Biological Samples - American

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