CC BY
10EDN: RSMVUZ
УДК 543.552.054.1: 543.64
Electrochemical Behavior of Tryptophan
and 5-hydroxytryptophan on an Electrode Modified
with Multi-Walled Carbon Nanotubes and Polyfolic Acid
Aleksey M. Altyev, Vladimir V. Shelkovnikov* and Mariya S. Fryanova
Tomsk State University Tomsk, Russian Federation
Received 26.04.2022, received in revised form 16.12.2022, accepted 24.01.2023
Abstract. A possible mechanism for the concentration and oxidation of tryptophan and 5-hydroxytryptophan on an electrode modified with carbon nanotubes and polyfolic acid has been proposed. The influence of the pH of the solution, the accumulation potential, and the potential sweep speed on the analytical signals of amino acids has been investigated. When the pH increases, the potentials of the anode peaks shift to the region of negative values; the I - pH dependence passes through the maximum with the pH of 6.8. Based on the processing of the dependences of the peak current and the peak potential on the potential sweep speed, it has been found that tryptophan and 5-hydroxytryptophan oxidize with the participation of two electrons and two protons. The transfer process of the first electron limits the electrochemical reaction. The process is controlled by diffusion and adsorption. The diffusion coefficients are equal to 9.710-6 cm2/s and 7.4 10-6 cm2/s for tryptophan and 5-hydroxytryptophan, the maximum adsorption value for tryptophan has been 1.6310-10 mol/cm2 and for 5-hydroxytryptophan it has been 6.41 10-1 mol/cm2. The optimal parameters of concentration and oxidation of tryptophan and 5-hydroxytryptophan in the compresence have been established: the pH of 6.8; the accumulation potential of 0.1 V, the electrolysis time of up to 120 s, the optimal scanning velocity of 120 mV/s. The limit of quantification has been 5 10-8 M.
Keywords: stripping voltammetry, tryptophan, 5-hydroxytryptophan, polyfolic acid, modified electrode.
Citation: Altyev, A.M., Shelkovnikov, V.V., Fryanova, M. S. Electrochemical behavior of tryptophan and 5-hydroxytryptophan on an electrode modified with multi-walled carbon nanotubes and polyfolic acid. J. Sib. Fed. Univ. Chem., 2023, 16(1), 36-46. EDN: RSMVUZ
© Siberian Federal University. All rights reserved
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC Corresponding author E-mail address: [email protected]
BY-NC 4.0).
Электрохимическое поведение триптофана и 5-гидрокситриптофана на модифицированном многостенными углеродными нанотрубками и полифолиевой кислотой электроде
А. М. Алтыев, В. В. Шелковников, М. С. Фрянова
Томский государственный университет Российская Федерация, Томск
Аннотация. В работе предложен возможный механизм концентрирования и окисления триптофана и 5-гидрокситриптофана на электроде, модифицированном углеродными нанотрубками и полифолиевой кислотой. Исследовано влияние рН раствора, потенциала накопления, скорости развертки потенциала на аналитические сигналы аминокислот. При повышении рН потенциалы анодных пиков смещаются в область отрицательных значений, зависимость I - рН проходит через максимум при рН 6,8. На основании обработки зависимостей тока пика и потенциала пика от скорости развертки потенциала установлено, что окисление триптофана и 5-гидрокситриптофана происходит при участии двух электронов и двух протонов. Лимитирует электрохимическую реакцию процесс переноса первого электрона. Процесс контролируется диффузией и адсорбцией. Коэффициенты диффузии равны 9,710-6 см2/с и 7,410-6 см2/с для триптофана и 5-гидрокситриптофана, величина предельной адсорбции для триптофана составила 1,63 10-10 моль/см2, а для 5-гидрокситриптофана - 6,4110-11 моль/см2. Установлены оптимальные параметры концентрирования и окисления триптофана и 5-гидрокситриптофана при совместном присутствии: рН 6,8; потенциал накопления -0,1 В, время электролиза до 120 с, оптимальная скорость развертки 120 мв/с. Нижняя граница определяемых концентраций составила 5 10-8 М.
Ключевые слова: инверсионная вольтамперометрия, триптофан, 5-гидрокситриптофан, полифолиевая кислота, модифицированный электрод.
