Theoretical Study of Pyridoxine (Vitamin B6) Photolysis - The Journal

Oct 13, 2011 - (4) Symptoms of serious vitamin B6 deficiency include muscle weakness, ..... Energy curves for stretching C1–C2, C3–C4, C4–C5, C5...
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Theoretical Study of Pyridoxine (Vitamin B6) Photolysis Min Wu,†,‡ Qi Xu,§ Åke Strid,|| Jaime M. Martell,§ and Leif A. Eriksson†,‡,* †

School of Chemistry, National University of Ireland, Galway, Ireland Department of Chemistry, University of Gothenburg, 412 96 G€oteborg, Sweden § Department of Chemistry, Cape Breton University, Sydney NS, Canada B1P 6L2 € € € Orebro Life Science Center, School of Science and Technology, Orebro University, 70182 Orebro, Sweden

)

‡

bS Supporting Information ABSTRACT: Two different reaction types for the photolysis of pyridoxine— aromatic ring-opening and photodissociation—have been studied in the Density Functional Theory (DFT) framework. Our results show that neither photolytic ring-opening, dehydroxymethylation, demethylation nor dehydroxylation from the aromatic ring can be induced spontaneously in UV-irradiated pyridoxine, due to the high barriers along the reaction coordinates in the excited states. However, the simultaneous dehydroxylation of the C4-bound hydroxymethyl group and dehydrogenation of the ring bound hydroxyl substituent, selectively generating ortho-quinone methide and water, does occur after UV exposure. The findings correlate very well with available experimental data. The geometries of pyridoxine, its various transition states and products are optimized in the ground and first excited states in vacuum within the TD-DFT formalism.

1. INTRODUCTION Pyridoxine, also known as Vitamin B6 (Figure 1), is an important nutrient in the human body. It is the precursor of the biologically active derivatives pyridoxal-50 -phosphate and pyridoxamine-50 -phosphate, with functional roles in many enzymes.1 It is one of eight water-soluble B vitamins, with plants being the major source of intake for humans. It furthermore acts as a coenzyme, involved in approximately 100 reactions in the metabolism of carbohydrates, fats, and amino acids to manufacture hormones, neurotransmitters, enzymes, and prostaglandins.2 Pyridoxine exhibits synergistic effects with vitamins B12 (riboflavin) and B9 (folic acid) to control homocysteine levels in the blood for protection of the heart muscle.3 In addition, pyridoxine plays a critical role in reducing the risk of developing colorectal cancer.4 Symptoms of serious vitamin B6 deficiency include muscle weakness, nervousness, irritability, depression, concentration difficulties, and short-term memory loss.3 Antioxidants capable of scavenging reactive oxygen species (ROS) have attracted much attention in photochemistry. It has been found that pyridoxine deficient mutants of fungi and yeast are sensitive to ROS including singlet oxygen5,6 and hydrogen peroxide.7 Since oxygen radicals have an extremely high chemical reactivity, their overproduction causes severe damage such as lipid peroxidation in cellular membranes.8 Pyridoxine and its derivatives are known to alleviate oxidative stress in living organisms, human blood cells,9 and eukaryotes,10 although they are not formally classified as antioxidant compounds and their antioxidant mechanisms are unclear. It has been suggested that the r 2011 American Chemical Society

