Carotenoid Molecules Research Articles

A quantum chemistry study of binding carotenoids in the bacterial light-harvesting complexes.

Carotenoids play the dual function of light harvesting and photoprotection in photosynthetic organisms. Despite their functional importance, the molecular basis for binding of carotenoids in the photosynthetic proteins is poorly understood. We have discovered that all carotenoids are surrounded either by aromatic residues or by chlorophylls in all known crystal structures of the photosynthetic pigment-protein complexes. The intermolecular pi-pi stacking interactions between carotenoids and the surrounding aromatic residues in the light-harvesting complex II (LH-II) of Rhodospirillum molischianum were analyzed by high level ab initio electronic structure calculations. Intermolecular interaction energies were calculated with the second-order Møller-Plesset perturbation method (MP2) using the modified 6-31G*(0.25) basis set with diffuse d-polarization by Hobza and co-workers. The MP2/6-31G*(0.25) calculations yield a total stabilization energy of -15.66 kcal/mol between the carotenoid molecule and the four surrounding aromatic residues (alpha-Trp-23, beta-Phe-20, beta-Phe-24, beta-Phe-27). It is thus concluded that pi-pi stacking interactions between carotenoids and the aromatic residues play an essential role in binding carotenoids in the LH-II complex of Rhodospirillum molischianum. The physical nature of the pi-pi stacking interactions was further analyzed, and the dispersion interactions were found to be the dominant intermolecular attraction force. There is also a substantial electrostatic contribution to the overall intermolecular stabilization energy.

Further insight into the molecular basis of carotenoid–albumin interactions: circular dichroism and electronic absorption study on different crocetin–albumin complexes

Binding of the achiral carotenoid crocetin to the serum albumins of seven different species in borate buffer at pH 8.5 was studied in detail by UV–vis circular dichroism and absorption spectroscopy. The circular dichroism spectra yielded new information about this carotenoid–protein interaction showing the inter- and intramolecular types of induced chirality. In the visible region, human and pig albumin–crocetin complexes exhibited intense, bisignate, vibrationally coupled circular dichroism bands with opposite chirality proving intermolecular excitonic interaction between the bound crocetin molecules, left-handed for human and right-handed for pig albumin, respectively. This sign inversion and the very weak excitonic bands found with dog, horse and rat albumin were interpreted with the model of two carotenoid molecules fitted to the fatty acid binding sites of human albumin in subdomain IIIA. Positive and negative Cotton effects measured around 315–325 nm indicate that the cis-isomers of crocetin also bind to serum albumin and they become optically active due to the distortion around the cis-bond induced by the surrounding protein structure and producing inherently chiral, non-planar conformers. To determine the sense of the resulting molecular helicity the C 2-chirality rule was applied. In all cases, albumin binding caused a bathochromic shift of the visible absorption band of crocetin due to dispersion interactions although the bandwidth and the vibrational fine structure were distinctly altered depending on the kind of albumin used. For human, bovine, rabbit and rat serum albumins the better resolved vibrational structure suggests a few, high affinity binding sites with rigid environments restricting conformational mobility of their ligands while the increased bandwidth and the suppressed fine structure obtained by titration of crocetin solution with horse, pig and dog albumins point to heterogeneous binding sites with lower affinity.

The supramolecular structure of self-assembly formed by capsanthin derivatives.

Carotenoids form structured self-assembly upon aqueous dilution of their organic solutions. In order to test the proposal predicting carotenoid aggregates to be organized in closely packed H-type (card-pack) manner by intermolecular hydrogen bonds, the trihydroxy derivative of capsanthin (1), (6'R)-capsanthol (2) ((all-E,3R,3'S,5'R,6'R)-beta,kappa-carotene-3,3',6'-triol) was acetylated to obtain all varieties of mono-, di- and triacetates and the corresponding supramolecules were studied by UV/Vis- and CD spectroscopy. It was verified that derivatives lacking hydroxyl functions at either of the end-groups form the loosely organized J-type (head-to-tail) aggregates. A model for the structure of the J-type self-assembly is proposed. Evidence was also obtained suggesting that close contacts of carotenoid molecules are not confined to hydrogen bonding.

Comparative X-ray studies on the interaction of carotenoids with a model phosphatidylcholine membrane.

