Showing papers on "Radical ion published in 2001"
TL;DR: It is demonstrated that PC is a potent ABTS(*)(+) scavenger even when bound to protein and that the complexes may act as a radical sink within the gastrointestinal tract.
Abstract: The 2,2'-azinobis(3-ethylbenzothiazoline 6-sulfonic acid) radical cation (ABTS(*)(+)) decolorization assay has been used to determine the antioxidant activity of the polyphenol epicatechin(16) (4 --> 8) catechin (procyanidin, PC) alone or in complex with the model proteins bovine serum albumin (BSA) or gelatin. PC had a molar antioxidant capacity of approximately 54, 92, or 108 radicals at pH values of 3.0, 4.9, or 7.4, respectively. Radical scavenging occurred via a rapid step followed by a slow step. Interaction with gelatin reduced the rate of rapid scavenging by 50% (PC-BSA mixtures reduced by 15%). Inhibition paralleled formation of precipitable PC-protein complexes over a range of protein/PC ratios. However, inhibition was virtually overcome in 10 min. Reaction with ABTS(*)(+) converted the PC-protein complexes from a dissociable form to a form resistant to dissociation by strong denaturants such as SDS. This study demonstrates that PC is a potent ABTS(*)(+) scavenger even when bound to protein and that the complexes may act as a radical sink within the gastrointestinal tract.
264 citations
TL;DR: The oxidation potentials have been established, showing that, in Triton X-100 micelles, lycopene is the easiest carotenoid to oxidize to its radical cation and astaxanthin is the most difficult.
259 citations
TL;DR: Isomeric flavonoid O-diglycosides were analyzed by positive and negative nano-electrospray ionization (ESI) ion trap mass spectrometry to evaluate whether the two most common interglycosidic linkage types, i.e. 1 --> 2 and 1 --> 6, found for glycosides containing a rhamnosylglucose glycan part can be differentiated.
Abstract: Isomeric flavonoid O-diglycosides were analyzed by positive and negative nano-electrospray ionization (ESI) ion trap mass spectrometry (ITMS} in order to evaluate whether the two most common interglycosidic linkage Types, i.e. 1 --> 2 and 1 --> 6, found for glycosides containing a rhamnosylglucose glycan part can be differentiated. In the positive ion mode the degree of internal glucose residue loss was found to be strongly dependent on the aglycone type and was very pronounced for aglycones of the flavanone type. The relative abundance of the Y-type ions formed by fragmentation at glycosidic bonds only allows one to infer the interglycosidic linkage types in the case of flavone O-diglycosides. In contrast, the negative ion mode makes a clear differentiation between a rutinoside (1 --> 6) and a neohesperidoside (1 --> 2) glycan residue possible for all aglycone types. The neohesperidose-containing compounds could be characterized by additional product ions. When the compounds were dissolved in pure methanol a molecular radical ion tvas found to be the base peak in nano-ESI. Copyright (C) 2001 John Wiley & Sons, Ltd.
222 citations
TL;DR: In contrast with all the previous results on porphyrin-fullerene dyads, the photoexcitation of a zinc chlorin-C60 dyad results in an unusually long-lived radical ion pair which decays via first-order kinetics with a decay rate constant of 9.1 x 10(3) x s(-1).
Abstract: The photochemical and electrochemical properties of four chlorin−C60 or porphyrin−C60 dyads having the same short spacer between the macrocycle and the fullerene are examined In contrast with all the previous results on porphyrin−fullerene dyads, the photoexcitation of a zinc chlorin−C60 dyad results in an unusually long-lived radical ion pair which decays via first-order kinetics with a decay rate constant of 91 × 103 s-1 This value is 2−6 orders of magnitude smaller than values reported for all other porphyrin or chlorin donor−acceptor of the molecule dyad systems The formation of radical cations of the donor part and the radical anion of the acceptor part was also confirmed by ESR measurements under photoirradiation at low temperature The photoexcitation of other dyads (free-base chlorin−C60, zinc porphyrin−C60, and free-base porphyrin−C60 dyads) results in formation of the ion pairs which decay quickly to the triplet excited states of the chlorin or porphyrin moiety via the higher lying radical i
182 citations
TL;DR: The radical cation 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonate), (ABTS*+) was utilized in an on-line HPLC method for the detection of radical scavengers in complex matrixes and can be applied to perform kinetic studies.