effect could be related to coenzymatic activities, but this has not been verified to date.9 In a previous study, we investigated the reactivity of pyridoxine toward addition of several oxygen radicals ( 3 OH, 3 OOH, and 3 O2) to the aromatic ring, and by hydrogen abstraction by said radicals from pyridoxine.11 It was suggested that 3 OH, as being the most reactive species, could form adducts at the C2 and C6 positions, followed in reactivity by the 3 OOH radical, and 3 O2 displaying no reactivity with pyridoxine at all. Hydrogen removal from the CH2OH groups and the ring-bound OH group were the energetically most favorable reactions. In a further work, we explored the scavenging capacity of pyridoxine under high hydroxyl radical concentrations,12 and we have reported on the reactions between pyridoxine and singlet oxygen.13 It was there concluded that 1,4-addition with the formation of hydroperoxide ketones was the most probable reaction, in line with experimental data.14 Pyridoxine, like riboflavin, absorbs in the near-ultraviolet range (UVA; 320400 nm) and is known to degrade upon long exposure at 254 nm. After UVA irradiation, pyridoxine displays a strong cytotoxic effect that is maintained for at least 60 min15 and the heteroaromatic absorbance peak at 323 nm disappears which reveals at least one of the targets for singlet oxygen is most likely the core of the aromatic ring.10 Pyridoxine is also very susceptible to ionizing radiation in aqueous solution, and degrades due Received: June 18, 2011 Revised: October 12, 2011 Published: October 13, 2011 13556

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Figure 1. Pyridoxine (Vitamin B6), including atom labeling.

Scheme 1. Generation of QMs from Pyridoxine via Exposure to UV Radiation; (a) o-QM, (b) m-QM

to its high reactivity toward the free radicals formed by water radiolysis.16 In both of these instances pyridoxine can be described as using its antioxidant property to sacrifice itself. In plants, pyridoxine and pyridoxine synthesis enzymes are specifically regulated by UVB radiation (280315 nm).1719 The aim of this work is to explore the possibility of spontaneous ring-opening reactions by UV-radiation, and the possible catabolites which are produced by pyridoxine photodegradation. Pyridoxine is known to generate quinone methides (QMs) by photolysis in aqueous solution.20,21 QMs widely occur as reactive intermediates in the chemistry of phenols and related compounds. They are believed to be fundamental intermediates in the bioformation of lignin as well as critical intermediates in the mechanism of action of many anticancer drugs.2224 In thermal chemistry, ortho- and para-QMs are the most common isomers. The meta-QM is an example of a non-Kekule molecule which is only accessible via photochemical routes.21 Due to the presence of CH2OH groups ortho and meta to the aromatic hydroxyl group in pyridoxine (Scheme 1)20,21 competition will in this case exist between o-QM vs m-QM formation. Because of the QMs’ electrophilic nature, the formation of QMs could furthermore have toxicological consequences, resulting in self-polymerization or alkylation of DNA and amino acids, in both normal and cancerous cells.2428