The interaction of structurally different carotenoids with a membrane molecular model was examined by X-ray diffraction. The selected compounds were beta-carotene, lycopene, lutein, violaxanthin, zeaxanthin, and additionally carotane, a fully saturated derivative of beta-carotene. They present similarities and differences in their rigidity, the presence of terminal ionone rings and hydroxy and epoxy groups bound to the rings. The membrane models were multibilayers of dipalmitoylphosphatidylcholine (DPPC), chosen for this investigation because the 3 nm thickness of the hydrophobic core of its bilayer coincides with the thickness of the hydrophobic core of thylakoid membranes and the length of the carotenoid molecules. Results indicate that the six compounds induced different types and degrees of structural perturbations to DPPC bilayers in aqueous media. They were interpreted in terms of the molecular characteristics of DPPC and the carotenoids. Lycopene and violaxanthin induced the highest structural damage to the acyl chain and polar headgroup regions of DPPC bilayers, respectively.

Molecular structures of carotenoids as predicted by MNDO-AM1 molecular orbital calculations

Semi-empirical molecular orbital calculations using AM1 Hamiltonian (MNDO-AM1 method) were performed for a number of biologically important carotenoid molecules, namely all- trans-β-carotene, all- trans-zeaxanthin, and all- trans-violaxanthin (found in higher plants and algae) together with all- trans-canthaxanthin, all- trans-astaxanthin, and all- trans-tunaxanthin in order to predict their stable structures. The molecular structures of all- trans-β-carotene, all- trans-canthaxanthin, and all- trans-astaxanthin predicted based on molecular orbital calculations were compared with those determined by X-ray crystallography. Predicted bond lengths, bond angles, and dihedral angles showed an excellent agreement with those determined experimentally, a fact that validated the present theoretical calculations. Comparison of the bond lengths, bond angles and dihedral angles of the most stable conformer among all the carotenoid molecules showed that the displacements are localized around the substituent groups and hence around the cyclohexene rings. The most stable conformers of all- trans-zeaxanthin and all- trans-violaxanthin gave rise to a torsion angle around the C6–C7 bond to be ±48.7 and −84.8°, respectively. This difference is a key factor in relation to the biological function of these two carotenoids in plants and algae (the xanthophyll cycle). Further analyses by calculating the atomic charges and using enpartment calculations (division of bond energies between component atoms) were performed to ascribe the cause of the different observed torsion angles.

Photooxidation of Carotenoids in Mesoporous MCM-41, Ni-MCM-41 and Al-MCM-41 Molecular Sieves

Photooxidation of β-carotene and canthaxanthin in mesoporous MCM-41, Ni-MCM-41, and Al-MCM-41 molecular sieves was studied by 9−220 GHz electron paramagnetic resonance (EPR) and 9 GHz electron nuclear double resonance (ENDOR). X-ray powder diffraction (XRD) measurements established that the MCM-41 pore size (33 A) was large enough to accommodate carotenoids. Mesoporous MCM-41 molecular sieves are found to be promising hosts for long-lived photoinduced charge-separation between carotenoid radical cations (Car•+) and the MCM-41 framework. Incorporating metal ions into siliceous MCM-41 enhances efficiency of carotenoid oxidation. The photoyield and stability of generated carotenoid radical cations increased in the order MCM < Ni-MCM < Al-MCM. Formation of carotenoid radical cations within the Me-MCM-41 is due to electron transfer between incorporated carotenoid molecules and metal ions, which act as electron acceptor sites. Detected EPR signals of Ni(I) species provide direct evidence for the reduction of Ni...

An unusual pathway of excitation energy deactivation in carotenoids: singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna.

Carotenoids are important biomolecules that are ubiquitous in nature and find widespread application in medicine. In photosynthesis, they have a large role in light harvesting (LH) and photoprotection. They exert their LH function by donating their excited singlet state to nearby (bacterio)chlorophyll molecules. In photosynthetic bacteria, the efficiency of this energy transfer process can be as low as 30%. Here, we present evidence that an unusual pathway of excited state relaxation in carotenoids underlies this poor LH function, by which carotenoid triplet states are generated directly from carotenoid singlet states. This pathway, operative on a femtosecond and picosecond timescale, involves an intermediate state, which we identify as a new, hitherto uncharacterized carotenoid singlet excited state. In LH complex-bound carotenoids, this state is the precursor on the reaction pathway to the triplet state, whereas in extracted carotenoids in solution, this state returns to the singlet ground state without forming any triplets. We discuss the possible identity of this excited state and argue that fission of the singlet state into a pair of triplet states on individual carotenoid molecules constitutes the mechanism by which the triplets are generated. This is, to our knowledge, the first ever direct observation of a singlet-to-triplet conversion process on an ultrafast timescale in a photosynthetic antenna.

