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Spectroscopic study of Mentha oils
Spectrochimica Acto, Vol. Printed in Great Britain
46A.No.8,
05%8539/90 $3.00+0.00 @ 1990 Pergamon Press plc
pp. 1269-1272. IWO
RESEARCH NOTE
Spectroscopic study of Mentha oils (Received for publication 27 March 1990) Abstract-The visible fluorescence and excitation spectra of Mentha oils (Japanese mint oil, peppermint oil and spearmint oil) have been recorded. Different physical constants which are characteristic of the fluorescent molecules have been calculated for all three oils. Results reveal that the same group of organic compounds dominate in the oils of peppermint and spearmint, whereas some different compound is present in Japanese mint oil. It is also found that the fluorescence intensity of these oils is comparable to that of Rhodamine 6G dye in methanol solution. Our studies suggest that Mentha oils may be a useful lasing material in the 450-600 nm wavelength range.
INTRODUCTION Mint species (Labiatae) are one of the most important and widely cultivated crops in the world. The essential oils obtained from these species are important ingredients in many cosmetics, flavour and pharmaceutical preparations. During the past several years much work has been done on the agricultural aspects and constituents of these species [l, 21. Biosynthetic pathways have been studied by CROUTEAU [3] and AKHILA [4]. MANFREDINIand MONTES[5], KOBAYASHIet al. [6] and SM~~A et al. [7] have reported different constituents of these oils using gas chromatography, infrared and ultraviolet, while gas chromatography mass spectrometry and NMR studies have been carried out by YOSHIRO [8] and RAO [9]. The emission and absorption spectra of these oils are, however, not available. LEWIS and KASHA [lo] suggested that organic molecules might be identified by their emission spectra. Studies of fluorescence have been demonstrated to be a powerful technique for investigating radiative pathways in excited biological molecules [ll, 121 while the nonradiative pathway can be studied with the help of photoacoustic spectroscopy [13]. Studies on excitation transfer in organic molecules are of great importance in the field of organic dye lasers and also in photodynamic therapy. F~~RSTER[14] has shown that the efficiency of nonradiative excitation transfer is proportional to the overlap of the emission spectrum of the donor and absorption spectrum of the acceptor. In view of the importance of the emission and absorption spectroscopy of these molecules, we have studied the spectral charactersistics at various biological molecules from medicinal and aromatic plants. The present article describes the results of tluorescence and excitation spectroscopy of Mentha oils Mentha arvensk (Japanese mint oil), M. piperitu (peppermint oil) and M. spicutu (spearmint oil).
EXPERIMENTAL The fresh leaves of the plants grown in the Research and Demonstration farm of CIMAP Regional Centre, Pantnagar, were hydrodistilled by Cleavernger’s apparatus and the oils thus obtained were dried over anhydrous sodium sulphate and utilized in the present investigation. The fluorescence and excitation spectra were recorded on a JY3 spectrofluorometer. The light source was a 150 W Xe arc with a quartz envelope. The same type of monochromator, having concave holographic grating (1200 grooves/mm) blazed in the U.V. for excitation and in the visible for emission, is used. The emission monochromator exit slit is fitted with a detector housing containing a photomultiplier tube (Hamamatsu R928S).
RESULTS
AND
DISCUSSION
The excitation spectra of the oils, shown in Fig. 1, exhibit a single broad envelope (FWHM = 50 nm) in all the oils with peak position at ==418 nm in the case of peppermint and spearmint oil and at ~395 nm in the case of Japanese mint oil. The excitation spectra of peppermint and spearmint are almost identical and much different from that of Japanese mint oil. The fluorescence spectra of all three oils are shown in Fig. 2. The shape of the fluorescence profiles in all the three cases are similar, indicating that they belong to the same group of organic 1269
1270
Research
Note
2.2-
o-0
2.1 -
bd
Spearmint
W
Psppermmt
20-
Japanese mint oil 0bI 011
I .s l.61.7 I61.5l41.3 1.2 s
II-
e (D E H
0.9 -
l.O-
OB0706050.4 0.3 0.2 0.1 t 550
400
450
Wavelength
lnm)
Fig. 1. Excitation spectra of mint oils: excitation spectra have been recorded by fixing the emission monochromator at 473,494 and 496 nm (strongest fluorescence signal) and by scanning the excitation monochromator from 360 to 460 nm, from 380 to 480 nm and from 360 to 480 nm for Japanese mint oil, peppermint oil and spearmint oil, respectively.
