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Photocatalytic Degradation of Tyrosine.

Byline: Jaime JimACopyrightnez-Becerril and Diana Jilote


Tyrosine is an amino acid that can be found in some wastewaters because it is part of the decomposition of proteins from organic material, and it can be a precursor of persistent and recalcitrant organic compounds. In this work, some experiments were carried out in order to determine kinetic parameters using a photocatalytic degradation reaction. It was found that degradation reaction is an interfacial process, adjusted to Langmuir-Hinshelwood model, where reaction rate depends on the concentration of tyrosine in the solution.


Amino acid degradation; Tyrosine degradation; Wastewater.


Biocontaminated water has been reported to contain protein serum albumin, ovalbumin, and gamma globulin, which are key components of microorganisms such as bacteria, fungi, viruses, and mold and are the cause of the formation of odorous compounds after chlorination treatment [1]. In general, the gamma ray, ethylene oxide, and autoclaving methods are commonly used for medical implant sterilization. Recent progress has been made in the synthesis of highly antibacterial nanomaterials such TiO2, ZnO, and Ag, for use as photocatalysts in killing cancer cells and bacteria and deactivating viruses under ultraviolet illumination [2].

Of particular importance are amino acids, which are the building blocks of proteins. Moreover, amino acids are present in several drinking water plants; they are found to be present in the filtration process and come from natural organic matter or microorganisms. Amino acids are not toxic compounds; however, they lead to the production of compounds that are stable enough to persist in tap water, and that have odor [3].

Although proteins consist exclusively of L- amino acids, the presence of D-isomers is thought to result from racemization after exposing these peptides to UV-B. Irradiation with a 100 W high- pressure Hg lamp of neutral aqueous solutions of some aromatic amino acids as tryptophan, phenylalanine, and tyrosine in the presence of nitrite ions and in contact with the atmosphere caused the formation of some strongly mutagenic products [4]. On the other hand, tyrosine is degraded by UV light in the presence of air to 3,4-dihydroxyphenylalanine (DOPA), ammonia, and a yellow-brown pigment.

The rate of production of the pigment is very pH- dependent, and production is most rapid in alkaline solution. The degradation of tyrosine is less pH dependent. When the tyrosine was totally degraded, the pigment was bleached by further exposure to the irradiation. In an atmosphere of nitrogen, the degradation of tyrosine is slower, and no pigment is formed. Thus, decomposition may proceed through either of two alternative mechanisms. In that mechanism in which pigment is produced, bleaching occurs simultaneously to, but at a slower rate than, color production. It is concluded that there is not necessarily a quantitative relationship between amino acid destruction and pigment production [5].

Photocatalysis is an advanced oxidation process that uses radical hydroxyl generated in a semiconductor irradiated with UV light. Some efforts to improve the process have been made by coupling ultrasound [6, 7] or doping semiconductors [8].

The aim of this research was to degrade tyrosine by a photocatalytic process in an aqueous solution.


The tyrosine used was analytical grade (Aldrich) and TiO2 commercial Degussa P25, and solutions were prepared with deionized water. The degradation of tyrosine was conducted in a totally dark room using 50 mg of TiO2 suspended in 50 mL of tyrosine solution at different concentrations (3.5, 10, 15, and 21.5 mg/mL) in a glass flask, with an air flow of 20 mL/min and constant agitation at natural pH for 30 minutes. A 3 mL sample was taken with a syringe, and the experiment began once the 8 W UV lamp was turned on. A 3 mL sample was taken every 15 minutes, and each was filtered with a Millipore membrane of 0.45 m pore diameter. The tyrosine remaining in solution samples was measured using a Perkin Elmer UV/VIS lambda 35 spectrophotometer at = 223 nm.

Results and Discussion

Experiments of tyrosine degradation were carried out at different concentrations Fig. 1. In experiments without a catalyst, the direct photolysis of tyrosine was negligible. Fig. 2 shows the degradation rate of tyrosine as a function of substrate concentration. Degradation rate increases with the initial concentration. The initial rate is proportional to the tyrosine concentration in solution, and the kinetics can be considered of first order. The influence of the TiO2 itself, in the absence of UV irradiation, on the removal of tyrosine from water showed that over a period of 30 min, the tyrosine concentrations decreased by a mean of only 3%. Hence physical adsorption of the tyrosine on the catalyst surface played a negligible role in the removal of the amino acid and is indicative of the essential role of the UV irradiation in achieving degradation.