Efficient Energy Transfer from the Carotenoid S 2 State in a Photosynthetic Light-Harvesting Complex

Previously, the spatial arrangement of the carotenoid and bacteriochlorophyll molecules in the peripheral light-harvesting (LH2) complex from Rhodopseudomonas acidophila strain 10050 has been determined at high resolution. Here, we have time resolved the energy transfer steps that occur between the carotenoid’s initial excited state and the lowest energy group of bacteriochlorophyll molecules in LH2. These kinetic data, together with the existing structural information, lay the foundation for understanding the detailed mechanisms of energy transfer involved in this fundamental, early reaction in photosynthesis. Remarkably, energy transfer from the rhodopin glucoside S 2 state, which has an intrinsic lifetime of ∼120 fs, is by far the dominant pathway, with only a minor contribution from the longer-lived S 1 state.

Chlorophyll and carotenoid radicals in photosystem II studied by pulsed ENDOR.

The stable carotenoid cation radical (Car(*+)) and chlorophyll cation radical (Chl(Z)(*+)) in photosystem II (PS II) have been studied by pulsed electron nuclear double resonance (ENDOR) spectroscopy. The spectra were essentially the same for oxygen-evolving PS II and Mn-depleted PS II. The radicals were generated by illumination given at low temperatures, and the ENDOR spectra were attributed to Car(*)(+) and Chl(Z)(*+) on the basis of their characteristic behavior with temperature as demonstrated earlier [Hanley et al. (1999) Biochemistry 38, 8189-8195]: i.e., (a) the Car(*)(+) alone was generated by illumination at < or =20 K, while Chl(Z)(*+) alone was generated at 200 K, and (b) warming of the sample containing the Car(*+) to 200 K resulted in the loss of the signal attributable to Car(*+) and its replacement by a spectrum attributable to the Chl(Z)(*+). A map of the hyperfine structure of Car(*+) in PS II and in organic solvent was obtained. The largest observed hyperfine splitting for Car(*+) in either environment was in the order of 8-9 MHz. Thus, the spin density on the cation is proposed to be delocalized over the carotenoid molecule. The pulsed ENDOR spectrum of Chl(Z)(*)(+) was compared to that obtained from a Chl a cation in frozen organic solvent. The hyperfine coupling constants attributed to the beta-protons at position 17 and 18 are well resolved from Chl(Z)(*+) in PS II (10. 8 and 14.9 MHz) but not in Chl a(*+) in organic solvent (12.5 MHz). This suggests a more defined conformation of ring IV with respect to the rest of the tetrapyrrole ring plane of Chl(Z)(*+) than Chl a(*+) probably induced by the protein matrix.

Antioxidant activity of polar carotenoids including astaxanthin-beta-glucoside from marine bacterium on PC liposomes

SUMMARY: This study was aimed at evaluating the antioxidant activity of polar carotenoids (zeaxanthin, astaxanthin, and astaxanthin-β-glucoside) in free radical-mediated oxidation of phosphatidylcholine (PC) liposomes. In 1-palmitoyl-2-linoleoyl-PC liposomes, astaxanthin-β-glucoside was the most active, followed by astaxanthin, zeaxanthin, and β-carotene, although there was no significant difference in the antioxidant activity of these carotenoids in chloroform solution. Thus, it is suggested that the different antioxidant activity of four kinds of carotenoids found in the PC liposomes would depend not on their ability to scavenge free radicals, but on other factors such as their location and orientation in PC liposome systems and the degree of their incorporation into PC bilayers. These physicochemical properties would be strongly correlated with the structural acceptability between the PC molecule and carotenoid molecule, especially between two polar groups of the PC bilayer and the carotenoid. Furthermore, two polar groups of the carotenoid can act as trans-bilayer rivets. The tightly packed conformation of PC bilayers caused by this function of the dipolar carotenoid would also be correlated with their antioxidant activity. This was suppported by the result that cholesterol, known as acting like a peg through one half of the bilayer, also increased the oxidative stability of PC liposomes.