compounds. In all three cases, the spectra show two peaks, which indicate that more than two states may be involved in this emission. The intensity of the lower wavelength (higher energy) peak in all three cases is stronger than the higher wavelength (lower energy) peak. For each excited singlet state in an organic molecule there is a triplet state with lower energy than the corresponding singlet electronic state [15]. We have recorded the emission spectra of all three oils after bubbling oxygen through them. It is found that the signal strength at the higher wavelength peak in the region of 514-530 nm decreases slightly with respect to the spectrum of the oil without bubbling of
W
Japanese
H
Spearmint
mint oil oil
M
Peppermmt
oil
0.9 3E
0.6 -
3
0.7-
’
0.60.5-
I
I
I
I
I
450
500
550
600
650
Wavelength
(nm)
Fig. 2. Fluorescence spectra of mint oils: fluorescence spectra have been recorded by fixing the excitation monochromator at 400, 41.5 and 418 nm (strongest absorption) and by scanning the emission monochromator from 410 to 650 nm, 420 to 650 nm and 430 to 650 nm for Japanese mint oil, peppermint oil and spearmint oil, respectively.
Research Note
1271
oxygen. On the basis of these observations one may assume that the peak arising at lower wavelengths (=l60 nm in Japanese mint oil, and at =495 nm in peppermint and spearmint oil) is due to singlet emission (fluorescence peak) and the peak at higher wavelengths (lower energy) at 425 nm in all the oils is due to triplet emission (phosphorescence peak). The magnitude of the signals are in the following order: peppermint oil>Japanese mint oil>spearmint oil. Japanese mint oil has a fluorescence and phosphorescence peak at ~460 and 525 nm. The peak at higher wavelengths (525 nm in Japanese mint oil) is not very well defined, whereas in peppermint and spearmint oils it is clearly present. Peppermint oil and spearmint oil have the fluorescence and phosphorescence peak at the same wavelength i.e. at 495 and 525 nm; though the peak intensity in peppermint oil is larger than the spearmint oil. In both the oils (peppermint and spearmint) the phosphorescence signals are constant from 512 to 525 nm. For comparison of the fluorescence and excitation intensity, we have also recorded the fluorescence and excitation spectra of Rh6G in methanol at a concentration of 5 X lo-’ M. Comparison of the spectra shows that the quantum efficiency of the fluorescence of these oils is comparable to that of Rh6G. Therefore, in addition to the medicinal use these oils may also be used as lasing material in dye lasers. A characteristic feature of fluorescent molecules is the difference between the wavelength of the excitation maximum and the wavelength of the fluorescence maximum, which is referred to as the “Stokes shift”. The values for Stokes shift are of structural significance, and their application to structure determination has been discussed by LEEMAN et al. [16]. The Stokes shift (in cm-‘) can be calculated from the following equation:
Stokes shift = 10’
where Iz,,, and &“, are the corrected excitation and fluorescence maxima, respectively, expressed in nm. From Figs 1 and 2, the Stokes shift for Japanese mint oil is ~3577 cm-’ and for peppermint and spearmint oil it is ~3721 cm-‘. The energy of the singlet and triplet state can be calculated by the following formulae:
where jlflu.and Ai,,,,, are the wavelength of the corrected fluorescence and phosphorescence maxima and A,,, is wavelength of the corrected excitation maxima from Figs 1 and 2. For Japanese mint oil and Etriplet=20900cm-‘. For peppermint and spearmint oil Esinglctx &sin&,=23591cm-’ 22 083 cm-’ and Errir,iet= 20 929 cm-‘. Acknowledgements-The authors are grateful to Professor S. N. THAKUR, Department of Physics, Banaras Hindu University, Varanasi, India for fruitful discussions and going through the manuscript, and also to Dr R. P. S. KUSHWAHA for helping in the experimental work. We are also thankful to Dr R. S. THAKUR, Director, CIMAP and Dr KAMLA SINGH,Scientist-in-Charge, CIMAP, Pantnagar, India for providing facilities and their keen interest. This work was supported in part by a grant from the University project No BS-368 on “Study of absorption and emission spectra of mint species”. Department of Physics, G.B. Pant University of Agriculture and Technology, Pantnagar-263 145, Nainital, India Central Institute of Medicinal and Aromatic Plants Regional Centre, P.O. Dairy Farm, Nagla-263 149, Nainital, India
A. K. RAI
A.K.