Цитирование: Алтыев, А.М., Шелковников, В.В., Фрянова, М. С. Электрохимическое поведение триптофана и 5-гидрокситриптофана на модифицированном многостенными углеродными нанотрубками и полифолиевой кислотой электроде. Журн. Сиб. федер. ун-та. Химия, 2023, 16(1). С. 36-46. EDN: RSMVUZ
Introduction
Tryptophan (ß- (ß-indolyl) -a-aminopropionic acid (Fig. 1a) represents an aromatic alpha-amino acid that exists in two optically isomeric forms, L and D, and in the form of racemate (racemic mixture) (DL) [1]. L-tryptophan is a proteinogenic amino acid that is included in the proteins of all known living organisms. It belongs to a number of hydrophobic amino acids as it contains an aromatic indole ring. L-tryptophan participates in hydrophobic and stacking interactions [2]. 5-hydroxytryptophan (Fig. 1b) is a chemical precursor and metabolic intermediate in the biosynthesis of the serotonin
Figure 1. Structural formulas of tryptophan (a) and 5-hydroxytryptophan(b)
neurotransmitter. Tryptophan is a precursor in the metabolism of 5-hydroxytryptophan and serotonin. A small proportion of tryptophan is used for synthesizing protein and producing neurotransmitters. The remaining part, more than 95 % of free tryptophan released during food digestion, is a substrate for the kynurenine cycle of tryptophan, which generates several metabolites with different biological activities [3-4].
Since amino acids enter the human body mainly through food, medicines and biologically active supplements (BAS), reliable methods of their control in these objects are needed.
There are various methods that allow assessing the content of tryptophan and 5-hydroxytryptophan in a wide range of concentrations: capillary electrophoresis [5], fluorescence spectroscopy [6], chromatographic methods [6-10], mass spectrometry [11], NMR [12]. However, the application of the mentioned methods requires long-term sample preparation, expensive equipment and reagents, thorough cleaning of the latter. In this connection, electrochemical methods attract much attention of experts because of their simplicity, high selectivity, sensitivity and low cost of equipment. Voltammetric methods provide massive opportunities for specifying amino acids [13]. In these electrochemical studies, carbon electrodes that are characterized by a low background current, a wide area of ideal polarizability are widely used. They are chemically inert, cheap and suitable for determining organic and biologically important compounds.
When using voltammetric methods on inert electrodes, the detection limit is high enough; therefore, the electrodes are modified to increase sensitivity. Nanoparticles, metal oxides [14-16], carbon nanotubes and fullerenes [17-20], cyclodextrins [21-22], graphene and graphene oxide [23-24], amino acids [25] are most commonly used for this purpose. In [26], a glassy-carbon electrode modified with AuNPs and carbon nanotubes was used. The linear range of determined concentrations is from 5.0 x 10-6 M to 100.0 x 10-6 M with a detection limit of 3.0 x 10-6 M. In [27], the authors presented the identification of tryptophan using an electrode, in the manufacture of which ionic liquid and multi-walled carbon nanotubes (MCNT) were used as the main components. The electrode surface was activated by potential cycling. The region of working concentrations is 5.0 x 10-6-1.0 x 10-3 M. A highly selective electrochemical sensor representing a hybrid structure of aluminum oxide, graphene and copper is presented in [28]. This electrode was prepared by electrodeposition of copper nanoparticles on aluminum oxide nanofibers encapsulated in graphene. The electrode was used for the simultaneous determination of adrenaline, acetaminophen and tryptophan with a low detection limit of 0.027; 0.012; 0.009 jM, respectively. One of the methods for determining tryptophan on a glassy-carbon electrode modified with argentum was described in [29]. Under optimal experimental conditions, the peak current linearly depends on the Trp
concentration in the range from 1.0 x 10-7 to 1.0 x 10-4 M with a detection limit of 4.0 x 10-8 M. All the methods presented above have their own advantages, but they are also not free from shortcomings associated with expensive materials and a long process of forming the active layer surface. An available determination method is the application of films of the vitamin B complex and amino acids [30]. In [31], a technique was proposed for the simultaneous determination of tryptophan and 5-hydroxytryptophan on a carbon-containing electrode modified with MCNT and a vitamin B 9 film (polyfolic acid). Proposing this technique for determining amino acids, we have proceeded from the fact that folic acid takes part in the kynurenine cycle as an enzyme. It is necessary to clarify the mechanism for optimizing the electrochemical process with a view of increasing the resolving power of this method and reducing the detection limit of tryptophan and 5-hydroxytryptophan.