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2. COMPUTATIONAL METHODOLOGY Along the reaction pathways, all S0, S1, and T1 structures were optimized in vacuum at the hybrid B3LYP HartreeFock DFT level of theory,2931 in conjunction with the 6-31+G(d,p) basis set. Harmonic vibrational frequency calculations were performed at the same level on the optimized geometries, in order to ensure the systems found were minima or transition states (TS) on their respective energy surfaces, and to extract zero-point vibrational energies (ZPE) and Gibbs energy contributions at 298 K. The numbering of atoms in pyridoxine as shown in Figure 1 is used throughout the study. Optimizations of the stationary points in the first excited singlet state were done using the same level of theory combined with the time-dependent (TD) formalism.3235 Product formation was investigated by stretching the bond length in question in steps of 0.1 Å in the ground state and at each point reoptimizing the remaining structural parameters. Single-point energy calculations for vertical singlet and triplet excitations were calculated at the B3LYP/6-31+G(d,p) level at each point along the pyridoxine ring-opening pathways, and along pyridoxine photodegradation pathways, using TD-DFT. On the basis of these scans, stationary points were optimized on the ground and excited state surfaces and verified employing intrinsic reaction coordinate (IRC) calculations to make sure they connect to the correct reactants and products. All calculations were performed using the Gaussian09 program package.35 3. RESULT AND DISCUSSION In Figure 2, the optimized structures of the pyridoxine molecule in the ground state and first excited triplet state are displayed with bond lengths labeled, also involving intramolecular hydrogen bonding between the hydroxyl groups on C3 and C8. The aromatic ring is in the ground state essentially planar, with the largest dihedral angle being 1.4°. This increases to 10.4° in the triplet state. The O10, C7, C8, and C9 atoms in the substituents lie in the ring-plane in the ground state, but are out of plane in the triplet state. The N1C2C3O10, C2C3C4C8, C3C4C5C9 and C6N1N2C7 dihedral angles are in the triplet state 164.3°, 159.6°, 175.6°, and 173.1°, respectively. The O11H and O12H groups at C4 and C5 are rotated out of the aromatic ring plane in all cases. The nonplanarity of the triplet structure is induced by the localization of the two unpaired electrons. The unpaired spin components of the optimized pyridoxine in the triplet state are mainly distributed on N1, C3, C4, C6, and O10, with values 0.288, 0.502, 0.763, 0.487, and 0.175e, respectively, leading to puckering of the aromatic ring. In the ground state, NBO analysis reveal that the double bonds as expected are located at the N1C2, C3C4, and C5C6 bonds. The σ orbital of N1C2 has 1.982 electrons with 58.6% N1 character in a sp2.2 hybrid and 41.4% C2 character in an sp1.78 hybrid, whereas the π orbital carries 1.734 electrons with same distribution between the atoms as above. The σ orbital of C3C4 has 1.976 electrons and displays 49.4% C3 character in a sp1.63 hybrid and 50.6% C4 character in a sp1.99 hybrid, and the π orbital of this bond has 1.644 electrons in p-character orbitals. The σ orbital of C5C6, finally, carries 1.980 electrons distributed as 52.0% C5 character in a sp1.89 hybrid and 48.0% C6 character in a sp1.64 hybrid; and 1.677 electrons in the corresponding π orbital. In the triplet state, NBO analysis give that C3C4 and C5C6 bonds remain as double bonds with similar properties in their ground state. The N1C2 bond, however, is now a single bond 13557

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Table 1. Calculated Pyridoxine Energies at the B3LYP/ 6-31+G(d,p) Level in Gas-Phasea state

E(vacuum)b

ZPE

ΔGcorr298

ΔG(vacuum)298 c

S0

591.930 356

0.187 358

0.149 674

591.780 682

T1

591.809 657

0.182 443

0.142 025

591.667 632

All data in atomic units. b Gas-phase optimized structure. c ΔG(vacuum)298= E(vacuum) + ΔGcorr298. a

Figure 3. Energy curves for stretching C2C3 bond in (a) ground state and five lowest singlet excited states and (b) three lowest triplet states.

Figure 2. Optimized structures of pyridoxine at the B3LYP/6-31+G(d, p) level: (a) S0 ground state; (b) first excited triplet state T1.

with 58.2% N1 character in a sp2.1 hybrid and 41.8% C2 character in a sp1.78 hybrid. Within the pyridoxine aromatic ring in the ground state, the bonds present a fully conjugated structure with CC distances between 1.39 and 1.41 Å and CN distances 1.342 and 1.355 Å. In the first excited triplet pyridoxine structure, the distances for C2C3, C3C4 and C6N1 bonds are longer than in the ground state. The largest difference is for the C3C4 bond at 0.106 Å, followed by a 0.075 Å difference for the C6N1 bond. In contrast, the distances for N1C2, C4C5, and C5C6 bonds are shorter in the T1 state than in the ground state. The intramolecular hydrogen bond is in the triplet state 0.057 Å shorter. In Table 1, the zero-point energies and the free energies of pyridoxine and triplet pyridoxine are listed in gas-phase. The Gibbs energy difference between the ground state and the T1 state in gas phase is 70.9 kcal/mol. 3.1. Pyridoxine Ring-Opening Reactions. A scanning approach was employed to investigate the singlet and triplet excited state energies along the different reaction pathways, in order to explore the possibility of aromatic ring-opening reactions for vitamin B6 induced by UV-radiation. The C2C3 bond (cf. Figure 1) was