Abstract: The radical cation 2,2‘-azinobis-(3-ethylbenzothiazoline-6-sulfonate), (ABTS•+) was utilized in an on-line HPLC method for the detection of radical scavengers in complex matrixes. The HPLC-separated analytes react postcolumn with the preformed ABTS•+, and the induced bleaching is detected as a negative peak by an absorbance detector at 734 nm. An optimized instrumental and experimental setup is presented. The method is suitable for both isocratic and gradient HPLC runs using mobile phases containing 100% organic solvent or its solution in water, weak acids, or buffers (pH 3−7.4). The method is sensitive, selective, relatively simple, applicable to compounds of different chemical natures; uses common instruments and inexpensive reagents; and has a time-saving, nonlaborious experimental protocol. It can also be used for quantitative analysis. The method was applied to several pure natural antioxidants and plant extracts. The minimum detectable concentration varied from 0.02 to 0.13 μg/mL, depending on the c...
169 citations
TL;DR: It is shown that the CO⨪3 anion can selectively oxidize guanines in the self-complementary oligonucleotide duplex d(AACGCGAATTCGCGTT) dissolved in air-equilibrated aqueous buffer solution by employing nanosecond laser flash photolysis methods.
149 citations
TL;DR: O2 acts as a novel catalyst in accelerating the BET of the C60.--ZnP.+ radical ion pairs and facilitates an intermolecular electron transfer (ET) from C60.- to O2.
Abstract: Dioxygen accelerates back electron transfer (BET) processes between a fullerene radical anion (C60) and a radical cation of zinc porphyrin (ZnP) in photolytically generated ZnP.+-C60.- and ZnP.+-H2P-C60.- radical ion pairs. The rate constant of BET increases linearly with increasing oxygen concentration without, however, forming reactive oxygen species, such as singlet oxygen or superoxide anion. When ferrocene (Fc) is used as a terminal electron donor moiety instead of ZnP (i.e., Fc-ZnP-C60), no catalytic effects of dioxygen were, however, observed for the BET in Fc+-ZnP-C60.-, that is, from C60.- to the ferricenium ion. In the case of ZnP-containing C60 systems, the partial coordination of O2 to ZnP.+ facilitates an intermolecular electron transfer (ET) from C60.- to O2. This rate-determining ET step is followed by a rapid intramolecular ET from O2.- to ZnP.+ in the corresponding O2.--ZnP.+ complex and hereby regenerating O2. In summary, O2 acts as a novel catalyst in accelerating the BET of the C60.--ZnP.+ radical ion pairs.
129 citations
TL;DR: In this paper, it was shown that the cis isomer of the o-cresol radical cation shows a low-frequency shift of the OH stretching attributed to an intramolecular hydrogen bond.
Abstract: Considering the findings of Fujii et al. showing that the cis isomer of the o-cresol radical cation shows a low-frequency shift of the OH stretching attributed to an intramolecular hydrogen bond wi...
122 citations
TL;DR: The photophysical and photochemical deactivation pathways of electronically excited methyl viologen (1,1'-dimethyl-4,4'bipyridinium, MV2+) were studied in several polar solvents at room temperature using a variety of ultrafast time-resolved and steady-state spectroscopic techniques.
Abstract: The photophysical and photochemical deactivation pathways of electronically excited methyl viologen (1,1‘-dimethyl-4,4‘-bipyridinium, MV2+) were studied in several polar solvents at room temperature using a variety of ultrafast time-resolved and steady-state spectroscopic techniques. The results highlight the very strong electron accepting character of the lowest singlet excited state of MV2+. Transient absorption was measured between 270 and 740 nm as a function of delay time after excitation of the strong π−π* transition of MV2+ by a 150 fs, 265 nm pump pulse. In methanol, the radical cation of methyl viologen (MV•+) appeared within our time resolution, indicating that forward electron transfer from a nearby donor quenches electronically excited MV2+ in < 180 fs. Identical dynamics within experimental uncertainty were observed for the chloride salt of MV2+ and for the salt prepared with tetrafluoroborate counterions. This latter “superhalide” ion has a condensed-phase detachment threshold that is too hi...
110 citations
TL;DR: Second generation product ions are formed as a result of elimination of the tyrosyl and tryptophan-containing peptide side chains, many of which are typically absent in the fragmentation of protonated peptide ions.
