INTRODUCTION:

Photochemistry has seen tremendous upsurge of interest and activity. Organic photochemistry gives us the way for new chemical reactions^1-3. Some of these reactions are unique, i. e. they occur only by photochemical means. The photo-oxygenation ^4-11 of heterocycles leads to a variety of products and serves as an important tool in the synthesis of many natural products or compounds of special interest. Photo-oxygenation can be described as a reaction in which substrate gives initial oxygen adduct by combination with oxygen in presence of light and a sensitizer. The role of oxygen is important to determine the type of the mechanism. The photo-oxygenation of purine represents a versatile and widely accepted tool for introducing oxygenated functions in a mild, simple and selective way. Currently great attention has been paid to the photo-oxygenation of biomolecules due to their involvement in the photobiological and photodynamic phenomenon as well as due to the role of the photo-oxygenation reaction in the light induced degradation of drugs, biomolecules or pesticides.^12-20Purines are basic chemicals of life. A very little work has been done on photo-oxygenation of purines by singlet molecular oxygen²¹. Therefore, in the present article we report the photo-oxygenation of adenosine under different conditions as it is an important biomolecule.

MATERIAL AND METHODS:

1. Photo-oxygenation of Adenosine by singlet oxygen in neutral medium:

To the solution of adenosine (2.0 g.) in doubly distilled water (100 ml.) and distilled alcohol (100ml.), methylene blue (0.01 g.) was added as sensitizer. The solution was then irradiated with low pressure mercury vapor lamp (SAIC make) placed inside the immersion well photo reactor; air was continuously bubbled through the solution with the help of an aerator. The progress of the reaction was monitored after every hour by TLC using butanol: acetic acid: water (12:3:5). After 20 hrs the spot of the substrate disappeared, therefore the irradiation was stopped and the dye was removed by animal charcoal treatment. The solution was concentrated on water bath under reduced pressure and then placed as such overnight in refrigerator. The colored solid appeared was filtered, washed with water and recrystallised with ethanol to give colorless crystals (Product II).

Found: C-36.17%, H-2.43%, N- 42.19% C₁₀H₈N₁₀O₄ requires: C-36.14%, H- 2.39%, N- 42.17%.

2. Photo-oxygenation of Adenosine by singlet oxygen in alkaline medium:

The reaction was carried out in the same manner as given above; the reaction mixture was made alkaline by adding dil. NaOH. The reaction completed in 40 hrs. The pale yellow product obtained was filtered, washed with distilled water and recrystallised with ethanol to give crystals. It was found to be the same as above (Product II) by co-TLC.

Table 1

S. No.	Time of irradiation (in hrs )	Product	Melting Point (° C)	Yields (in gms)	Medium
1.	20	II	278-280	~1.1	Neutral
2.	42	II	278-280	~0.8	Alkaline
3.	15	III	140	~1.0	Acidic

3. Photo-oxygenation of Adenosine by singlet oxygen in acidic medium:

The reaction was carried out in the same manner as given above. The reaction mixture was made acidic by adding dil HCl. The reaction completed in 15 hrs. The yellow solid appeared which were filtered, washed with water and recrystallised with ethanol to give colorless crystals.(product III)

Found: C-35.98%, H-1.80%, N- 33.51% C₁₀H₆N₁₀O₄ requires: C-35.92%, H- 1.83%, N- 33.54%.

RESULTS AND DISCUSSION:

The photo-oxygenation can take place by any of the two mechanisms- Backstrom type I and type II, depending upon the sensitizer being used. With benzophenone as sensitizer the substrate reacts in the excited sate and oxygen reacts in the ground state (type I mechanism) and with dye as sensitizer, the oxygen reacts in the excited state i.e. as singlet oxygen (type II mechanism) and the substrate reacts in the ground state. In the present study, methylene blue has been used as sensitizer, therefore type II mechanism has been proposed. When neutral or alkaline solution of Adenosine (I) was irradiated with mercury vapor lamp in presence of methylene blue, it gives the product (II) as shown in Scheme 1.

The dimer formed initially, reacts with the singlet oxygen to give hydroperoxide (II). The structure of the product (II) has been confirmed by spectral and elemental analysis. The results of element analysis are given in experimental section and are in accordance with the calculated values. The IR spectrum of product shows the following important peaks- 3327cm^-1(N-H streching), 3138 cm^-1 (O-H streching), 1633 cm^-1 (N-H bending), 1180 cm^-1 (C-O streching), 885 cm^-1 (O-O streching) etc. The ¹H NMR spectrum of the show peaks at δ 8.16 aromatic proton, δ 7.83 NH₂ protons and δ7.50 -OH protons. The mass spectrum of the product (II) shows the molecular ion peak at m/e 332 corresponding to the molecular weight of the product. Other important peaks are observed at m/e 298, 266, 234,166, 149,117 etc.