Organization of the pigment molecules in the chlorophyll a/b/c containing alga Mantoniella squamata (Prasinophyceae) studied by means of absorption, circular and linear dichroism spectroscopy

In order to obtain information on the organization of the pigment molecules in chlorophyll (Chl) a/b/c-containing organisms, we have carried out circular dichroism (CD), linear dichroism (LD) and absorption spectroscopic measurements on intact cells, isolated thylakoids and purified light-harvesting complexes (LHCs) of the prasinophycean alga Mantoniella squamata. The CD spectra of the intact cells and isolated thylakoids were predominated by the excitonic bands of the Chl a/b/c LHC. However, some anomalous bands indicated the existence of chiral macrodomains, which could be correlated with the multilayered membrane system in the intact cells. In the red, the thylakoid membranes and the LHC exhibited a well-discernible CD band originating from Chl c, but otherwise the CD spectra were similar to that of non-aggregated LHC II, the main Chl a/b LHC in higher plants. In the Soret region, however, an unusually intense (+) 441 nm band was observed, which was accompanied by negative bands between 465 and 510 nm. It is proposed that these bands originate from intense excitonic interactions between Chl a and carotenoid molecules. LD measurements revealed that the Q Y dipoles of Chl a in Mantoniella thylakoids are preferentially oriented in the plane of the membrane, with orientation angles tilting out more at shorter than at longer wavelengths (9° at 677 nm, 20° at 670 nm and 26° at 662 nm); the Q Y dipole of Chl c was found to be oriented at 29° with respect to the membrane plane. These data and the LD spectrum of the LHC, apart from the presence of Chl c, suggest an orientation pattern of dipoles similar to those of higher plant thylakoids and LHC II. However, the tendency of the Q Y dipoles of Chl b to lie preferentially in the plane of the membrane (23° at 653 nm and 30° at 646 nm) is markedly different from the orientation pattern in higher plant membranes and LHC II. Hence, our CD and LD data show that the molecular organization of the Chl a/b/c LHC, despite evident similarities, differs significantly from that of LHC II.

In vivo analysis of the electron transfer within photosystem I: are the two phylloquinones involved?

Electron transfer within PS I reaction centers has been analyzed in vivo in a mutant of Chlorella sorokiniana which lacks most of the PS II and of the peripheric antennae, using a new spectrophotometric technique with a time resolution of approximately 5 ns. Absorption changes associated with the oxidation of semiphylloquinone (acceptor A(1)(-)) have been characterized in the 371-545 nm spectral range. The oxidation of A(1)(-) and the reduction of an iron-sulfur cluster (F(X), F(A)F(B)) is monitored by an absorption decrease at 377 nm (semiphylloquinone absorption band) and by the decrease of two positive absorption bands around 480 and 515 nm, respectively, very likely associated with a local electrochromic shift induced by A(1)(-) on a carotenoid molecule localized in its vicinity. A(1)(-) undergoes a two-phase oxidation of about equal amplitude with half-times of approximately 18 and approximately 160 ns, respectively. Two hypotheses are proposed to interpret these data: (1) Photosystem I reaction centers are present under two conformational states which differ by the reoxidation rate of A(1)(-). (2) The two phylloquinones corresponding to the two branches of the PS I heterodimer are involved in the electron transfer. The similar amplitude of the two phases implies that the rates of electron transfer from P700 to each of the phylloquinones are about equal. The two different rate constants measured for A(1)(-) oxidation suggests some asymmetry in the relative position of the two phylloquinones with respect to F(X).

Surface Modification of TiO2 Nanoparticles with Carotenoids. EPR Study

EPR measurements demonstrate efficient charge separation on carotenoid-modified titanium dioxide nanoparticles (7 nm). Strong complexation of carotenoids containing terminal carboxy groups (−CO2H) with the TiO2 surface leads to electron transfer from the adsorbed carotenoid molecule to the surface trapping site. For these systems, EPR signals of the carotenoid radical cations Car•+ and the electrons trapped on the TiO2 are observed before irradiation (77 K). Their UV−visible spectra show an absorption band with a maximum near 650 nm that is characteristic of the trapped electrons. Surface modification of the TiO2 by other carotenoids results in the formation of a complex with an optical absorption band near 545 nm. These systems form charge-separated pairs [Car•+···TiO2(e-tr)surf. TiO2(e-tr)latt.] only upon 365−600 nm illumination at 77 K. Complexation of the TiO2 colloids with carotenoids enhances spatial charge separation, shifts the absorption threshold into the visible region, and thus greatly improves the reducing ability of the semiconductor. Photoreduction of acceptor molecules such as 2,5-dichloro-1,4-benzoquinone, nitrobenzene, and oxygen is demonstrated.