When the photolysis of tyrosine was checked without photocatalyst, amino acid could not be degraded, as shown in Fig. 1. With addition of photocatalyst, the reduction rate of tyrosine increased.

According to the Langmuir-Hinshelwood (LH) model, the reaction rate varies proportionally with the coverage rate as a concentration function, and a linear expression can be conveniently obtained by plotting the reciprocal initial rate against the reciprocal initial concentration:

r = kKC/(1 + KC) (1)

1/r = 1/k + (1/kK)(1/C) (2)

Where C is the concentration of organic at equilibrium, K is the adsorption constant, and k is the rate constant [9-11].

The corresponding value of K was found equal to 0.06 L/mg, and the k was equal to 0.093 mg/L min. This indicates that the reaction takes place on the surface. By experimental observations, the value of K derived from a kinetic study is not directly equivalent to the isotherm adsorption Langmuir constant, as these values are usually minor. In addition, it has been found that k is proportional to the intensity of the light absorbed and proportional to the fraction of adsorbed O2 [12]. The results correspond to an unimolecular elementary process, when degradation is greater at a lower concentration.

This could be useful for example in the reduction of mite allergen activities that might be attributable to the following processes: (A) the decomposition of amino acid sequences, and (B) the degradation of amino acids.


The amino acid tyrosine was degradated by a photocatalytic method and reaction parameters were obtained adjusting to the LH model as a heterogeneous process. This method could be useful in elimination of this compound in environmental samples containing derivates of natural residues.


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2. J. Y. Wu, C. W. Li, C. H. Tsai, C. W. Chou, D. R. Chen and G. J. Wang, Synthesis of Antibacterial TiO2/PLGA Composite Biofilms. Nanomedicine, 10, 1097 (2014).

3. L. Elsellamia, S. Pigeot-RACopyrightmy, F. Dappozze, F. Vocanson, A. Houas and C. Guillard, Comparison of Initial Photocatalytic Degradation Pathway of Aromatic and Linear Amino Acids. Environ. Technol., 31, 1417 (2010).

4. M. M. Halmann, In Photodegradation of water pollutants, CRC Press, USA, p.320 (1995).

5. R. S. Asquith, L. Hirst and D. E. Rivett, A Study of the Ultraviolet Yellowing of Amino Acids, Peptides, and Soluble Proteins, Textile Research Journal, 40, 285 (1970).

6. M. Siddique, R. Farooq and A. Shaheen, Removal of Reactive Blue 19 from Wastewaters by Physicochemical and Biological Processes-A Review, J. Chem. Soc. Pakistan, 33, 284 (2011).

7. B. Saygi and T. Tekin, Effect of Ultrasound Energy on the Photocatalytic Activity of TiO2, J. Chem. Soc. Pakistan, 34, 1115 (2012).

8. D. Li, B. Fang, K. Zhang and C. Hu, Preparation and Characterization of Carbon and Nitrogen Co-doped TiO2 with Enhanced Visible Light Activity, J. Chem. Soc. Pakistan, 34, 333 (2012).

9. H. Lachheb, F. Dappozze, A. Houas and C. Guillard, Adsorption and Photocatalytic Degradation of Cysteine in Presence of TiO2, J. Photochem. Photobiol. A: Chem., 246, 1 (2012).

10. D. Chen and A. K. Ray, Photodegradation Kinetics of 4-nitrophenol in TiO2 Suspension, Water Res., 32, 3223 (1998).

11. T. Yetim and T. Tekin, Sonophotocatalytic Degradation Kinetics of an Azo Dye Amaranth, J. Chem. Soc. Pakistan, 34, 1397 (2012).

12. A. Mills and S. Le Hunte, An Overview of Semiconductor Photocatalysis, J. Photochem. Photobiol. A: Chem., 108, 1 (1997).
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Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Date:Jun 30, 2015
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