When acidic solution of adenosine (I) was irradiated by low pressure mercury vapor lamp in presence of methylene blue it gives product (III). The initially formed dimer undergoes deamination in presence of acid followed by oxygenation with singlet oxygen giving the hydroperoxide. The structure of the product (III) has been confirmed by elemental and spectral analysis. The IR spectrum of product (III) shows the following important peaks- 3438 cm^-1(N-H stretching), 3134 cm^-1 , (O-H stretching), 1654 cm^-1(C = N stretching), 1126 cm^-1(C-N bending), 815 cm^-1(O-O stretching), 690 cm^-1(O-H bending) etc. The ¹H NMR spectrum of the product (III) shows peaks at δ 7.88 aromatic proton, δ 5.90 (O-O –H) protons and δ4.78 (OH) protons. The mass spectrum of the product (III) shows the molecular ion peak at m/e 334. Other important peaks are observed at m/e 306, 300, 291,289, 268, 265, 167, 124, 81, 54 etc. The results of element analysis are given in experimental section and are in accordance with the calculated values.

CONCLUSION:

Adenosine is found to be oxidized photochemically in acidic, alkaline and neutral media by singlet molecular oxygen. In the alkaline and neutral medium the same product is obtained but in acidic condition, different product is obtained. The rate of reaction is fast in acidic condition compared to neutral and alkaline. The reaction is slowest in alkaline medium and the yield is also less, indicating that adenosine is less reactive or more stable in alkaline condition.

ACKNOWLEDGEMENT:

The author is thankful to the authorities of R.S.I.C., C.D.R.I. Lucknow for providing the spectral and elemental analysis of compounds.

REFERENCES:

1. Gorman AA and Rodgers MAJ. CRC Handbook of Organic Photochemistry, 1989; II, CRC Press: 229-247.

2. Coyle JD. Introduction to organic photochemistry. 1991; Wiley Publications.

3. Albini A and Fasani E. Drugs: Photochemistry and Photostability.The Royal Society of Chemistry: Cambridge (UK): 1998.

4. Bunsho Ohtani et al. Photooxygenation of methyl linoleate sensitized porphyrins and dyes in acetonitrile solution and aqueous emulsion systems. Photochem. Photobiol. A: Chem. 2008; 44(6): 725- 732.

5. Shubha Jain et al. Dye sensitized photooxygenation of symmetrical biphenyl thiourea. Acta Chimica Hungarica. 1985; 120(3): 207-208.

6. Shubha Jain et al. Dye sensitized photooxygenation of t-cinnamic acid. Science and Culture. 1985; 52: 379-381.

7. Iesce MR et al. Photooxygenation of heterocycles. Current Organic Chemistry, 2005; 9(2): 109-139.

8. George MV and Bhat V. Photooxygenation of nitrogen heterocycles. Chem. Rev.1979; 79 (5): 447–478.

9. Jawaid Iqbal, Adil Husain, and Anamika Gupta. Sensitized photooxygenation of Tinosponone, a Clerodane Diterpene from Tinospora Cordifolia.Acta Chim. Slov. 2005; 52: 455–459.

10. Axel GG and Anna B. Sustainable photochemistry: solvent-free singlet oxygen- photooxygenation of organic substrates embedded in porphyrin-loaded polystyrene beads. Chem Comm. 2002; 1594–1595.

11. Amitbhod Upadhyaya, Deepali Pradhan, Jain S. Photooxygenation of some alkaloids with singlet molecular oxygen. Ind. J. Chem. Sec. B, 40, 1255, 2001.

12. Jain S, Rakha Chauhan and Pandey RB. pH-dependent phototransformation of formyl piperidine. Int. J. Chem. Sci. 2007; 5(1): 51-54.

13. Melvin IS and Helen VV. The dye- sensitized photooxidation of purines of and pyrimidine derivatives. Archives of Biochemistry and Boipphysics, 1964; 105: 197-207.

14. Swaranga MS and Asinarayana M. Photooxidation of purines in presence of peroxydisulphate in aqueous solution. Ind. J Chem. Sec. A 2002; 41: 2096-2100

15. Straight RC and Spikes JD. In Singlet Oxygen.CRC Press: Boca Raton (FI), 1985; IV, 91-143.

16. Burrows HD et al.Effects of sensitizers on the photodegradationof the fungicide triadimenol. Photochem. Photobiol. B: Biol. 2002; 67: 71.

17. Jain S, Pandey RB, Chaurey M, Benzophenone and Methylene Blue sensitized photooxidation of Piperine: A comparative study of photooxidation of piperine by ground state and singlet oxygen. Ind. J. Chem. Sec. B. 2004; 43: 2245-48.

18. Marx JL. Oxygen free radical related to disease. Science.1987; 235: 529-531.

19. Halliwell B and Gutteridge JMC. Free Radicals in Molecular Biology and Medicine, II: Clarendon. Oxford. 1989.

20. Floyd RA. Oxidative damage to behaviour during aging. Science. 1991; 254: 1597.

21. Shubha Jain and Rakha Nagwanshi. Photooxidation of Pyrazoline by singlet molecular oxygen. 2009; 32(2) :330-334.

Received on 16.09.2009 Modified on 09.11.2009

Asian J. Research Chem. 3(1):Jan.- Mar. 2010 page 110-112