scanned from 1.55 Å in pyridoxine, to 2.85 Å, with step size 0.1 Å. In each new point, the structure was reoptimized both in the ground state and triplet state, and the vertical singlet and triplet excitation energies recalculated. The five lowest singlet excited states and three lowest triplet excited states were included in the calculations along the reaction pathways, given that the induced energy from natural UV radiation is not sufficient to reach higher excited states. The singlet states (including ground state) and the triplet state energy curves along the reaction coordinates obtained at the TD-B3LYP/6-31+G(d,p) level are shown in Figure 3. As seen, all curves are highly endothermic throughout the scan of the C2C3 bonds. It is thus suggested that UV-induced ringopening will not occur via C2C3 bond scissions. The S1 state, for example, displays a transition barrier of 70 kcal/mol at a C2C3 distance of 2.65 Å, which is far too high for ring-opening to occur as a result of UV-irradiation. The triplet state energies also increase along the reaction coordinate, which means that a possible intersystem crossing between singlet and triplet state will not be able to induce ring-opening either. For the other five bonds in the pyridoxine aromatic ring, similar situations are noted; the curves are available in the Supporting Information (Figure S1S5). The energy barriers are all higher than 50 kcal/ mol in both the singlet and triplet states. This means that pyridoxine ring-opening reactions will not be induced spontaneously by exposure to UV radiation. Other mechanisms must thus be in place, to account for the changed absorption noted experimentally. 3.2. Pyridoxine Photodegradation Reactions. In a second set of calculations, the bonds between the aromatic ring and the 13558

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Figure 4. Energy curves for bond scission between the aromatic ring and the ring-bound functional groups in the six lowest excited singlet states (including the ground state) calculated at TD-B3LYP/6-31+G(d,p) level; (a) C2C7, (b) C4C8, (c) C5C9, and (d) C3O10.

various ring-bound functional groups (methyl group, hydroxymethyl groups, and hydroxyl group), i.e., the C2C7, C5C9, C4C8, and C3O10 bonds, were scanned outward from 1.55 Å to 2.85 Å, with step size 0.1 Å. At each new point, the structures were reoptimized both in the ground state and the first triplet state, and the vertical excitation energies calculated. The five lowest singlet excited states and three lowest triplet excited states were included in the calculations at the TD-B3LYP/6-31+G(d,p) level. The resulting energy curves of demethylation, dehydroxymethylation, and dehydroxylation reactions in the lowest singlet states are shown in Figure 4. In the demethylation reaction, Figure 4a, no sign of spontaneous photoscission occurring between the pyridoxine aromatic ring and its groups can be seen, since all the surfaces are endothermic throughout the scan. For the other ring-bound functional group photodegradations, most of the singlet state surfaces are strictly endothermic. The exceptions are the S2 states in all of the cases, which display obvious transition barriers with a maximum at the C3O10 distance of 1.85 Å (Figure 4d) and at the distance 2.35 Å for C4C8 and C5C9 (Figure 4b,c). However, the barriers for these reactions are very high, about 45 kcal/mol. The lowest triplet state surfaces are shown in Supporting Information.

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Figure 5. Energy curves for the dehydroxylation reactions of the two ring-bound hydroxymethyl groups: (a) C8O11 bond, (c) C9O12bond in the six lowest singlet states (including ground state); (b) C8O11 bond, (d) C9O12 bond in the three lowest triplet states, calculated at the TD-B3LYP/6-31+G(d,p) level.

The energy barriers are all higher than ∼35 kcal/mol, and possible intersystem crossing will thus not help in inducing the reactions. It can therefore be concluded that, just as for the ringopening reactions, UV induced dehydroxymethylation, demethylation and dehydroxylation reactions from the pyridoxine aromatic ring are not likely to occur neither in the singlet nor in the triplet excited states. A scanning approach was also employed in the same manner as described above, in order to determine the energy barrier for the dehydroxylation reactions from the two ring-bound hydroxymethyl groups (i.e., scission of the C8O11 and C9O12 bonds; Figure 1) in the ground state and the lowest excited states, at the B3LYP/6-31+G(d,p) levels. The results are presented in Figure 5. In the case of C8O11 elongation, the lowest energy barrier from the singlet excited states is located on the S1 surface at the distance of 1.75 Å. The energy barrier for the dehydroxylation is only 7.3 kcal/mol, followed by a 24.1 kcal/mol energy barrier in the third singlet excited state. The energy barrier in the ground state is 40.4 kcal/mol and occurs at ∼2.15 Å. In all three triplet states studied, the energy barriers are around 7.5 kcal/mol at the distance of ∼1.75 Å. For the dehydroxylation reaction on the C9 atom, the lowest energy barrier is about 17.1 kcal/mol and occurs in the S1 state. The energy maximum is at a C9O12 13559