99 citations
TL;DR: This is the first study that demonstrates the formation of a protonated trihydrobiopterin radical with the constitutive isoforms of NOS, and the first time the radical was obtained without exogenous BH4.
Abstract: Nitric oxide synthase (EC 1.14.13.39; NOS) converts L-arginine into NO and L-citrulline in a two-step reaction with Nomega-hydroxy-L-arginine (NOHLA) as an intermediate. The active site iron in NOS has thiolate axial heme-iron ligation as found in the related monooxygenase cytochrome P450. In NOS, tetrahydrobiopterin (BH4) is an essential cofactor for both steps, but its function is controversial. Previous optical studies of the reaction between reduced NOS with O2 at -30 degrees C suggested that BH4 may serve as an one-electron donor in the first cycle, implying formation of a trihydrobiopterin radical. We investigated the same reaction under identical conditions with electron paramagnetic resonance spectroscopy. With BH4-containing full-length neuronal NOS we obtained an organic free radical (g-value 2.0042) in the presence of Arg, and a similar radical was observed with the endothelial NOS oxygenase domain in the presence of Arg and BH4. Without substrate the radical yield was greatly (10x) diminished. Without BH4, or with NOHLA instead of Arg, no radical was observed. With 6-methyltetrahydropterin or 5-methyl-BH4 instead of BH4, radicals with somewhat different spectra were formed. On the basis of simulations we assign the signals to trihydropterin radical cations protonated at N5. This is the first study that demonstrates the formation of a protonated trihydrobiopterin radical with the constitutive isoforms of NOS, and the first time the radical was obtained without exogenous BH4. These results offer strong support for redox cycling of BH4 in the first reaction cycle of NOS catalysis (BH4 BH3.H+).
TL;DR: A periodic structured organosilica solid of the MCM-41 type containing 4,4′-bipyridinium presumably inserted on the walls that is stable after surfactant removal has been prepared as mentioned in this paper.
TL;DR: The present state of research on the formation and reactivity of certain sulfur-centered radicals [RS*, RSS*, RS*+, (RSSR)*+] and their implications for biological systems are discussed.
Abstract: Sulphur compounds play a central role in the structure and activity of many vital systems. In the living cell, sulfur constitutes an essential part of the defense against oxidative damage and is transformed into a variety of sulfur free radical species. Many studies of the chemistry of sulfur-centered radicals using pulse radiolysis and photolysis techniques to detect and measure the kinetics of these radicals have been published and reviewed. This paper discusses the present state of research on the formation and reactivity of certain sulfur-centered radicals [RS·, RSS·, RS·+, (RSSR)·+] and their implications for biological systems.Key words: sulfur-centered radicals, thiylradicals, sulfur-centered radical cation, cation radicals.
TL;DR: The π-radical cation as well as the dication species of a series of quinonoid πextended TTF have been generated by means of time-resolved and steady-state radiation chemical techniques as mentioned in this paper.
Abstract: The π-radical cation as well as the dication species of a series of quinonoid π-extended TTF have been generated by means of time-resolved and steady-state radiation chemical techniques. The spectr...
TL;DR: In this paper, it was shown that a small energy barrier separates the unbound FS+/N2 state from the bound FS2+/n2-state, which explains the facile reversibility of the electron transfer process.
Abstract: Upon nitrogen adsorption at low temperature onto the surface of magnesium oxide and calcium oxide containing FS+ centers (single electron trapped in a suitable surface vacancy), electron transfer occurs from the solid to the adsorbed molecule. About 90% of the total electron density is localized on the adsorbed molecule. The 11-electron N2- radical anion so formed has been detected by electron paramagnetic resonance for both 14N2 and 15N2. The electron transfer is reversible and, when the pressure is lowered or the temperature increased, the N2 molecules desorb, regenerating the original FS+ center. The diatomic radical ion lies parallel to the surface, and the electron density is mainly confined in the πy* orbital. Theoretical calculations at the DFT level indicate that a small energy barrier separates the unbound FS+/N2 state from the bound FS2+/N2- state. This explains the facile reversibility of the electron-transfer process. The calculated spin densities are in excellent agreement with those derived ...