Materials and methods
All electrochemical measurements were carried out on a TA-LAB voltammetric analyzer (NPO "Tomanalit") in a direct current mode in a three-electrode cell. An indicator electrode is modified carbon-containing, auxiliary and reference - silver chloride in 3 M KCl. Folic acid (vitamin B 9) (Sigma), tryptophan, 5-hydroxytryptophan (Sigma, USA) were used in the work. MCNT were purchased from the "Aldrich" company. All the reagents were prepared using deionized water obtained from "Sartorius" of the "arium®pro" brand. All the experiments were carried out at room temperature.
Preparation of the modified electrode
The MCNT suspension was prepared in the following way: the MNT sample was oxidized in a mixture with the 1:1 ratio of HNO3(conc) and H2O2(conc) until the complete evaporation of the solution. Then the oxidized nanotubes were washed with deionized water and dissolved in dimethylformamide. The suspension aliquot (5 mcl) was applied to the electrode surface and dried until the complete evaporation of the solvent. The folic acid film was applied by cyclic voltammetry in the range of potentials equal to - 1.4 V - 0.5 V.
Results and discussion
The process of concentrating tryptophan and 5-hydroxytryptophan is significantly influenced, in addition to the electrode, by the composition of the background electrolyte, pH, conditions of concentration on the electrode and the concentrate electrodissolution. The process of simultaneous oxidation of tryptophan and 5-hydroxytryptophan is shown in the Fig. 2.
pH influence
To study the electrochemical behavior of tryptophan and 5-hydroxytryptophan, voltamperograms were recorded in the Britton-Robinson solution in the pH ranges from 1.75 to 9.18. Fig. 3(a) shows the dependences of the anode currents of tryptophan and 5-hydroxytryptophan on pH. The highest sensitivity is observed at pH = 6.8. The state diagrams [31] have shown that tryptophan and 5-hydroxytryptophan are in the form of zwitter ions at such pH value. Since COO-groups are present in polyfolic acid at such pH value, amino acids concentrate due to the electrostatic interaction between COO- and NH3+. During the transition to a more alkaline region, these amino acids deprotonate, as a result of which the electrostatic interaction is impossible. In the acid medium, polyfolic acid is protonated, which slows down the electrostatic interaction between the film and amino acids.
Figure 2. Voltamperograms of tryptophan (TRP) and 5-hydroxytryptophan (5-HTP): 1) 0,5 ^M TRP h 0,2 ^M 5-HTP, 2) 0,7 ^M TRP h 0,5 ^M 5-HTP
When the pH of the buffer solution increases, the peak potentials shift to values that are more
negative. The linear dependences can be observed between the peak potentials and pH as shown in Fig. 3(b).
The slope values of 53.8 mV/pH for tryptophan and 50.6 mV/pH for 5-hydroxytryptophan are close to the Nernst slope (59.0 mV/pH), which indicates an equal number of electrons and protons involved in the electrochemical process.
The influence of the potential sweep speed on the currents and potentials of anode peaks
The ratio between the peak current/potential and the potential sweep speed provides constructive information about the mechanism of the electrochemical reaction. The electrochemical behavior of
tryptophan and 5-hydroxytryptophan was studied by the voltammetry method at various potential sweep speeds in the range from 25 to 280 mV/s.
An acceleration in the potential scanning velocity increased the currents of electrooxidation of tryptophan and 5-hydroxystryptophan. The dependences of the peak current on W12 were plotted to evaluate the oxidation mechanism (Fig. 4).
Linear dependences in these coordinates are typical of both reversible and irreversible processes. The peak potentials shift to more positive values when the potential sweep speed increases for both amino acids, which indirectly confirms the irreversibility of the oxidation process [32]. The proof of such irreversibility is also the linear dependence between the peak potential and lgW (Fig. 5c).
The linear dependence of the current on the potential sweep speed (Fig. 5a) indicates that the process is controlled by either diffusion or adsorption.