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Table 2. Calculated Energies for QMs in Gas Phase (B3LYP/ 6-31+G(d,p) Level)a E(vacuum)b

compound

ZPE

ΔGcorr298 ΔG(vacuum)298 c

515.419 785 0.155271 0.118 586 515.301 199

m-QM

biradical

o-QM

zwitterion 515.415 012 0.156 283 0.119 871 515.295 141 515.449 697 0.156 871 0.120 253 515.329 444

All data in atomic units. b Gas-phase optimized structure. c ΔG(vacuum)298= E(vacuum)+ ΔGcorr298. a

distance of 1.85 Å. All of the energy barriers are approximately 14 kcal/mol in the lowest triplet states, at distances of about 1.75 Å. In the ground state, the transition barrier is approximately 60 kcal/mol at the C9O12 distance of ∼2.65 Å. Again, intersystem crossing is not expected to assist in reducing the transition barrier in these cases. According to the optimized ground state reaction coordinates, pyridoxine degradation will generate QM molecules and water. Competition has been proposed to exist between formation of meta-QM and ortho-QM,21 depending on which of the two hydroxymethyl groups that undergoes scission. The energy curves above illustrate that generation of the m-QM compound (C9O12H bond broken) requires more energy than the o-QM compound (C8O11H bond rupture), both in the ground and excited states. In summary the computed data hence suggest that, based on kinetic energy constraints, the favored reaction is excited state dehydroxylation from carbon C8. During the o-QM reaction in the ground state, a water molecule is formed via concerted abstraction by the leaving O11H hydroxy group, of the hydrogen on the ring bound O10H group. For the m-QM formation, the HO11 bond is instead elongated to yield the corresponding water molecule from the leaving O12H group + H(O11). The hydrogen on the ring bound O10 is simultaneously transferred to O11 in a concerted TS, resulting in the formation of the non-Kekule m-QM structure. Due to the formation of the water molecule, the ground state energy surfaces change abruptly after the respective TS’s, but are then essentially planar. Comparing the two reactions on the ground state surface, formation of the m-QM compound requires ∼20 kcal/mol more energy, whereas the difference in barrier heights on the scanned first singlet and triplet excited state surfaces are ∼10 kcal/mol in favor of o-QM formation (7.3 vs 17.1 kcal/mol and 5.3 vs 15.2 kcal/mol) . Due to the non-Kekule structure of m-QM, it furthermore has two possible electron distributions: biradical or zwitterionic (see Scheme 1). The absolute electronic and Gibbs energy corrected energies of the different QMs in gas phase are listed in Table 2. The Gibbs energy difference between the m-QMbiradical and the m-QM-zwitterion is in absence of the formed water molecule 2.8 kcal/mol. Inclusion of the water molecule increases the Gibbs energy difference between the two molecules slightly, to about 3.8 kcal/mol. Both the m-QM-biradical and the m-QM-zwitterion could thus be the product in m-QM generation. In the m-QM-biradical, positive spin densities are mainly distributed on C2, C5, C6 and O10 atoms (pyridoxine labeling), with values 0.416, 0.339, 0.157, and 0.101e, and negative spin components are mainly found on N1, C4 and C9 with values 0.109, 0.407, and 0.521e, respectively. The C3O10 and C5C9 bonds are according to NBO analysis double bonds. The σ orbital of C3O10 has 36.7% C3 character in an sp2.24 hybrid and 63.3% O10 character in an sp1.61 hybrid. The π orbital of

Figure 6. Reaction paths (ΔΔG(vacuum)298) for the formation of o-QM and m-QM in the ground state and the first singlet excited state obtained at B3LYP/6-31+G(d,p) level in gas phase.