SINGH
1272
Research Note REFERENCES
[1] E. Guenther, The Essential Oils 111, pp.586-676. Van Nostrand, New York (1949). [2] C. K. Atal and B. M. Kaput, Cultivation and Utilization of Aromatic Plants, pp.241-296. Publication Information Directorate CSIR, New Delhi (1982). [3] R. Crouteau, Perfumer and Flavorist, pp. 35-39 (1980). [4] A. Akhila, J. Plant Physiol. 126, 379 (1986). [5] T. A. A. Manfredini and A. C. Montes, Anals. Assoc. Qins Agric. 50, 279 (1962). [6] M. Kobayashi, H. Toshigi and J. Kasuhara, Lory O, 60, 27 (1972). [7] D. M. Smita, W. Shakum and L. J. Levi, J. Agr. Food Chem. H, 268 (1968). [8] M. Yoshiro, Analysis of Essential Oils by Gas Chromatogrphy Mass Spectrometry. John Wiley, New York (1976). [9] J. M. Rao, Ind. Perfum. 32, 109 (1988). [10] G. N. Lewis and M. J. Kasha, J. Am. Chem. Soc. 66, 2100 (1944). [11] L. Brand and J. R. Gohlke, Ann. Rev. Biochem. 41,843 (1972). [12] T. A. Moore, E. P. O'Hara, D. M. Anjo, R. Tom and D. Benin, Z Phys. C6, 339 (1983). [13] S. Rai, A. K. Rai and S. N. Thakur, Ind. J. Pure Appl. Phys. 26, 649 (1988). [14] Th. FOrster, Discuss. Faraday Soc. 27, 7 (1959). [15] S. Udenfriend, FluorescenceAssay in Biology and Medicine, Vol. II. Academic Press, London (1969). [16] H. G. Leeman, K. Stitch and M. Thomas, Progress in Drug Research (edited by E. Jucker), Vol. 6, p. 152. Birkh~iuser, Basel (1963).
46A.No.8,
05%8539/90 $3.00+0.00 @ 1990 Pergamon Press plc
pp. 1269-1272. IWO
RESEARCH NOTE
Spectroscopic study of Mentha oils (Received for publication 27 March 1990) Abstract-The visible fluorescence and excitation spectra of Mentha oils (Japanese mint oil, peppermint oil and spearmint oil) have been recorded. Different physical constants which are characteristic of the fluorescent molecules have been calculated for all three oils. Results reveal that the same group of organic compounds dominate in the oils of peppermint and spearmint, whereas some different compound is present in Japanese mint oil. It is also found that the fluorescence intensity of these oils is comparable to that of Rhodamine 6G dye in methanol solution. Our studies suggest that Mentha oils may be a useful lasing material in the 450-600 nm wavelength range.
INTRODUCTION Mint species (Labiatae) are one of the most important and widely cultivated crops in the world. The essential oils obtained from these species are important ingredients in many cosmetics, flavour and pharmaceutical preparations. During the past several years much work has been done on the agricultural aspects and constituents of these species [l, 21. Biosynthetic pathways have been studied by CROUTEAU [3] and AKHILA [4]. MANFREDINIand MONTES[5], KOBAYASHIet al. [6] and SM~~A et al. [7] have reported different constituents of these oils using gas chromatography, infrared and ultraviolet, while gas chromatography mass spectrometry and NMR studies have been carried out by YOSHIRO [8] and RAO [9]. The emission and absorption spectra of these oils are, however, not available. LEWIS and KASHA [lo] suggested that organic molecules might be identified by their emission spectra. Studies of fluorescence have been demonstrated to be a powerful technique for investigating radiative pathways in excited biological molecules [ll, 121 while the nonradiative pathway can be studied with the help of photoacoustic spectroscopy [13]. Studies on excitation transfer in organic molecules are of great importance in the field of organic dye lasers and also in photodynamic therapy. F~~RSTER[14] has shown that the efficiency of nonradiative excitation transfer is proportional to the overlap of the emission spectrum of the donor and absorption spectrum of the acceptor. In view of the importance of the emission and absorption spectroscopy of these molecules, we have studied the spectral charactersistics at various biological molecules from medicinal and aromatic plants. The present article describes the results of tluorescence and excitation spectroscopy of Mentha oils Mentha arvensk (Japanese mint oil), M. piperitu (peppermint oil) and M. spicutu (spearmint oil).