The linear dependence of the peak potential on the square root of the potential sweep speed is typical of the processes accompanied by diffusion control [33] (Fig. 5b). The diffusion coefficients were calculated using the Randles-Sevcik equation and equaled 9.710-6 cm2/s and 7.4 ■ 10-6 cm2/s for tryptophan and 5-hydroxytryptophan, respectively. Based on the slope angle of the dependence of lgl on lgW (Fig. 5d), the value of the Semerano criterion was 0.64 for tryptophan and 1.04 for 5-hydroxytryptophan. For the processes controlled purely by diffusion, this criterion should be 0.5. Since the Semerano criterion is greater than 0.5, it is possible to conclude that this process is also of adsorption nature.
To assess the influence of adsorption, the dependences of the peak current on the potential sweep speed were plotted (Fig. 5a). The linearity of these dependences indicates the contribution of adsorption to the electron-transfer process. The maximum adsorption was calculated according to the following equation [33]:
Too = 4.10sAn>, where b is a tangent of the slope angle I - W, A is an electroactive surface area of the electrode, n is the number of electrons involved in the process.
I, nikA 0,9
0.S
1 6 11 16 21
Wlfl
Fig. 4. Dependences of the electrooxidation current of tryptophan (1) and 5-hydroxytryptophan (2) on the square root of the sweep speed W1/2
1, inkA °-9
0,8 0,7 0.6 0.5 0.4 0.3 0.2 0,1 0
0
E.V 03
0,7 0,6 05
0,4
03 0,2 0,1
0 -
1,5
Figure 5. The influence of the potential sweep speed on the electrochemical behavior of 7.0 x 10-7 M tryptophan (1) and 5-hydroxytryptophan (2)
The maximum adsorption value for tryptophan was 1.63 ■ 10-10 mol/cm2, and for 5-hydroxytryptophan it was 6.41 10-11 mol/cm2. This fact may explain the higher sensitivity coefficient when determining 5 -hydroxytryptophan.
The proposed adsorption mechanism of concentration is in good agreement with the mechanism of electrooxidation of tryptophan on graphite electrodes, which was reported in [34-35].
To determine the number of electrons and the limiting stage of the electrode reaction, the transfer
coefficient for the irreversible process (oxidation of tryptophan and 5-hydroxytryptophan) was calculated
CCF
using the Tafel dependence according to the formula: b — —„ where b is a tangent of the slope angle of the dependence ln|Iox| - Ep, a is a transfer coefficient.
As a result of the calculations, the values of effective transfer coefficients equal to 1.47 and 1.52 for tryptophan and 5-hydroxytryptophan, respectively, were obtained.
For the irreversible process, the number of electrons was calculated according to the formula:
47 7
|En - En| = — . The values equal to 1.94 and 2.18 were obtained for tryptophan and 5-hydroxytryptophan; that is, they are close to 2.
The calculated values of the effective transfer coefficients that are close to 1.5 indicate that the limiting stage is the process of transferring the first electron.
The above-mentioned results confirm the mechanism of concentration and oxidation of tryptophan and 5 -hydroxytryptophan on electrodes modified with MCNT and vitamin B 9 that we proposed earlier [31].
Selection of optimal conditions for recording an analytical signal
To select the optimal conditions for recording an analytical signal, the influence of the potential and accumulation time was studied. The values of the analytical signals practically do not depend
on the electrolysis potential in the range from -1.0 to + 0.2 V, which proves the fact of adsorption concentration of amino acids on the electrode modified with polyfolic acid. Let us propose a potential of -0.1 V as an optimal one for simultaneous determination of amino acids. When accumulating more than 120 s, the analytical signals reach the limit, which may be conditioned by the complete filling of the electrode surface with adsorbed precipitate; therefore, 120 s was chosen as the optimal time.
The dependences of peak currents on the concentration are linear in the range of (0.5-50) * 10-7 and obey the equations I = 0.017C - 0.012 and I = 0.003C+0.001 for tryptophan and 5-hydroxytryptophan, respectively.
In the compresence of tryptophan and 5-hydroxytryptophan, analytical signals are not distorted even at a ratio of 1:250, 250:1. The influence of other amino acids on the results of the determination of tryptophan and 5-hydroxytryptophan was studied. The ultimate transfer concentration was determined as a maximum concentration of a substance-obstacle that causes an error in determining amino acids of no more than 5 % [36, 37]. Accordingly, at least a 10-fold excess of ascorbic acid, dopamine, histidine, glycine, methionine, arginine, cysteine and tyrosine does not have any noticeable distorting effect on the determination.