C3O10 has 31.4% C3 character and 68.6% O10 character in p-character orbitals. The σ orbital of C5C9 bond has 52.5% C5 character in an sp1.84 hybrid and 47.5% C9 character in an sp1.71 hybrid. The π orbital of this bond has 43.2% C5 character and 56.8% C9 character. In the m-QM-zwitterion molecule, negative charges of approximately 0.654 and 0.451 e are located on the ketone group and C5-bound CH2 group, respectively. The positive charges are mainly localized on C4, C5 and C2 on the aromatic ring, with values 0.622, 0.252 and 0.213e . N1C6, C3O10, and C5C9 bonds are in this case found to be double bonds. N1C6 holds 59.8% N1 character in an sp1.66 hybrid and 40.2% C6 character in a sp2.11 hybrid in the σ orbital, with 1.986 electrons, whereas the π-bond has 60.7% N1 character and 39.3% C6 character with 1.792 electrons. The double bonds of C3O10 and C5C9 have the similar properties as in the m-QM-biradical molecule. In order to visualize the dehydroxylation reactions, the optimized geometries of the stationary points in the o-QM and mQM pathways from the ground state and the first singlet excited states were calculated using the TD-B3LYP/6-31+G(d,p) formalism. The energy curves are depicted in Figure 6, and the corresponding geometries for o-QM are displayed in Figure 7; the structures for m-QM are available in the Supporting Information (Figures S7 and S8). All reactions are concerted, as can be seen from Figure 6. The o-QM product has lower energy than either of the m-QM forms in the ground state. However, in the excited state, the order is reversed. No biradical solution could be identified for m-QM in the excited state, despite extensive search. The changes in bondlengths within the aromatic ring are in general small between the S0 and the S1 states. In pyridoxine, most of the aromatic ring bonds are longer in the first excited state than in the ground state, except the N1C7 and N1C2 bonds. The bonds between the ring-bound functional groups and the aromatic ring are shorter than in the ground state, whereas the two bonds between the hydroxyl groups and the methylenes are in the excited state slightly longer than in the ground state; cf. Figure 7a,b. In the first singlet excited o-QM compound, however, the aromatic ring bonds are shorter than in the ground state. The bonds between the ringbound functional groups and the aromatic ring are all longer than in the ground state, whereas the bond to the hydroxyl on the methylene group is shorter; cf. Figure 7e,f. The optimized structures of the transition states for the o-QM compound in the first singlet excited and ground states are 13560

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Figure 7. Optimized geometries for o-QM generation. (a) and (b) reactant in singlet ground state and first singlet excited state; (c) and (d) TS structure in ground state and first excited state; (e) and (f) product in ground state and first excited state, respectively.

displayed in Figure 7c,d. The imaginary frequency for the excited transition state is 556i cm1, and 448i cm1 for the ground state. The excited state TS of o-QM (TS1*) has a C8O11 distance of 1.751 Å, and an energy barrier of 4 kcal/mol. In the ground state, TS1 is formed with a C8O11 distance of 1.949 Å. The structure of the transition state for the m-QM compound in the excited state is displayed in Supporting Information (Figure S8). The TS for C9O12 dissociation (TS2) is found at a distance 2.347 Å, with an imaginary frequency of 258i cm1 in the ground state, and a distance of 1.937 Å with an imaginary frequency of 237i cm1 in the first singlet excited state. The energy barrier for the generation of the m-QM molecule in the first excited state is after optimization 12.1 kcal/mol, i.e., 8.1 kcal/mol higher than for o-QM. In Table S1 (Supporting Information), the energetics and electron density difference plots are shown for the S1 reaction leading to o-QM. The electron density differences are calculated