TL;DR: The one‐electron reduction potentials of the radical cations of five dietary carotenoids in aqueous micellar environments have been obtained from a pulse radiolysis study of electron transfer between the carotanoids and tryptophanradical cations as a function of pH, and lie in the range of 980–1060 mV, consistent with the observation that the carotinoids oxidise tyrosine and cysteine.
TL;DR: These IET paths indicate that quite different IET mechanisms may operate in radical cations, revealing an unexpectedly enriched and flexible mechanistic spectrum.
Abstract: The structure of the potential energy surface for the intramolecular electron transfer (IET) of four different model radical cations has been determined by using reaction path mapping and conical intersection optimization at the ab initio CASSCF level of theory. We show that, remarkably, the calculated paths reside in regions of the ground-state energy surface whose structure can be understood in terms of the position and properties of a surface crossing between the ground and the first excited state of the reactant. Thus, in the norbornadiene radical cation and in an analogue compound formed by two cyclopentene units linked by a norbornyl bridge, IET proceeds along direct-overlap and super-exchange concerted paths, respectively, that are located far from a sloped conical intersection point and in a region where the excited-state and ground-state surfaces are well separated. A second potential energy surface structure has been documented for 1,2-diamino ethane radical cation and features two parallel conc...
TL;DR: The radical pair mechanism is developed theoretically and shown to account satisfactorily for both the magnetic field effect and the associated magnetic isotope effect.
Abstract: We present measurements of the spectrum (1--80 MHz) of the effect of a weak (approximately 500 microT) radio frequency magnetic field on the electron-hole recombination of radical ion pairs in solution. Distinct spectra are observed for the pyrene anion/dimethylaniline cation radical pair in which one or both of the radicals are perdeuterated. The radical pair mechanism is developed theoretically and shown to account satisfactorily for both the magnetic field effect and the associated magnetic isotope effect.
TL;DR: The reaction of ONOO(-) with CO(2) at alkaline pH was recently reported to form a transient absorption, and the rate constant of the decay of peroxynitrite was found to be identical to that of the formation of ferricyanide, indicating that ONOOC(O)(-) does not accumulate.
Abstract: The reaction of ONOO- with CO2 at alkaline pH was recently reported to form a transient absorption with a maximum at 640 nm and a half-life of ca. 4 ms at 10 degreesC [Meli et al. (1999) Helv. Chim ...
TL;DR: The 4-aminophenyl triplet cation formed under these conditions is trapped by iodide more efficiently than by the best pi nucleophiles, but in contrast to the singlet cation, it does not form ethers with alcohols, by which it is rather reduced.
Abstract: 4-Chloroaniline and its N,N-dimethyl derivative are photostable in cyclohexane but undergo efficient photoheterolysis in polar media via the triplet state and give the corresponding triplet phenyl cations. CASSCF and UB3LYP calculations show that the 4-aminophenyl triplet cation has a planar geometry and is stabilized by >10 kcal mol(-1) with respect to the slightly bent singlet. The triplet has a mixed carbene-diradical character at the divalent carbon. This species either adds to the starting substrate forming 5-chloro-2,4'-diaminodiphenyls (via an intermediate cyclohexadienyl cation) or is reduced to the aniline (via the aniline radical cation) in a ratio depending on the hydrogen-donating properties of the solvent. Transients attributable to the triplet aminophenyl cation as well as to the ensuing intermediates are detected. Chemical evidence for the generation of the phenyl cation is given by trapping via electrophilic substitution with benzene, mesitylene, and hexamethylbenzene (in the last case the main product is a 6-aryl-3-methylene-1,4-cyclohexadiene). Relative rates of electrophilic attack to benzene and to some alkenes and five-membered heterocycles are measured and span over a factor of 15 or 30 for the two cations. The triplet cation formed under these conditions is trapped by iodide more efficiently than by the best pi nucleophiles. However, in contrast to the singlet cation, it does not form ethers with alcohols, by which it is rather reduced.
TL;DR: The dication 1(2+) has been characterised by a spectroelectrochemical study [lambdamax (CH2Cl2) = 377, 392, 419, 479 nm] and by an X-ray crystal structure of the salt 1( 2-) (ClO4)2, which was obtained by electrocrystallisation.
