Photochemistry and Photobiology, 2015, 91: 280–290
Spectroscopic Properties of Morin in Various CH3OH–H2O and CH3CN–H2O Mixed Solvents Hyoung-Ryun Park1, Seo-Eun Im1, Jung-Ja Seo1, Bong-Gon Kim2, Jin Ah Yoon2 and Ki-Min Bark*3 1
Department of Chemistry and Research Institute of Basic Science, Chonnam National University, Gwangju, Korea Department of Chemical Education, Gyeongsang National University, Chinju, Korea 3 Department of Chemical Education, and Research Institute of Life Science, Gyeongsang National University, Chinju, Korea 2
Received 21 August 2014, accepted 14 December 2014, DOI: 10.1111/php.12407
Morin (3,20 ,40 ,5,7-pentahydroxyflavone), one of the most common flavonoids, has attracted the attention of many researchers because of its broad spectrum of biological and pharmaceutical activities (17–19). The flavonoids that contain an –OH group at position 5 (C-5) have been known as nonfluorescent molecules. However, it was discovered that some flavonoids such as quercetin glycoside, quercetin and apigenin, which also contains an – OH at C-5, exhibit a new significant fluorescence emission in hydro-organic mixed solvents and AOT reverse micelle (20–22). Compared with quercetin, morin has the –OH at C(20 ) position instead of C(30 ) position (see Fig. 1). Without this difference, quercetin and morin have exactly the same molecular structures. As the number and position of the –OH groups in flavonoids will influence their chemical properties, it will be very interesting to study the difference of chemical properties for several flavonoids with similar molecular structures such as quercetin and morin. The geometrical molecular structure and properties of the polyphenolic compounds such as morin are very sensitive to changes in solvent properties. These surroundings would alter the solubility, hydrophobicity and spectroscopic properties of morin and induce the change in its antioxidant capacity. Therefore, the study of the physical and chemical properties of morin in various environments, especially in vivo, is fundamentally important. Many investigations have been performed in biological mimetic systems such as aerosol-OT (AOT) reverse micelles or hydro-organic mixed solvents (20,22–25). Hydro-organic solvent mixtures have been found to be very suitable in emulating biological conditions because they simultaneously show low polarity and a partially aqueous content. In CH3OH–H2O and CH3CN–H2O mixed solvents, H2O and CH3OH have similar solvatochromic parameters, but CH3CN and H2O have quite different solvatochromic properties, which provide mixed solvents offering a wide diversity of features and behavior. In this study, the spectroscopic properties of morin and the effect of solvents were investigated in various hydro-organic solvent mixtures.
ABSTRACT The specific fluorescence properties of morin (3,20 ,40 ,5,7-pentahydroxyflavone) were studied in various CH3OH–H2O and CH3CN–H2O mixed solvents. Although the dihedral angle is large in the S0 state, morin has an almost planar molecular structure in the S1 state owing to the very low rotational energy barrier around the interring bond between B and the A, C ring. The excited state intramolecular proton transfer (ESIPT) at the S1 state cannot occur immediately after excitation, S1 ? S0 fluorescence can be observed. Two conformers, Morin A and B have been known. At the CH3OH–H2O, Morin B will be the principal species but at the CH3CN– H2O, Morin A is the principal species. At the CH3OH–H2O, owing to the large Franck–Condon (FC) factor for S2 ? S1 internal convernal (IC) and flexible molecular structure, only S1 ? S0 fluorescence was exhibited. At the CH3CN–H2O, as the FC factor for S2 ? S1 IC is small and molecular structure is rigid, S2 ? S0 and S1 ? S0 dual fluorescence was observed. This abnormal fluorescence property was further supported by the small pK1 value, effective delocalization of the lone pair electrons of C(20 )–OH to the A, C ring, and a theoretical calculation.
INTRODUCTION Flavonoids are a group of polyphenolic molecules found in the majority of plants, particularly in the leaves, fruits, nuts, skins, flowers and plant extracts such as red wine and tea (1–4). These naturally occurring substances are known to possess novel therapeutic activities such as anti-inflammetry, antihepatotoxic, antiallergic, antiatherogenic, antiosteoporotic and anticancer activities, with low systemic toxicity (5–11). The greater part of their beneficial effects has been explained in terms of antioxidant activity. Furthermore, flavonoids have attractive characteristic chemical properties and are one of the best known molecular systems showing excited state intramolecular proton transfer (ESIPT) and dual fluorescence behavior (12–16). To study the antioxidant and radical scavenging activity of flavonoids, the information on the molecular structure and the intra- and intermolecular interactions is very important.