between the ground state and first singlet excited states at their corresponding optimized geometries. The density difference plots show that no dramatic restructuring of the electron distributions result from the excitations; they are both major nπ* excitations mixed with minor ππ* excitation. The energy differences between the ground state and the first excited state are 92.0 kcal/mol for pyridoxine and 42.5 kcal/mol for o-QM. It is concluded that the formation of m-QM requires the system to overcome a much higher energy barrier than the o-QM generation, whether in the ground or excited states. This explains the experimental data according to which no m-QM is generated upon UV exposure, but that o-QM is predominantly formed. Given that QMs in general are reactive intermediates in chemical reactions, the formed compounds may in turn react with various biomolecules such as proteins or DNA in the cell. The exact fate of the pyridoxine derived o-QMs remain however to be verified. 13561

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4. CONCLUSIONS A range of potential photolysis reactions of pyridoxine are investigated in this work using computational chemistry techniques. By scanning the energy surfaces for bond scission between atoms within the aromatic ring, and for ring-bound substituents, we can conclude that neither aromatic ring-opening nor photodegradation reactions leading to loss of ring-bound functional groups (hydroxymethyl groups, methyl group and hydroxyl group) will be likely to occur upon exposure to UV-radiation. We also explore the rationale for the observation that orthoQuinone methide (o-QM) is predominantly generated by UV irradiated pyridoxine.21 The ortho-form is the result of dehydroxylation (rupture of the C8O11 bond) and O10H hydrogen abstraction giving water, whereas for the meta counterpart, the C9O12 bond is broken followed by water formation. Analyzing the ground and first excited state energy surfaces for o-QM vs the non-Kekule m-QM formation, first by scanning the surface by stretching the two CO bonds in question, and then full optimization of the excited state structures, we note that the energy barrier for photogeneration of o-QM is only 4 kcal/mol in the excited state, which is 8 kcal/mol lower than for m-QM in the excited state (the corresponding ground state reaction has a 20 kcal/mol lower barrier). This most likely accounts for why the o-QM compound is selectively produced after UV-irradiation of pyridoxine. ’ ASSOCIATED CONTENT

bS

Supporting Information. Energy curves for stretching C1C2, C3C4, C4C5, C5C6, and C6C1 bonds in the singlet and triplet state are found in Figures S1, S2, S3, S4, and S5. Energy curves for the aromatic ring-bound functional group photodegradation reactions in the triplet state are found in Figure S6. TS and product complex structures of the S0 dehydroxylation of pyridoxine at C8 and C9 atoms are found in Figure S7. The energies of the first singlet excited states, and the electron density differences between the ground and first singlet excited states for pyridoxine and o-QM are found in Table S1. The TS structure in the m-QM pathway is found in Figure S8. TS coordinates for QM generation in ground and first excited states are found in Table S2. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National University of Ireland, Galway, a Cape Breton University RP grant, and by the Faculty of € Business, Science and Technology at Orebro University. ’ REFERENCES (1) Schneider, G.; Kack, H.; Lindqvist, Y. Struct. Fold Des. 2000, 8, R1–R6. (2) http://www.oralchelation.com/technical/vitaminb6.html, and references therein. (3) http://www.umm.edu/altmed/articles/vitamin-b6-000337. html, and references therein.