Abstract: Flash photolysis of bis[4.5-di(methylsulfanyl) 1,3-dithiol-2-ylidene]-9,10(-dihydroanthracene (1) in chloroform leads to formation of the transient radical cation species 1.+ which has a diagnostic broad absorption band at lambdamax approximately 650 nm. This band decays to half its original intensity over a period of about 80 micros. Species 1.+ has also been characterised by resonance Raman spectroscopy. In degassed solution 1.+ disproportionates to give the dication 1(2+), whereas in aerated solutions the photodegradation product is the 10-[4,5-di(methylsulfanyl) 1,3-dithiol-2-ylidene]anthracene-9(10 H)one (2). The dication 1(2+) has been characterised by a spectroelectrochemical study [lambdamax (CH2Cl2) = 377, 392, 419, 479 nm] and by an X-ray crystal structure of the salt 1(2-) (ClO4)2, which was obtained by electrocrystallisation. The planar anthracene and 1,3-dithiolium rings in the dication form a dihedral angle of 77.2 degrees; this conformation is strikingly different from the saddle-shaped structure of neutral 1 reported previously.
TL;DR: It is demonstrated that neutral radicals, dGMP(-H)(*), derived by the one-electron oxidation and deprotonation of dG MP, can oxidize nitrite anions (NO2(-)) to the nitrogen dioxide radical (*)NO2, and it is shown that (*) NO2 can give rise to a one-Electron oxidation of 8-oxo-G, but not ofdGMP.
Abstract: The redox reactions of guanine and its widely studied oxidation product, the 8-oxo-7,8-dihydro derivative, are of significant importance for understanding the mechanisms of oxidative damage in DNA. Employing 2'-deoxyguanosine 5'-monophosphate (dGMP) and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) in neutral aqueous solutions as model systems, we have used nanosecond laser flash photolysis to demonstrate that neutral radicals, dGMP(-H)(*), derived by the one-electron oxidation and deprotonation of dGMP, can oxidize nitrite anions (NO2(-)) to the nitrogen dioxide radical (*)NO2. In turn, we show that (*)NO2 can give rise to a one-electron oxidation of 8-oxo-G, but not of dGMP. The one-electron oxidation of dGMP was initiated by a radical cation generated by the laser pulse-induced photoionization of a pyrene derivative with enhanced water solubility, 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (BPT). The dGMP(-H)(*) neutral radicals formed via deprotonation of the dGMP(*)(+) radical cations and identified by their characteristic transient absorption spectrum (lambda(max) approximately 310 nm) oxidize nitrite anions with a rate constant of (2.6 +/- 0.3) x 10(6) M(-1) s(-1). The 8-oxo-dG is oxidized by (*)NO2 with a rate constant of (5.3 +/- 0.5) x 10(6) M(-1) s(-1). The 8-oxo-dG(-H)(*) neutral radicals thus generated are clearly identified by their characteristic transient absorption spectra (lambda(max) approximately 320 nm). The rate constant of 8-oxo-dG oxidation (k(12)) by the (*)NO2 one-electron oxidant (the (*)NO2/NO2(-) redox potential, E degrees approximately 1.04 V vs NHE) is lower than k(12) for a series of oxidizing aromatic radical cations with known redox potentials. The k(12) values for 8-oxo-dG oxidation by different aromatic radical cations derived from the photoionization of their parent compounds depend on the redox potentials of the latter, which were in the range of 0.8-1.6 V versus NHE. The magnitude of k(12) gradually decreases from a value of 2.2 x 10(9) M(-1) s(-1) (E degrees = 1.62 V) to 5.8 x 10(8) M(-1) s(-1) (E degrees = 1.13 V) and eventually to 5 x 10(7) M(-1) s(-1) (E degrees = 0.91 V). The implications of these results, including the possibility that the redox cycling of the (*)NO2/NO2(-) species can be involved in the further oxidative damage of 8-oxo-dG in DNA in cellular environments, are discussed.
TL;DR: The Sc(3+)-promoted electron-transfer rate constants were determined for photoinduced electron transfer from the triplet excited states of C(60), C(70), and their derivatives to p-chloranil and the values are compared with the HOMO (highest occupied molecular orbital) levels of the fullerenes and their derivative.