Some metrological characteristics of the procedure
for determining tryptophan and 5-hydroxytryptophan
According to ISR 61-2010, the indicators of repeatability, intermediate precision, and accuracy were calculated (Tables 1, 2).
Table 1. Metrological characteristics of the procedure for determining tryptophan (p = 0.95, n = 2, l = 15)
Concentration *10-7, mol/dm3 Repeatability indicator, ar*,% Intermediate precision indicator, aRl*,% Accuracy indicator (limits of relative error at P = 0.95), ±A,%
10 9.14 9.97 20.85
20 8.66 8.91 19.96
30 7.63 7.70 19.53
40 7.21 7.67 19.15
50 6.89 7.39 18.79
Table 2. Metrological characteristics of the procedure for determining 5-hydroxytryptophan (p = 0.95, n = 2, l = 15).
Concentration *10-7, mol/dm3 Repeatability indicator, ar*,% Intermediate precision indicator, aRl*,% Accuracy indicator (limits of relative error at P = 0.95), ±A,%
10 8.24 8.74 20.35
20 8.16 8.25 19.26
30 7.93 8.04 19.03
40 7.81 7.92 18.95
50 7.69 7.85 18.89
Based on the obtained metrological characteristics of the developed procedure for determining tryptophan and 5-hydroxytryptophan, it is possible to conclude that the accuracy figure does not exceed 21 % for tryptophan and 5-hydroxytryptophan, and the repeatability and intermediate precision indicators do not exceed 10 % for tryptophan and 9 % for 5-hydroxytryptophan.
Conclusions
The regularities of electrooxidation of tryptophan and 5-hydroxytryptophan on modified electrodes have been investigated. Amino acids oxidize with the participation of two electrons and two protons. The transfer process of the first electron limits the electrochemical reaction. The process is controlled by diffusion and adsorption. According to the Randles-Sevcik equation, the diffusion coefficients have been calculated: 9.710-6 cm2/s and 7.410-6 cm2/s for tryptophan and 5-hydroxytryptophan. The maximum adsorption value has been 1.6310-10 mol/cm2 for tryptophan and 6.4110-11 mol/cm2 for 5-hydroxytryptophan. The optimal conditions for the determination of amino acids in the compresence have been chosen: the accumulation potential is -0.1 V, the electrolysis time is up to 120 s, the potential sweep speed is 120 mV/s. Some metrological indicators have been established for the procedure of determining tryptophan and 5-hydroxytryptophan: accuracy indicators, repeatability and intermediate precision. The lower limit of the determined concentrations has been 5 10-8 M.
References
[1] Jakubke H. D. Amino acids, peptides, proteins. Moscow, 1985. 75. (In Russ.)
[2] Kaluzna-Czaplinska J, Gatarek P, Chirumbolo S, Chartrand MS, Bjorklund G. How important is tryptophan in human health? Crit. Rev. Food Sci and Nutr. 2019. 59(1). 72-88.
[3] Kapalka G. M. Chapter 4 - substances involved in neurotransmission. In: Kapalka GM, editor. Nutritional and herbal therapies for children and adolescents 2010. San Diego: Academic Press. 71-99.
[4] Arranz LI. Chapter 13 - effects of obesity on function and quality of life in chronic pain. In: Watson RR, Zibadi S, editors. Nutritional modulators of pain in the aging population 2017. Academic Press. 151-170.
[5] Altria K D, Harkin P., Hindson M. G. Quantitative determination of tryptophan enantiomers by capillary electrophoresis. J. Chromatogr. B. 686(1), 103-110.
[6] GOST 32195-2013. Feed, compound feed. Method for determining the content of amino acids (In Russ.)
[7] Iizuka H., Ishii K., Hirasa Y., Kubo K. Fukushima T. Fluorescence determination of D- and L-tryptophan concentrations in rat plasma following administration of tryptophan enantiomers using HPLC with pre-column derivatization. J. Chromatogr B. 2011. 879. 3208-3213.