EXPERIMENTAL The fresh leaves of the plants grown in the Research and Demonstration farm of CIMAP Regional Centre, Pantnagar, were hydrodistilled by Cleavernger’s apparatus and the oils thus obtained were dried over anhydrous sodium sulphate and utilized in the present investigation. The fluorescence and excitation spectra were recorded on a JY3 spectrofluorometer. The light source was a 150 W Xe arc with a quartz envelope. The same type of monochromator, having concave holographic grating (1200 grooves/mm) blazed in the U.V. for excitation and in the visible for emission, is used. The emission monochromator exit slit is fitted with a detector housing containing a photomultiplier tube (Hamamatsu R928S).
RESULTS
AND
DISCUSSION
The excitation spectra of the oils, shown in Fig. 1, exhibit a single broad envelope (FWHM = 50 nm) in all the oils with peak position at ==418 nm in the case of peppermint and spearmint oil and at ~395 nm in the case of Japanese mint oil. The excitation spectra of peppermint and spearmint are almost identical and much different from that of Japanese mint oil. The fluorescence spectra of all three oils are shown in Fig. 2. The shape of the fluorescence profiles in all the three cases are similar, indicating that they belong to the same group of organic 1269
1270
Research
Note
2.2-
o-0
2.1 -
bd
Spearmint
W
Psppermmt
20-
Japanese mint oil 0bI 011
I .s l.61.7 I61.5l41.3 1.2 s
II-
e (D E H
0.9 -
l.O-
OB0706050.4 0.3 0.2 0.1 t 550
400
450
Wavelength
lnm)
Fig. 1. Excitation spectra of mint oils: excitation spectra have been recorded by fixing the emission monochromator at 473,494 and 496 nm (strongest fluorescence signal) and by scanning the excitation monochromator from 360 to 460 nm, from 380 to 480 nm and from 360 to 480 nm for Japanese mint oil, peppermint oil and spearmint oil, respectively.
compounds. In all three cases, the spectra show two peaks, which indicate that more than two states may be involved in this emission. The intensity of the lower wavelength (higher energy) peak in all three cases is stronger than the higher wavelength (lower energy) peak. For each excited singlet state in an organic molecule there is a triplet state with lower energy than the corresponding singlet electronic state [15]. We have recorded the emission spectra of all three oils after bubbling oxygen through them. It is found that the signal strength at the higher wavelength peak in the region of 514-530 nm decreases slightly with respect to the spectrum of the oil without bubbling of
W
Japanese
H
Spearmint
mint oil oil
M
Peppermmt
oil
0.9 3E
0.6 -
3
0.7-
’
0.60.5-
I
I
I
I
I
450
500
550
600
650
Wavelength
(nm)
Fig. 2. Fluorescence spectra of mint oils: fluorescence spectra have been recorded by fixing the excitation monochromator at 400, 41.5 and 418 nm (strongest absorption) and by scanning the emission monochromator from 410 to 650 nm, 420 to 650 nm and 430 to 650 nm for Japanese mint oil, peppermint oil and spearmint oil, respectively.
