UDC:541.123.2

THE TEMPERATURE INFLUENCE ON THE IONIZATION CONSTANTS OF α- AND β-ALANINE IN AQUEOUS SOLUTIONS

Hakimov J.​ N.*, Rahimova M.M., Samadov A.S., Faizullozoda E.F.

Tajik National University

Introduction. Amino acids, the foundational components of peptides and proteins, serve as indispensable chemical entities within the organism, essential for the harmonious conduct of metabolic processes and overall well-being. The scarcity of amino acids due to inadequate dietary intake or suboptimal endogenous synthesis may manifest in deficiency symptoms, including pronounced weight loss and compromised immune system function. Given the transient nature of amino acid storage within the body, maintaining a healthy lifestyle necessitates fulfilling the daily amino acid requirements either through nutritional intake or supplementation [1]. The presence of amino (–NH₂) and carboxyl (–COOH) functional groups endows amino

acids with ampholytic characteristics, underscoring their pivotal role across a spectrum of biological contexts [2,3].

Amino acids α - and β -alanine are one of the biologically active molecules and have a zwitter-ionic structure. α -Alanine is one of the irreplaceable proteinogenic amino acids of the protein composition and plays an important role in metabolism and genetic processes [3-6]. β - Alanine is also a non-protein amino acid that plays the main role in the metabolism of carnosine [4]. The study of protolytic processes of amino acids α - and β -alanine is of fundamental importance and useful for explaining the biological activity of proteins depending on pH [5].

The pH-metric (potentiometric) method, which has been used by researchers for a long time and is preferred for research, is considered the traditional, most common and simple method for determining the protonation (ionization) constant [7-10].

The investigation into the acid-base equilibrium of amino acids lays a solid foundation for exploring complexation processes within solutions. The interaction between transition metal ions and α-amino acids leads to the formation of stable, five-membered chelate complexes. These complexes have been effectively utilized in the creation of selective complexes rooted in amino carboxylic acids [8, 11].

The acid-base properties of α- and β -alanine and their protonation constants were studied in works [4, 8, 9, 12-16]. In this literature, the information on the ionization constants of α -alanine at a temperature of 25 ^oC is as follows: рК₂ = 9.89 ± 0.17 (–NH₂ ) [8], рК₂ = 9.04 (–NH₂ ), рК₁ = 2.67 (–СООН ) [9], pK₁ = 9.912 ± 0.013, pK₂ = 2.365 ± 0.015; pK₁ = 9.727 ± 0.010, pK₂ = 2.374 ± 0.013 0.1mol/l NaCl [12], pK₁ = 2.347 ± 0.004 (–СООН) [14]. The ionization constants of β -alanine in the mentioned literature are as follows: рК₂ = 10.33 (–NH₂ ) [13], рК₁ = 3.49 (–СООН), рК₂ = 9.92 (–NH₂ ) [15], рК₁ = 3.55 , pK₂ = 10.29 [15] and pK₁ = 3.60 [16]. Also in the work [4] on the microscopic ionization constants of α- and β -alanine at temperatures of 5, 20 and 35 ^o C data are provided, but there is no data on the macroscopic constants. Therefore, the research of the literature showed that the process of acid-base balance and ionization constants of α- and β -alanine at different temperatures have not been fully studied, and there is no certain law when the temperature increases. At the same time, in this work, the ionization constants of α- and β -alanine at temperatures of 5, 15, 25, 35 and 45 °C using the pH-metric titration method at a constant ionic strength (0.1000 mol∙l^–1 KCl) were determined. thermodynamic variables - Gibbs free energy, entropy and enthalpy were also studied. In this work, we want to consider the macroscopic constants of protonation and thermodynamic behavior of important microscopic substances - α - and β-alanine, and make our contribution to the world literature.

EXPERIMENTAL PART

In this study, all materials were meticulously selected to ensure high purity and reliability of results. L-alanine (α-Ala) was procured from REAKHIM, Russia, with a mass fraction purity exceeding 98.5%, while β-alanine (β-Ala) was obtained from Reanal Finechemical Private Ltd., Budapest, Hungary, with a purity greater than 99%. These reagents, belonging to the analytical grade, were utilized without any additional purification process. The preparation of hydrochloric acid and potassium hydroxide solutions was carried out using standard solutions (from fixanals), ensuring accuracy in the impregnation process.