[8] Zhen Q. N., Xu B. A., Ma L., Tian G, Tang X. F., Ding M. Simultaneous determination of tryptophan, kynurenine and 5-hydroxytryptamine by HPLC: application in uremic patients undergoing hemodialysis. Clin. Biochem. 2011. 44. 226-230.
[9] Zhang T., Holder E., Franco P., Lindner W. Method development and optimization on cinchona and chiral sulfonic acid-based zwitterionic stationary phases for enantiomer separations of free amino acids by high-performance liquid chromatography. J. Chromatogr. A 2013. 1363. 191-199.
[10] Taujenis L, Olsauskaite V, Padarauskas A. Enantio Selective determination of protein amino acids in fertilizers by liquid chromatography-tandem mass spectrometry on chiral teicoplanin stationary phase. J. Agric. Food Chem. 2014. 62. 11099.
[11] de Jong W. H. A. Plasma tryptophan, kynurenine and 3-hydroxykynurenine measurement using automated on-line solid-phase extraction HPLC-tandem mass spectrometry. J. Chromatogr. B 2009. 877. 603-609.
[12] Labuta J., Ishihara S., Sikorsky T., Futera Z., Shundo A., Hanykova L., Burda J. V., Ariga K., Hill J. P. NMR spectroscopic detection of chirality and enantiopurity in referenced systems without formation of diastereomers. Nat. Commun. 2013. 4. 2188.
[13] Mojtaba K., Abdollahi H., Bozorgzadeh S., Haghighi B. Second-order data obtained from differential pulse voltammetry: Determination of tryptophan at a gold nanoparticles decorated multiwalled carbon nanotube modified glassy carbon electrode. Electrochim. Acta 2011. 56(24). 8618-8624.
[14] Taleb M, Ivanov R., Bereznev S., Sayed H. K., Hussainova I. Alumina/graphene/Cu hybrids as highly selective sensor for simultaneous determination of epinephrine, acetaminophen and tryptophan in human urine. J. Electroanalyt. Chem. 2018. 823. 184-192.
[15] Mao S., Li W., Long Y., Tu Y., Deng A. Sensitive electrochemical sensor of tryptophan based on Ag@C core-shell nanocomposite modified glassy carbon electrode. Anal. Chim. Acta 2012. 738. 35-40.
[16] Neeraj K, Rosy R., Goyal N. Palladium nano particles decorated multi-walled carbon nanotubes modified sensor for the determination of 5-hydroxytryptophan in biological fluids. Sens. Actuators, B 2017. 239. 1060-1068.
[17] Rezaee E, Honarasa F. Determination of Tryptophan Using Activated Multi-Walled Carbon Nanotube Ionic Liquid Electrode. Russ. J. Electrochem. 2018. 54(10). 1073-1080.
[18] Tang X., Liu Y., Hou H., You T. Electrochemical determination of L-tryptophan, L-tyrosine and L-cysteine using electrospun carbon nanofibers modified electrode. Talanta 2010. 80, 2182-2186.
[19] Wu F.H., Zhao G. C., Wei X. W., Yang Z. S. Electrocatalysis of tryptophan on multiwalled carbon nanotube modified electrode. Microchim. Acta 2004. 144. 243-247.
[20] Yiyong W., Peihong D., Yaling T., Ziyu D., Guangli L., Jun L., Zavuga Z., Quanguo H. Rapid recognition and determination of tryptophan by carbon nanotubes and molecularly imprinted polymer-modified glassy carbon electrode. Bioelectrochemistry 2020. 131. 107393.
[21] Zilberg R., Maystrenko V., Yarkaeva Yu., Dubrovskii D. An Enantioselective Voltammetric Sensor System Based on Glassy Carbon Electrodes Modified by Polyarylenephthalide Composites with а-, в-, and y-Cyclodextrins for Recognizing D- and L-Tryptophans J. Anal. Chem 2019. 74(12). 941-952.