Research Note
1271
oxygen. On the basis of these observations one may assume that the peak arising at lower wavelengths (=l60 nm in Japanese mint oil, and at =495 nm in peppermint and spearmint oil) is due to singlet emission (fluorescence peak) and the peak at higher wavelengths (lower energy) at 425 nm in all the oils is due to triplet emission (phosphorescence peak). The magnitude of the signals are in the following order: peppermint oil>Japanese mint oil>spearmint oil. Japanese mint oil has a fluorescence and phosphorescence peak at ~460 and 525 nm. The peak at higher wavelengths (525 nm in Japanese mint oil) is not very well defined, whereas in peppermint and spearmint oils it is clearly present. Peppermint oil and spearmint oil have the fluorescence and phosphorescence peak at the same wavelength i.e. at 495 and 525 nm; though the peak intensity in peppermint oil is larger than the spearmint oil. In both the oils (peppermint and spearmint) the phosphorescence signals are constant from 512 to 525 nm. For comparison of the fluorescence and excitation intensity, we have also recorded the fluorescence and excitation spectra of Rh6G in methanol at a concentration of 5 X lo-’ M. Comparison of the spectra shows that the quantum efficiency of the fluorescence of these oils is comparable to that of Rh6G. Therefore, in addition to the medicinal use these oils may also be used as lasing material in dye lasers. A characteristic feature of fluorescent molecules is the difference between the wavelength of the excitation maximum and the wavelength of the fluorescence maximum, which is referred to as the “Stokes shift”. The values for Stokes shift are of structural significance, and their application to structure determination has been discussed by LEEMAN et al. [16]. The Stokes shift (in cm-‘) can be calculated from the following equation:
Stokes shift = 10’
where Iz,,, and &“, are the corrected excitation and fluorescence maxima, respectively, expressed in nm. From Figs 1 and 2, the Stokes shift for Japanese mint oil is ~3577 cm-’ and for peppermint and spearmint oil it is ~3721 cm-‘. The energy of the singlet and triplet state can be calculated by the following formulae:
where jlflu.and Ai,,,,, are the wavelength of the corrected fluorescence and phosphorescence maxima and A,,, is wavelength of the corrected excitation maxima from Figs 1 and 2. For Japanese mint oil and Etriplet=20900cm-‘. For peppermint and spearmint oil Esinglctx &sin&,=23591cm-’ 22 083 cm-’ and Errir,iet= 20 929 cm-‘. Acknowledgements-The authors are grateful to Professor S. N. THAKUR, Department of Physics, Banaras Hindu University, Varanasi, India for fruitful discussions and going through the manuscript, and also to Dr R. P. S. KUSHWAHA for helping in the experimental work. We are also thankful to Dr R. S. THAKUR, Director, CIMAP and Dr KAMLA SINGH,Scientist-in-Charge, CIMAP, Pantnagar, India for providing facilities and their keen interest. This work was supported in part by a grant from the University project No BS-368 on “Study of absorption and emission spectra of mint species”. Department of Physics, G.B. Pant University of Agriculture and Technology, Pantnagar-263 145, Nainital, India Central Institute of Medicinal and Aromatic Plants Regional Centre, P.O. Dairy Farm, Nagla-263 149, Nainital, India
A. K. RAI
A.K.
SINGH
1272
Research Note REFERENCES
[1] E. Guenther, The Essential Oils 111, pp.586-676. Van Nostrand, New York (1949). [2] C. K. Atal and B. M. Kaput, Cultivation and Utilization of Aromatic Plants, pp.241-296. Publication Information Directorate CSIR, New Delhi (1982). [3] R. Crouteau, Perfumer and Flavorist, pp. 35-39 (1980). [4] A. Akhila, J. Plant Physiol. 126, 379 (1986). [5] T. A. A. Manfredini and A. C. Montes, Anals. Assoc. Qins Agric. 50, 279 (1962). [6] M. Kobayashi, H. Toshigi and J. Kasuhara, Lory O, 60, 27 (1972). [7] D. M. Smita, W. Shakum and L. J. Levi, J. Agr. Food Chem. H, 268 (1968). [8] M. Yoshiro, Analysis of Essential Oils by Gas Chromatogrphy Mass Spectrometry. John Wiley, New York (1976). [9] J. M. Rao, Ind. Perfum. 32, 109 (1988). [10] G. N. Lewis and M. J. Kasha, J. Am. Chem. Soc. 66, 2100 (1944). [11] L. Brand and J. R. Gohlke, Ann. Rev. Biochem. 41,843 (1972). [12] T. A. Moore, E. P. O'Hara, D. M. Anjo, R. Tom and D. Benin, Z Phys. C6, 339 (1983). [13] S. Rai, A. K. Rai and S. N. Thakur, Ind. J. Pure Appl. Phys. 26, 649 (1988). [14] Th. FOrster, Discuss. Faraday Soc. 27, 7 (1959). [15] S. Udenfriend, FluorescenceAssay in Biology and Medicine, Vol. II. Academic Press, London (1969). [16] H. G. Leeman, K. Stitch and M. Thomas, Progress in Drug Research (edited by E. Jucker), Vol. 6, p. 152. Birkh~iuser, Basel (1963).