Table 1.
Crystal data of galangin●H2O.
Fig 1.
Galangin data crystal showing the color variation.
Fig 2.
A: The asymmetric unit of galangin monohydrate with atom labeling and the torsion angle about the C2-C11 bond at -45.1(2°; B: Offset stacking interactions among galangin molecules (about 3.35–3.38 Å). The repeat distance between water molecules of 3.831Å is equivalent to the unit cell a axis length; C: Distances of hydrogen bonding interactions in the galangin crystal structure.
Table 2.
Hydrogen bonds for galangin [Å and °].
Fig 3.
A: Galangin geometry obtained from X-ray coordinates after DFT minimization; B: Initial van der Waals separation between H7 and superoxide, 2.60 Å, decreases to 1.465 Å after DFT minimization, while hydroxyl separation is still a bond length, 1.065 Å. Superoxide: stick style; galangin 7-OH: ball and stick style; C: H7 was eliminated from the hydroxyl and the remaining species, the galangin radical, was DFT minimized and later placed 2.60 Å from HO2 for further geometry minimization. Fig 3C shows the converged energy minimum: the H atom of HO2 is not recaptured by the galangin radical, 1.412 Å. The energy of this minimum is 1.7 kcal/mol higher than that in Fig 3B, suggesting H7 of galangin is not captured by superoxide.
Fig 4.
Galangin, 3,5,7-trihydroxyflavone, and chrysin, 5,7-dihydroxyflavone, are important components of propolis.
Fig 5.
A: Converged DFT minimum for superoxide attack on galangin H3. The galangin O-H3 bond distance elongates to 1.507 Å and H3 advances to establish a bond distance to superoxide, 1.054 Å; B: Converged DFT minimum for initial van der Waals contact between HO2 and the radical obtained after excluding H3 from galangin. The galangin radical does not recapture H3, 1.544 Å; C: The galangin-superoxide complex shown in Fig 5A is van der Waals placed nearby a proton. This is a neutral radical that after minimization shows formation of H2O2 plus the galangin derivative radical, which locates the unpaired electron in the polyphenol.
Fig 6.
A: The π-π approach of a superoxide to galangin ring B allows for ring capture of its unpaired electron. O-O bond distance becomes similar to that in molecular oxygen, 1.273 Å, in contrast with the approached superoxide O-O, 1.373 Å. B: From previous Fig 6A O2 is eliminated and DFT minimization is performed; C: Reagent, TS and Product for galangin H5 extraction by superoxide. ΔG = -2.067 kcal/mol, E(barrier) = 1.3 kcal/mol; D: TS profile for galangin H5 extraction by superoxide. ΔG = -2.067 kcal/mol, E(barrier) = 1.3 kcal/mol.
Fig 7.
A: Galangin accepts a π-π approach by superoxide. Initially van der Waals separated molecules, 3.50 Å, are geometrically optimized. Both reagents become closer with centroid-centroid separation stabilized 3.190 Å apart. There is a slight shortening of O-O bond distance in superoxide, 1.354 Å, from the original 1.373 Å; B: Galangin π–π superoxide from previous Fig 7A is now approached by a proton that, after minimization, becomes stabilized by formation of an O-H bond, 0.985 Å, while superoxide O-O separation is lengthened, 1.407 Å (it was 1.354 Å in Fig 7A); C: From previous Fig 7B, an additional proton is placed 2.60 Å from HO2. Upon minimization H2O2 is formed, however, it is in a complex with the galangin ring, as shown by the centroid-centroid separation of 3.247 Å, shorter that the 3.50 Å π–π van der Waals separation.
Fig 8.
A: Rearrangement of H2O2 moiety as described by the Transition State search (TS): left, reagent; center TS; right, product, are displayed; B: From the product of Fig 8A proton is captured by the O2 moiety, total charge is 1; C: From Fig 8B an additional proton, small ball size, is captured by the HO2 moiety forming H2O2, while the other H2O2 located in the polyphenol plane, detaches from galangin, 3.472 Å. Total charge of this radical species is 2.
Fig 9.
A: Minimum reached after excluding H2O2 from Fig 8C. Total charge is still 2; B: Superoxide entering trans to H2O2 moiety, total charge 1. This is a non-radical species and the O-O separation for the newly arrived superoxide, 1.251 Å, is much shorter than the original 1.373 Å, and similar to O-O bond length in O2; C: After the arrangement shown in Fig 9B, three additional superoxide radicals are incorporated above and below the aromatic plane of galangin.
Fig 10.
A: A total of 5 runs of galangin were performed in this study. This CV compilation shows the initial blank, black color, and colored aliquots added of galangin; B: Antioxidant Efficiency of galangin, as function of molarity.
Fig 11.
A: Cyclovoltammograms of propolis when scavenging the superoxide radical, B: Collection efficiency of propolis; linear behavior considering all runs. The last run shows almost complete elimination of superoxide; C: Linear behavior of propolis considering the first 4 data on Fig 11A.