[22] Дубровский Д. И., Кабирова Л. Р., Хаблетдинова А. И., Зильберг Р. А., Майстренко В. Н. Вольтамперометрические сенсоры на основе композитов полиэлектролитного комплекса хитозана и а-, в-, у-циклодекстринов для определения и распознавания энантиомеров метионина. Вестник Башкирского университета. 2018. 23(3). 723. [Dubrovsky D. I., Kabirova L. R., Khabletdinova A. I., Zilberg R. A., Maistrenko V. N. Voltammetric sensors based on composites of polyelectrolyte complex of chitosan and а-, в-, y-cyclodextrins for determination recognition of methionine enantiomers. Bulletin of the Bashkir University. 2018. 23(3). 723-726. (In Russ.)]
[23] Somayeh N., Reza H., Hosseini S. O. A novel nanocomposite electrochemical sensor based on green synthesis of reduced graphene oxide/gold nanoparticles modified screen printed electrode for
determination of tryptophan using response surface methodology approach. Microchem. J. 2002. 154. 104634.
[24] Worapot P., Ibrar A., Piyapong A. Hydroxyapatite/Graphene oxide composite for electrochemical detection of L-Tryptophan. J. Taiwan Inst. Chem. Eng. 2019. 102. 415-423.
[25] Fang W., Wencheng G., Wang L., Zilin C. Selective recognition of d-tryptophan from d/l--tryptophan mixtures in the presence of Cu(II) by electropolymerized l-lysine film. Anal. Biochem. 2016. 492. 30-33.
[26] Mojtaba K., Abdollahi H., Bozorgzadeh S., Haghighi B. Second-order data obtained from differential pulse voltammetry: Determination of tryptophan at a gold nanoparticles decorated multiwalled carbon nanotube modified glassy carbon electrode. Electrochim. Acta 2011. 56(24). 8618-8624.
[27] Rezaee E., Honarasa F. Determination of Tryptophan by Using of Activated Multi-Walled Carbon Nanotube Ionic Liquid Electrode. Russ. J. Electrochem. 2018. 54(10), 1073-1080.
[28] Taleb M., Ivanov R., Bereznev S., Sayed H. K., Hussainova I. Alumina/graphene/Cu hybrids as highly selective sensor for simultaneous determination of epinephrine, acetaminophen and tryptophan in human urine. J. Electroanalyt. Chem. 2018. 823. 184-192.
[29] Mao S., Li W., Long Y., Tu Y., Deng A. Sensitive electrochemical sensor of tryptophan based on Ag@C core-shell nanocomposite modified glassy carbon electrode. Anal. Chim. Acta 2012. 738. 35-40.
[30] Shelkovnikov V., Altyev A., Vinogradov M. Determination of Methionine in Medicines by Stripping Voltammetry. J. Anal. Chem■2019. 74. 1239-1244.
[31] Altyev A.M. A voltammetric sensor for simultaneous determination of tryptophan and 5-hydroxytryptophan. JPCS 2020. 1611. 1-6.
[32] Hegde R.N., Kumara Swamy B. E., Shetti N. P. and Nandibewoor S. T. Electro-oxidation and determination of gabapentin on gold electrode. J. Electroanal. Chem. 2009. 635. 51.
[33] Nosova N.M., Zaitseva A. S., Arlyapov V. A. Application of the method of cyclic voltammetry to study the electrochemical behavior of electron transport mediators on a carbon-paste electrode. News of TulGU. Natural Sciences 2017. 3. 2-11 (In Russ.)
[34] Safavi A., Momeni S. Electrocatalytic oxidation of tryptophan on gold nanoparticle-modified carbon ionic liquid electrode. Electroanalysis 2010. 22. 2848.
[35] Nguyen, N.T., Wrona, M.Z., Dryhurst, G. Electrochemical oxidation of tryptophan. J. Electroanal. Chem. 1986. 199. 101-126.
[36] Keyvanfard M., Shakeri R., Karimi-Maleh H., Alizad K. Highly selective and sensitive voltammetric sensor based on modified multiwall carbon nanotube paste electrode for simultaneous determination of ascorbic acid, acetaminophen and tryptophan. Mater. Sci. Eng. 2013. 33. 811.
[37] Rajabzadeh N., Benvidi A., Mazloum-Ardakani M., Firouzabadi A. D., Vafazadeh, R. A Highly Sensitive Sensor Based on Reduced Graphene Oxide, Carbon Nanotube and a Co (II) Complex Modified Carbon Paste Electrode: Simultaneous Determination of Isoprenaline, Captopril, Tryptophan. Electroanal. 2015. 27. 2792.