MATERIALS AND METHODS Morin, methanol (spectrophotometric grade) and acetonitrile (spectrophotometric grade) were purchased from Sigma Aldrich (St. Louis, MO, USA) and used as received. Deionized and doubly distilled water was used. The mole fraction denotes the composition of various binary solvent mixtures. Two solvatochromic parameters, p* and a, of the mixed solvents were used in this study. The p* scale is an index of solvent
*Corresponding author email: [email protected] (Ki-Min Bark) © 2014 The American Society of Photobiology
280
Photochemistry and Photobiology, 2015, 91
Compounds
R1
R2
R3
R4
Morin
OH
H
OH
OH
Quercetin
H
OH
OH
OH
Kaempferol
H
H
OH
OH
Apigenin
H
H
OH
H
Figure 1. Atomic numbering (IUPAC nomenclature) and substitution pattern of several flavonoids. dipolarity–polarizability and the a scale is the solvent hydrogen-bond donor acidity. The p* and a values of the CH3OH–H2O mixed solvents will be proportion to the solvent composition within experimental errors. The p* and a of CH3CN–H2O mixed solvents was obtained from the literatures as described previously (26–28). All experiments were performed with low concentration solutions (below 29105 M) to prevent any primary and secondary interfilter effects. The dissolved oxygen in samples was removed by high-purity Ar gas purging for 20 min. The UV/visible absorption spectra were obtained with a JASCO (Tokyo, Japan) V-530 spectrophotometer. Steady-state fluorescence spectra were measured using a Hitachi (Tokyo, Japan) F-7000 spectrofluorometer. The fluorescence center of gravity, which was proportional to the average energy of emission, was used as the position of fluorescence emission band (29). Quantum yields (Φ) were measured using quinine sulfate (Φ = 0.546) as a reference (30). The fluorescence lifetimes were measured using the time-correlated single-photon counting method with the Fluorolog-3 spectrofluorometer (HORIBA, Edison, NJ, USA). The interchangeable NanoLED pulsed laser-diodes and LEDs having fixed wavelength were used as sources. Standard optical pulse durations were
Spectroscopic Properties of Morin in Various CH3OH–H2O and CH3CN–H2O Mixed Solvents Hyoung-Ryun Park1, Seo-Eun Im1, Jung-Ja Seo1, Bong-Gon Kim2, Jin Ah Yoon2 and Ki-Min Bark*3 1
Department of Chemistry and Research Institute of Basic Science, Chonnam National University, Gwangju, Korea Department of Chemical Education, Gyeongsang National University, Chinju, Korea 3 Department of Chemical Education, and Research Institute of Life Science, Gyeongsang National University, Chinju, Korea 2
Received 21 August 2014, accepted 14 December 2014, DOI: 10.1111/php.12407
Morin (3,20 ,40 ,5,7-pentahydroxyflavone), one of the most common flavonoids, has attracted the attention of many researchers because of its broad spectrum of biological and pharmaceutical activities (17–19). The flavonoids that contain an –OH group at position 5 (C-5) have been known as nonfluorescent molecules. However, it was discovered that some flavonoids such as quercetin glycoside, quercetin and apigenin, which also contains an – OH at C-5, exhibit a new significant fluorescence emission in hydro-organic mixed solvents and AOT reverse micelle (20–22). Compared with quercetin, morin has the –OH at C(20 ) position instead of C(30 ) position (see Fig. 1). Without this difference, quercetin and morin have exactly the same molecular structures. As the number and position of the –OH groups in flavonoids will influence their chemical properties, it will be very interesting to study the difference of chemical properties for several flavonoids with similar molecular structures such as quercetin and morin. The geometrical molecular structure and properties of the polyphenolic compounds such as morin are very sensitive to changes in solvent properties. These surroundings would alter the solubility, hydrophobicity and spectroscopic properties of morin and induce the change in its antioxidant capacity. Therefore, the study of the physical and chemical properties of morin in various environments, especially in vivo, is fundamentally important. Many investigations have been performed in biological mimetic systems such as aerosol-OT (AOT) reverse micelles or hydro-organic mixed solvents (20,22–25). Hydro-organic solvent mixtures have been found to be very suitable in emulating biological conditions because they simultaneously show low polarity and a partially aqueous content. In CH3OH–H2O and CH3CN–H2O mixed solvents, H2O and CH3OH have similar solvatochromic parameters, but CH3CN and H2O have quite different solvatochromic properties, which provide mixed solvents offering a wide diversity of features and behavior. In this study, the spectroscopic properties of morin and the effect of solvents were investigated in various hydro-organic solvent mixtures.