Effects of high light and desiccation on the operation of the xanthophyll cycle in two marine brown algae

Two brown algae, Pelvetia canaliculata and Laminaria saccharina, from the higher and lower mediolittoral belts respectively, have been tested for their capacity to overcome high-light stress in water and in air (in both fully hydrated and desiccated states). When exposed to supersaturating light irradiance in water, the two species developed non-photochemical quenching of fluorescence (NPQ) which was correlated with an increase in the de-epoxidation ratio (DR) of the xanthophyll cycle carotenoids (violaxanthin, antheraxanthin and zeaxanthin) and was followed by a slower decrease in oxygen evolution. NPQ reached values of up to 9 in P. canaliculata but only 4·5 in L. saccharina, at DRs of 0·65 and 0·5, respectively. In air, the xanthophyll cycle was also operative but the efficiency of de-epoxidation decreased linearly with the degree of hydration of the thallus. Photoprotection capacities in air also appeared higher in P. canaliculata than in L. saccharina, probably due to the higher molar content of the xanthophyll cycle pool size relative to chlorophyll a (Chl a) in the former (nearly double of that L. saccharina at 19 carotenoid molecules per 100 Chl a), which may be associated with a higher DR at the same level of desiccation. The concomitant higher accumulation of zeaxanthin in P. canaliculata might divert a higher percentage of the incident energy from the reaction centres, as demonstrated by the levels of NPQ, with steady-state fluorescence reduced to below the initial F0 level. Such differences, together with the unequal resistance to desiccation of the operation of the xanthophyll cycle, should be considered as possible factors responsible for the distribution for these two species on the shore.

Effects of high light and desiccation on the operation of the xanthophyll cycle in two marine brown algae

Two brown algae, Pelvetia canaliculata and Laminaria saccharina, from the higher and lower mediolittoral belts respectively, have been tested for their capacity to overcome high-light stress in water and in air (in both fully hydrated and desiccated states). When exposed to supersaturating light irradiance in water, the two species developed non-photochemical quenching of fluorescence (NPQ) which was correlated with an increase in the de-epoxidation ratio (DR) of the xanthophyll cycle carotenoids (violaxanthin, antheraxanthin and zeaxanthin) and was followed by a slower decrease in oxygen evolution. NPQ reached values of up to 9 in P. canaliculata but only 4·5 inL. saccharina, at DRs of 0·65 and 0·5, respectively. In air, the xanthophyll cycle was also operative but the efficiency of de-epoxidation decreased linearly with the degree of hydration of the thallus. Photoprotection capacities in air also appeared higher in P. canaliculata than in L. saccharina, probably due to the higher molar content of the xanthophyll cycle pool size relative to chlorophyll a (Chl a) in the former (nearly double of that L. saccharina at 19 carotenoid molecules per 100 Chl a), which may be associated with a higher DR at the same level of desiccation. The concomitant higher accumulation of zeaxanthin in P. canaliculata might divert a higher percentage of the incident energy from the reaction centres, as demonstrated by the levels of NPQ, with steady-state fluorescence reduced to below the initial F0 level. Such differences, together with the unequal resistance to desiccation of the operation of the xanthophyll cycle, should be considered as possible factors responsible for the distribution for these two species on the shore.

AM1, INDO/S and optical studies of carbocations of carotenoid molecules. Acid induced isomerization

The mechanism of trans–cis isomerization of carotenoid molecules through the formation of the carotenoid carbocation (CarH+) intermediate in the presence of acid is presented. Various cis-isomers of carotenoids (β-carotene, canthaxanthin and 8′-apo-caroten-8′-al) are predicted from AM1 calculations of rotation barriers for CarH+, as well as from the stabilities of CarH+. AM1 dipole moments D and INDO/S optical transitions of CarH+ were calculated for all protonation sites. Optical spectra of CarH+ solutions exhibit broad lines in the region of 700–1000 nm with extinction coefficients of 1–3 × 105 M–1 cm–1.

Biological functions of carotenoids--diversity and evolution.