Abstract: In the presence of scandium triflate, an efficient photoinduced electron transfer from the triplet excited state of C(60) to p-chloranil occurs to produce C(60) radical cation which has a diagnostic NIR (near-infrared) absorption band at 980 nm, whereas no photoinduced electron transfer occurs from the triplet excited state of C(60) (3C(60)) to p-chloranil in the absence of scandium ion in benzonitrile. The electron-transfer rate obeys pseudo-first-order kinetics and the pseudo-first-order rate constant increases linearly with increasing p-chloranil concentration. The observed second-order rate constant of electron transfer (k(et)) increases linearly with increasing scandium ion concentration. In contrast to the case of the C(60)/p-chloranil/Sc(3+) system, the k(et) value for electron transfer from 3C(60) to p-benzoquinone increases with an increase in Sc(3+) concentration ([Sc(3+)]) to exhibit a first-order dependence on [Sc(3+)], changing to a second-order dependence at the high concentrations. Such a mixture of first-order and second-order dependence on [Sc(3+)] is also observed for a Sc(3+)-promoted electron transfer from CoTPP (TPP(2-) = tetraphenylporphyrin dianion) to p-benzoquinone. This is ascribed to formation of 1:1 and 1:2 complexes between the generated semiquinone radical anion and Sc(3+) at the low and high concentrations of Sc(3+), respectively. The transient absorption spectra of the radical cations of various fullerene derivatives were detected by laser flash photolysis of the fullerene/p-chloranil/Sc(3+) systems. The ESR spectra of the fullerene radical cations were also detected in frozen PhCN at 193 K under photoirradiation of the fullerene/p-chloranil/Sc(3+) systems. The Sc(3+)-promoted electron-transfer rate constants were determined for photoinduced electron transfer from the triplet excited states of C(60), C(70), and their derivatives to p-chloranil and the values are compared with the HOMO (highest occupied molecular orbital) levels of the fullerenes and their derivatives.
TL;DR: In this article, the B3LYP and MP2 levels of theory were used to study the keto-enol isomerization of neutral and ionized acetaldehyde, both isolated and catalyzed by solvent molecules.
TL;DR: Transient absorption techniques allowed for the direct monitoring of the oxidant and product radical intermediates as a function of time on the 100 ns−1 ms time scale and revealed the existence of two-component oxidation kinetics.
Abstract: Oligonucleotide duplexes (15-mers) containing 8-oxo-7,8-dihydro-2‘-deoxyguanosine (8-oxo-dG) and the base analogue 2-aminopurine (2AP) separated from one another by zero, two, or four thymidine residues on the same strand, were synthesized. Selective two-photon excitation of 2AP with intense 308 nm XeCl excimer laser pulses (fwhm = 12 ns, ∼80 mJ pulse-1 cm-2) gives rise to 2AP radical cations that, in turn, oxidize 8-oxo-dG at a distance. Transient absorption techniques allowed for the direct monitoring of the oxidant and product radical intermediates as a function of time on the 100 ns−1 ms time scale and revealed the existence of two-component oxidation kinetics. The evolution in time of the rapid component (<100 ns), attributed to the oxidation of 8-oxo-dG by the radical cation 2AP•+, was not resolved. However, this component was observed only in the case of zero or two intervening thymidine bases and was not evident when the 2AP and 8-oxo-dG residues were separated by four thymidines. A slower kinetic...
TL;DR: In this article, a field dependent chemically induced dynamic nuclear polarization (CIDNP) spectroscopy has been used to measure the exchange interaction, J, in photochemically generated radical ion pairs of rigid donor-bridge-acceptor molecules in benzene and dioxane solution.
TL;DR: Pulse radiolysis was used to generate the radical cations of beta-carotene and two xanthophylls, zeaxanthin and lutein, in unilamellar vesicles of dipalmitoylphosphatidyl choline and suggest that for beta- carotene, a hydrocarbon carotenoid, theradical cation is able to interact with a water-soluble species even though the parent hydrocarbon
Abstract: Pulse radiolysis was used to generate the radical cations of β-carotene and two xanthophylls, zeaxanthin and lutein, in unilamellar vesicles of dipalmitoylphosphatidyl choline. The rate constants for the reaction (repair) of these carotenoid radical cations with the water-soluble vitamin C were found to be similar (∼1×107 M−1 s−1) for β-carotene and zeaxanthin and somewhat lower (∼0.5×107 M−1 s−1) for lutein. The results are discussed in terms of the microenvironment of the carotenoids and suggest that for β-carotene, a hydrocarbon carotenoid, the radical cation is able to interact with a water-soluble species even though the parent hydrocarbon carotenoid is probably entirely in the non-polar region of the liposome.