To maintain a constant ionic strength, essential for the integrity of our experiments, potassium nitrate (REAKHIM, Russia, with a mass fraction purity over 99%) was employed as the electrolytic background. This meticulous approach to maintaining ionic strength underscores the rigor with which experimental conditions were controlled.

The solutions of α- and β-alanine, alongside potassium nitrate, were precisely measured using an analytical balance, highlighting the precision in our experimental setup. For all solution preparations, deionized water was utilized to avoid any impurities that could affect the results. Furthermore, all solutions were contained in Class A glassware, ensuring that the highest standards of quality and accuracy were upheld throughout the research. This careful selection of materials and strict adherence to precision underpin the scientific rigor of our work, contributing to the reliability and validity of our findings. pH-metric titration was carried out with the help of a pH-meter "EXPERT-001" (FR-2023). To measure the environment of the solution, a combination glass electrode of the brand "ESK-10603/7" was used. The pH-meter with an electrode before the start of each experiment through standard solutions whose pH is 1.68 (KH₃(C₂O₄), 6.86 (KH₂РО₄ + Na₂HPO₄) and 9.18 (Na₂B₄O₇) were calibrated at 25 °C. The accuracy of the pH measurement of the instrument was equal to ±0.03. The temperature of the system was kept constant during the titration process using a thermostatic cell to an accuracy of ± 0.1 K.

During pH-metric titration, solutions of 0.0100 mol∙l^-1 HCl and 0.0100 mol∙l^-1 HCl+0.0100 mol∙l^-1 amino acids (α - and β -alanine) were taken and titrated with a solution of 0.2000 mol∙l ^-1 KOH have been In the first stage, only 0.0100 mol∙l^-1 HCl solution, then the mixture of 0.0100 mol∙l^-1 HCl+0.0100 mol∙l^-1 amino acid solution was titrated with alkaline solution. For each amino acid, we performed titration at least three times, the number of titration points for each of them included more than 50 values. In each titration series, the volume of the investigated solution was equal to 25-50 ml. All calculations of the ionization constants of the investigated amino acids and graphs were performed on a computer using MS Excel 2016 and OriginLab Corporation . Also, all mathematical formulas and images of amino acid molecules were created using MathType 7.4.8.0 and ChemSketch 2021 2.1 apps.

The structural formulas of the amino acids investigated in this work are presented in Scheme 1:

The ionization constants were studied by pH-metric titration of the corresponding amino acids α - and β -alanine. In this case, α- and β -alanine were first completely protonated with a certain amount of hydrochloric acid, and then potassium alkali solution was used as a titrant. Protonation constants of investigated amino acids were calculated by the titration equation of weak diprotic acids presented in [17, 18]. Also, to confirm the constants calculated using Berrum's method (equation 1), they were calculated and confirmed each other.

                                     (1)

Here: V _i – added volume of alkali, V ₀ – total volume of the investigated solution, V _e – equivalent volume of alkali, α _H ⁺ – activity of hydrogen ions, γ – coefficient of hydrogen activity, C _b – concentration of alkali, К _W – product of multiplication water ions (taken from the work of [19]) and  – Berrum derivation function.

By means of equation (2), Berrum's derivation function was calculated and the values of calculated constants were approximated empirically.

                                                                   (2)

Here: K ₁ and K ₂ - constants proton of α- and β -alanine .

We confirmed the correctness of the experimental works through the titration curve of a strong acid with a strong base (3), which is theoretically calculated.

                                                                 (3)

DISCUSSION OF RESULTS

Amino acids are amphoteric compounds, their molecule has both an acidic - carboxylic group (-COOH) and a basic - amine group (-NH₂). They exist in aqueous solution in the form of zwitter-ions, which are formed as a result of the transfer of a proton from a carboxyl group to an amine. In a highly acidic environment, the carboxyl group is not completely dissociated, while the amino group is protonated, and vice versa, in a highly basic environment, the amino group is present in the form of a free base, but the carboxyl group is present in the form of a carboxylate ion. A possible equilibrium diagram of α- and β -alanine in aqueous solution is given below:

It is convenient to determine the ionization constants of acids or bases by titrating them with a standardized strong acid or base. The pH is measured at various neutralization stages during the titration, but the ratio of the concentrations of the basic or acidic form is calculated from the compound to be titrated and the added titrant. The dissociation of these amino acids can be written in the general form as:

                                                                                (4)

                                                                        (5)

The dissociation constant of these processes can be calculated as follows:

                                                                                                   (6)

                                                                                                       (7)

Figure 1 is a curve pH-metric titration of α - alanine is presented.