Carotenoids first emerged in archaebacteria as lipids reinforcing cell membranes. To serve this function their long molecules have extremely rigid backbone due to the linear chain of usually 10 to 11 conjugated C=C bonds in transconfiguration--the length corresponding the thickness of hydrophobic zone of membrane which they penetrate as "molecular rivets". Carotenoids retain their membrane-reinforcing function in some fungi and animals. The general structure of carotenoid molecule, originally having evolved for mechanical functions in membranes, possess a number of other properties that were later used for independent functions. The most striking fact is that these properties proved to fit some new functions to perfection. The polyene chain of 9-11 double bonds absorbs light precisely in the gap of chlorophyll absorption--function as accessory light-harvesting pigments in all plants; Unique arrangement of electronic levels owing to the by polyene chain structure makes carotenoids the only natural compounds capable of excitation energy transfer both (i) from carotenoid excited state to chlorophyll in the light-harvesting complex and (ii) from triplet chlorophyll or singlet oxygen to carotenoid in photosynthetic reaction centers--protection of RC from photodamage. The linear system of conjugated C=C bonds provides high reducing potential of carotenoid molecules making them potent antioxidants in lipid formations. Still, there is a lack of evidence of the chemical antioxidant function of carotenoids, especially in higher organisms; most data demonstrate an antioxidant ability rather than a function. Carotenoids have many other independent biological functions, including: specific coloration patterns in plants and animals, screening from excessive light and spectral filtering, defense of egg proteins from proteases in some invertebrates; the direct carotenoid derivative--retinal--acts as visual pigment in all animals and as chromophore in bacteriorhodopsin photosynthesis, retinoic acid in animals and abscisic acid in plants serve as hormones. All these functions utilize various properties (mechanical, electronic, stereospecific) of a single structure evolved in bacteria for a single membrane-reinforcing function, thus demonstrating an example of pure evolutionary preadaptation. One of the practical conclusions that can be reached by reviewing uniquely diverse properties and functions of carotenoids is that, when considering possible mechanisms of their effects in organisms (e.g., anticarcinogenic action), all their functional traits should be taken into account.

Carotene as a Molecular Wire: Conducting Atomic Force Microscopy

A conducting atomic force microscope was used to measure the electrical properties of carotenoid molecules attached to a gold electrode. The thiolated carotene molecules were embedded in insulating n-alkanethiol self-assembled monolayers. At a contact force of a few nanoNewtons, a carotenoid molecule behaves ohmically with a resistance of approximately 4.2 ± 0.7 GΩ, over a million times more conductive than an alkane chain of similar length. Modes of electron transport are discussed.

Organization of the pigment molecules in the thylakoids and the chlorophyll a/c light-harvesting complex of a xanthophyte alga, Pleurochloris meiringensis. A linear dichroism study

We have studied the orientation of the absorption transition dipoles of the pigment molecules in thylakoid membranes, in the isolated core complex of photosystem I (CCI) and in the chlorophyll a/c light-harvesting complex of photosystem II (Chl a/c LHC) of the xanthophyte alga Pleurochloris meiringensis. In thylakoids, linear dichroism (LD) spectra reveal two distinct orientations of the Q y transition dipoles of the Chl a molecules: the long-wavelength Chl a molecules tend to lie in the plane of the thylakoid membrane, while those at shorter wavelengths tilt out of the membrane plane. Our data show that in CCl, the orientation of Q y dipoles of Chl a, and the overall orientation pattern of all transition dipoles in the visible region, are very similar to those in higher-plant photosystem I particles. The Chl a molecules with their Q y dipoles tilting out of the membrane plane are associated with the LHC, and constitute a substantial part of the Chl a population. The majority of the carotenoid molecules of LHC also tend to tilt out of the membrane plane. These data further corroborate our earlier conclusion that the molecular organization of thylakoid membranes and most particularly of the Chl a/c LHC differs significantly from the organization of higher-plant thylakoids and the main Chl a/b complexes, respectively.

EPR and ENDOR studies of carotenoid–solid Lewis acid interactions

EPR and ENDOR results provide strong support for the formation of carotenoid radical cations on activated alumina and silica–alumina as a consequence of electron transfer from the adsorbed carotenoid molecules to the Lewis acid sites on these surfaces. This assignment is confirmed by the correlation between the AlIII Lewis acid site content on the activated surfaces and the maximum concentration of the generated carotenoid radical cations. The results are compared with those obtained for a solution of carotenoid and AlCl3, which serves as a model system. The ENDOR line at ca. 6.5 MHz obtained for the carotenoids adsorbed on solid supports and carotenoid–AlCl3 solution, is attributed to the high-frequency feature of a hyperfine doublet, centred about the 27Al Larmor frequency. ENDOR detection of the hyperfine doublet, instead of the single line at the Larmor frequency, indicates the formation of strong complexes between carotenoid molecules and Lewis acid sites on the surface.