Elsevier

Fitoterapia

Volume 76, Issues 3–4, June 2005, Pages 316-323
Fitoterapia

Quenching of singlet molecular oxygen by Commiphora myrrha extracts and menthofuran

https://doi.org/10.1016/j.fitote.2005.03.017Get rights and content

Abstract

The quenching activity against singlet oxygen, an actor of lipid peroxidation and DNA degradation, of the essential oil and resinoid of Commiphora myrrha from Somalia has been studied and compared to dl-α-tocopherol using 1,3-diphenylisobenzofuran (DPBF) as a probe. To insure that the furan ring was the site of the reaction, experiments were conducted with menthofuran. The essential oil and menthofuran show a higher activity than dl-α-tocopherol, suggesting their potential usefulness to neutralise this deleterious form of molecular oxygen.

Introduction

Singlet oxygen produced in vivo is a potential source of damage to cells. It can be generated photochemically or chemically and the reactivity of this molecule is a threat for several biomolecules such as DNA, proteins and lipids [1], [2].

Ground state dioxygen has two electrons unpaired in two separate orbitals with parallel spins and opposed angular momentum.

This bi-radical conformation prevents the reaction of oxygen with singlet state molecules and molecular oxygen in its ground state is rather inert.

This state is known as 3Σg. Photo excitation with a sensitizer such as Rose Bengal brings the oxygen into two different excited states: 1Σg and 1Δg. The 1Σg state rapidly turns into 1Δg, a singlet oxygen form with a relative long life time (approximately 4 μs in water) (Fig. 1).

The quenching of singlet oxygen by carotenoids and dl-α-tocopherol is well documented [3], [4], [5], [6], [7]. Indeed, there are two types of mechanisms known for the quenching of this higher energy state of oxygen: first is physical quenching where the substrate absorbs the energy of the excited oxygen as happens with carotenoids. The second way is chemical quenching where singlet oxygen reacts on electron-rich molecules such as an unsatured compound for the ene reaction to form the corresponding hydroperoxide or with a conjugated diene to react in a hetero Diels Alder to produce an endoperoxyde as shown in Fig. 2 [8]. Thus, singlet oxygen can react on vital targets such as DNA, proteins and lipids which can lead to cytopathological effects.

In the present study we investigated the relative singlet oxygen quenching abilities of two extracts from Commiphora myrrha using the DPBF method [9].

Section snippets

Tested material

Menthofuran, essential oil and resinoid of C. myrrha from Somalia were produced in our company. dl-α-Tocopherol 98%, ethanol 99% and 1,3-diphenylisobenzofuran (DPBF) were purchased from Sigma Aldrich.

Principle

Singlet oxygen is produced by photosensitization of Rose Bengal in EtOH with an appropriate wavelength of light. This sensitizer is excited and transfers its energy to molecular oxygen to produce singlet oxygen. The probe is DPBF which undergoes direct reaction with singlet oxygen resulting in a

Results

Fig. 3, Fig. 4, Fig. 5, Fig. 6 show the decrease in the probe concentration for various irradiation times and different substrate concentrations. From these data, the ratio S0/S of the different substrates tested is plotted in Fig. 7. In ethanol kd is reported to be approximately 6.7 × 104 s−1 and kQ for DPBF to be 8 × 108mol−1 s−1 [9]. We calculate from the plot of S0/S for α-tocopherol kr = 1.6 × 108 L mol−1 s−1 which is not very different from the one found by Koka et al. [11] in ethanol (1 × 108 L mol

Discussion

Fig. 3, Fig. 4, Fig. 5, Fig. 6 show the quenching of singlet oxygen by the different substrates at various concentrations.

As shown in Fig. 7 the slope of S0/S for C. myrrha essential oil is better than dl-α-tocopherol, which means a better quenching capacity than dl-α-tocopherol whereas the resinoid is equal or marginally more efficient.

The fact that the oil and the resinoid are very rich in natural furans is a possible explanation for this result. Table 1 indicates the concentration of the

Conclusions

This study shows that C. myrrha essential oil is a better quencher of singlet oxygen than α-tocopherol. The fact that the oil is more efficient than the resinoid is probably due to the higher concentration of furans in the oil. Experiments were made on menthofuran in order to confirm that singlet oxygen reacts with the furans of myrrh. This suggests that C. myrrha extracts as well as menthofuran or extract containing menthofuran can be used in conjunction with other natural free radical

Acknowledgements

Thanks are due to Dr. D. Joulain for helpful discussion and review of this article. We also want to thank the analytical Department of Robertet for its help, especially N. Barat.

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