 

Figure 1. Dependence of pH on the volume of potassium hydroxide (V_i , l KOH) for α -alanine at temperatures of 278.2 K ( 1 ), 298.2 K (2 ) and 318.2 K (3), ionic strength of 0.1 M KCl. Points – experimental data, lines – calculated data.

Scientific inquiry into the acid-base equilibrium of amino acids in aqueous solutions highlights a notable pH optimization range of 6-7. This equilibrium is subject to alteration through the introduction of requisite quantities of acids or bases, thereby modulating the system's pH either upwards or downwards. To accurately ascertain the constants of acidity and basicity, it becomes imperative to undertake dual titration sequences. Incorporating an equimolar quantity of a potent acid into an amino acid solution effectively diminishes the system's pH. Subsequent titration with a strong base enables the precise determination of ionization constants across a titration series. This methodological approach necessitates accounting for a stabilization period within the buffer zone of titration points. The alignment of calculated (represented by a line) and experimental values within weak acid-strong base titration curves (as depicted in Figure 1) serves as a testament to the method's precision. Furthermore, the empirical calculation of the Berrum derivative function, as evidenced in Figure 2, underscores the robustness of the experimental outcomes. Consequently, employing the titration curve equation for weak diprotic acids alongside Berrum's function facilitates the determination of α- and β-alanine ionization constants across varied temperatures, adhering to the outlined scientific protocol.

Figure 2. Berrum's derivative function for α -alanine at temperatures of 278.2 K ( 1 ), 298.2 K ( 2 ) and 318.2 K ( 3 ), ionic strength 0.1 M KСl. Points – experimental data, lines – calculated data.

The values of ionization constants and thermodynamic values for α - and β -alanine obtained at temperatures of 278.2 - 318.2 K are presented in table 1.

T, K	pK1	pK2	ΔG, kJ / mol (1 and 2)		ΔH0 , kJ /mol	ΔS0 , J /mol * K
L-alanine (α- Ala )
278.2	2.82 ± 0.05	9.99 ±0.06	15.02±1.20	53.21 ±0.43	(–СООН) 53.79 ± 1.13	(–СООН) 137.94 ± 1.50
288.2	2.60 ± 0.07	9.77 ±0.04	14.34±0.24	53.91 ±0.35
298.2	2.28 ± 0.03	9.58 ±0.06	13.02±0.50	54.69 ±0.75	(–NH2) 35.00 ± 0.70	(–NH2) –65.67 ± 1.18
308.2	1.95 ± 0.08	9.40 ±0.05	11.50±0.13	55.47 ±0.63
318.2	1.55 ± 0.05	9.14 ±0.03	9.44±0.26	55.68 ±0.65
β-alanine (β- Ala )
278.2	4.08 ± 0.12	10.41 ±0.03	21.73±1.06	55.45 ±0.32	(–СООН) 46.76 ± 1.02	(–СООН) 88.90 ± 1.20
288.2	3.87 ± 0.12	10.15 ±0.15	21.35±0.34	56.01 ±0.28
298.2	3.60 ± 0.06	9.98 ±0.06	20.55±0.60	56.98 ±0.45	(–NH2) 46.85 ± 0.90	(–NH2) –31.92 ± 0.98
308.2	3.30 ± 0.08	9.65 ±0.05	19.47±0.21	56.95 ±0.53
318.2	2.98 ± 0.05	9.27 ±0.03	18.16±0.27	56.48 ±0.45

As can be seen from Table 1, the increase in temperature leads to a decrease in pK values for both carboxyl groups and amino groups of α- and β -alanine, which indicates an increase in the degree of dissociation of molecules with an increase in temperature, according to Brønsted–Lowry theories of acidity and basicity agrees. It can be seen from the pK values of α -alanine that when the temperature increases from 278.2 K to 318.2 K, the pK of the carboxyl group of this amino acid decreases by 1.27 logarithmic units, the degree of its dissociation increases, and its acidity property increases 18.6 times. This value for the amino group of α -alanine is approximately equal to 7.1, that is, the basicity of α -alanine decreases by 7.1 times when the temperature changes from 318.2 K to 278.2 K. Similar differences can also be observed for β -alanine. For example, the acidity property increases by 12.6 times with increasing temperature, and the basic property decreases by 13.8 times.