ABSTRACT The specific fluorescence properties of morin (3,20 ,40 ,5,7-pentahydroxyflavone) were studied in various CH3OH–H2O and CH3CN–H2O mixed solvents. Although the dihedral angle is large in the S0 state, morin has an almost planar molecular structure in the S1 state owing to the very low rotational energy barrier around the interring bond between B and the A, C ring. The excited state intramolecular proton transfer (ESIPT) at the S1 state cannot occur immediately after excitation, S1 ? S0 fluorescence can be observed. Two conformers, Morin A and B have been known. At the CH3OH–H2O, Morin B will be the principal species but at the CH3CN– H2O, Morin A is the principal species. At the CH3OH–H2O, owing to the large Franck–Condon (FC) factor for S2 ? S1 internal convernal (IC) and flexible molecular structure, only S1 ? S0 fluorescence was exhibited. At the CH3CN–H2O, as the FC factor for S2 ? S1 IC is small and molecular structure is rigid, S2 ? S0 and S1 ? S0 dual fluorescence was observed. This abnormal fluorescence property was further supported by the small pK1 value, effective delocalization of the lone pair electrons of C(20 )–OH to the A, C ring, and a theoretical calculation.
INTRODUCTION Flavonoids are a group of polyphenolic molecules found in the majority of plants, particularly in the leaves, fruits, nuts, skins, flowers and plant extracts such as red wine and tea (1–4). These naturally occurring substances are known to possess novel therapeutic activities such as anti-inflammetry, antihepatotoxic, antiallergic, antiatherogenic, antiosteoporotic and anticancer activities, with low systemic toxicity (5–11). The greater part of their beneficial effects has been explained in terms of antioxidant activity. Furthermore, flavonoids have attractive characteristic chemical properties and are one of the best known molecular systems showing excited state intramolecular proton transfer (ESIPT) and dual fluorescence behavior (12–16). To study the antioxidant and radical scavenging activity of flavonoids, the information on the molecular structure and the intra- and intermolecular interactions is very important.
MATERIALS AND METHODS Morin, methanol (spectrophotometric grade) and acetonitrile (spectrophotometric grade) were purchased from Sigma Aldrich (St. Louis, MO, USA) and used as received. Deionized and doubly distilled water was used. The mole fraction denotes the composition of various binary solvent mixtures. Two solvatochromic parameters, p* and a, of the mixed solvents were used in this study. The p* scale is an index of solvent
*Corresponding author email: [email protected] (Ki-Min Bark) © 2014 The American Society of Photobiology
280
Photochemistry and Photobiology, 2015, 91
Compounds
R1
R2
R3
R4
Morin
OH
H
OH
OH
Quercetin
H
OH
OH
OH
Kaempferol
H
H
OH
OH
Apigenin
H
H
OH
H
Figure 1. Atomic numbering (IUPAC nomenclature) and substitution pattern of several flavonoids. dipolarity–polarizability and the a scale is the solvent hydrogen-bond donor acidity. The p* and a values of the CH3OH–H2O mixed solvents will be proportion to the solvent composition within experimental errors. The p* and a of CH3CN–H2O mixed solvents was obtained from the literatures as described previously (26–28). All experiments were performed with low concentration solutions (below 29105 M) to prevent any primary and secondary interfilter effects. The dissolved oxygen in samples was removed by high-purity Ar gas purging for 20 min. The UV/visible absorption spectra were obtained with a JASCO (Tokyo, Japan) V-530 spectrophotometer. Steady-state fluorescence spectra were measured using a Hitachi (Tokyo, Japan) F-7000 spectrofluorometer. The fluorescence center of gravity, which was proportional to the average energy of emission, was used as the position of fluorescence emission band (29). Quantum yields (Φ) were measured using quinine sulfate (Φ = 0.546) as a reference (30). The fluorescence lifetimes were measured using the time-correlated single-photon counting method with the Fluorolog-3 spectrofluorometer (HORIBA, Edison, NJ, USA). The interchangeable NanoLED pulsed laser-diodes and LEDs having fixed wavelength were used as sources. Standard optical pulse durations were