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(4) Wei, E. K.; Giovannucci, E.; Selhub, J.; Fuchs, C. S.; Hankinson, S.; Ma, J. Cancer Epidem. Biomar. 2004, 13, 1854s–1854s. (5) Ehrenshaft, M.; Jenns, A. E.; Chung, K. R.; Daub, M. E. Mol. Cell 1998, 1, 603–609. (6) Osmani, A. H.; May, G. S.; Osmani, S. A. J. Biol. Chem. 1999, 274, 23565–23569. (7) Rodriguez-Navarro, S.; Llorente, B.; Rodriguez-Manzaneque, M. T.; Ramne, A.; Uber, G.; Marchesan, D.; Dujon, B.; Herrero, E.; Sunnerhagen, P.; Perez-Ortin, J. Yeast 2002, 19, 1261–1276. (8) Halliwell, B.; Gutteridge, J. M. C. Free Radicals in Biology and Medicine; Clarendon Press: New York, 2007. (9) Stocker, P.; Lesgards, J. F.; Vidal, N.; Chalier, F.; Prost, M. Biochim. Biophys. Acta -Gen. Subjects 2003, 1621, 1–8. (10) Bilski, P.; Li, M. Y.; Ehrenshaft, M.; Daub, M. E.; Chignell, C. F. Photochem. Photobiol. 2000, 71, 129–134. (11) Matxain, J. M.; Ristila, M.; Strid, A.; Eriksson, L. A. J. Phys. Chem. A. 2006, 110, 13068–13072. (12) Matxain, J. M.; Padro, D.; Ristil€a, M.; Strid, Å.; Eriksson, L. A. J. Phys. Chem. B: Lett. 2009, 113, 9629–9632. (13) Matxain, J. M.; Ristilla, M.; Strid, A.; Eriksson, L. A. Chem.— Eur. J. 2007, 13, 4636–4642. (14) Ohta, B. K.; Foote, C. S. J. Am. Chem. Soc. 2002, 124, 12064–12065. (15) Sato, K.; Taguchi, H.; Maeda, T.; Yoshikawa, K. J. Invest. Dermatol. 1993, 100, 266–270. (16) Albarran, G.; Ramirez-Cahero, F.; Aliev, R. Radiat. Phys. Chem. 2008, 77, 605–611. (17) Brosche, M.; Schuler, M. A.; Kalbina, I.; Connor, L.; Strid, Å. Photochem. Photobiol. Sci. 2002, 1, 656–664. (18) Chen, H.; Xiong, L. M. Plant J. 2005, 44, 396–408. (19) Ristil€a, M.; Strid, H.; Eriksson, L. A.; Strid, Å.; S€avenstrand, H. Plant Physiol. Biochem. 2011, 49, 284–292. (20) Wan, P.; Brousmiche, D. W.; Chen, C. Z.; Cole, J.; Lukeman, M.; Xu, M. S. Pure Appl. Chem. 2001, 73, 529–534. (21) Brousmiche, D.; Wan, P. Chem. Commun. 1998, 491–492. (22) Wan, P.; Barker, B.; Diao, L.; Fischer, M.; Shi, Y. J.; Yang, C. Can. J. Chem. 1996, 74, 465–475. (23) Shevchenko, S. M.; Apushkinsky, A. G. Usp. Khim. 1992, 61, 195–245. (24) Thompson, D. C.; Thompson, J. A.; Sugumaran, M.; Moldeus, P. Chem.-Biol. Int. 1992, 86, 129–162. (25) Bolton, J. L.; Sevestre, H.; Ibe, B. O.; Thompson, J. A. Chem. Res. Toxicol. 1990, 365–370. (26) Bolton, J. L.; Valerio, L. G.; Thompson, J. A. Chem. Res. Toxicol. 1992, 5, 816–822. (27) Bolton, J. L.; Acay, N. M.; Vukomanovic, V. Chem. Res. Toxicol. 1994, 7, 443–450. (28) Bolton, J. L.; Thatcher, G. R. J.; McCracken, P. G. J. Org. Chem. 1997, 62, 1820–1825. (29) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (30) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter. 1988, 37, 785–789. (31) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623–11627. (32) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454–464. (33) Casida, M. E.; Casida, K. C.; Salahub, D. R. Int. J. Quantum Chem. 1998, 70, 933–941. (34) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998, 109, 8218–8224. (35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A. Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; 13562

dx.doi.org/10.1021/jp205724k |J. Phys. Chem. A 2011, 115, 13556–13563

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ARTICLE

Millam, J. M.; Klene,M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, v. B.02, Gaussian, Inc., Pittsburgh, 2009.

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