Table 1 also shows that at a temperature of 298.2 K, the ionization constant of β-alanine is higher than that of α -alanine (2.28 (рК₁ ) and 9.58 (рК₂) when I = 0.1 mol/l KCl ). The transition of –NH ₂ from the α -state to the β -state in the alanine molecule increases the stability of the proton bond to the carboxyl group, which is related to the effect of the negative inductive effect of the amine group. Therefore, the property of acidity increases in the order: propionate acid (pK₁ = 4.89, [20]) < β -aminopropionate acid (pK₁ = 3.60) < α -aminopropionate acid (pK₁ = 2.28).  On the contrary, the basicity property of α –Ala ^– ( pK₂ = 9.58) compared to β –Ala ^– рК₂ = 10.18 weakens, i.e. the property of basicity β -Ala^± is more than α -Ala.

The pK_a values of the researched amino acids (table 1) were compared to the literature values [4, 8, 9, 12-16], presented in the introduction section to confirm the working methodology. The value of pK_a (298.2 K) given in the literature is slightly larger than the value of pK_a obtained. A slight difference in such constants can be attributed to the diversity of the work methodology, and in general, it can be said that the processing of the received results is very reliable.

It is known that it is impossible to measure the values of enthalpy and entropy by the potentiometric method at the same transition temperature of the experiment, but different experimental experiments at different temperatures allow to calculate thermodynamic quantities. Thus, we calculated the change of Gibbs free energy using equation (8), enthalpy and entropy based on the dependence of pK of the studied amino acids on the inverse temperature using the thermal coefficient method using the Van't-Hoff isobar formula (9).

                                                                                                    (8)

                                                                                  (9)

Here: ΔH⁰ is the enthalpy (kJ/mol), ΔS⁰ is the entropy (J/(mol ^. K)) and ΔG is the free energy (Gibbs energy) of the ionization processes of the researched amino acids. The dependence of pK_i on the inverse temperature is presented in Fig. 3. As from ras. 3 can be seen that this relationship is linear and shows a satisfactory correlation (R² ≥ 0.95).

Fig. 3. Dependence constant ionization of α - alanine (a) and β- alanine (b) ( рК_i ) from temperature on the contrary  (1/T) when T = 278.2 – 318.2 K , pK₁ (1) and pK₂ – (2).

Thermodynamic variables allow to explain ionization processes based on the laws of thermodynamics depending on the obtained values. As it follows from the values of enthalpy and Gibbs free energy, these values are unfavorable for ionization processes of amino and carboxyl functional groups of α- and β -alanine (table 1). A positive value of the enthalpy of all ionization processes is evidence that such processes are endothermic. The analysis of the obtained results with the values of glycine [21] shows that the addition of a non-polar lateral radical (CH₃-, for the carboxyl group of α -alanine) to the glycine molecule almost doubles the enthalpy of the process (∆(∆Н) = 24 kJ/mol) increases and indicates that the degree of ionization of α -alanine decreases compared to glycine. The opposite can be observed for the amino group of α -alanine, that is, in this case, the enthalpy of the NH₂ group of α -alanine is smaller than that of glycine ∆(∆Н) = 43 kJ/mol. The transition of the lateral radical CH₃ - from the α state to the β state slightly changes the equilibrium of the system and increases the basicity of β -alanine.

The magnitude of entropy describes the interaction of the molecule with the solvent and is highly dependent on the structure of the lateral radical of the amino acid molecule. Molecules with non-polar radicals have a weak interaction with water (ΔS > 0) . The entropy values for the carboxyl group of α- and β -alanine are positive and indicate such interaction. The entropy values are negative for the amino group of α- and β -alanine, which probably indicates the presence of lyophilic bonds formed in the alkaline regions of the system.

For all processes in the system, the values of enthalpy and entropy are unfavorable, and the value of Gibbs free energy shows the non-spontaneity of the ionization process at the investigated temperatures. However, the studies carried out in the temperature range of 278.2 - 318.2 K show that an increase in temperature for the carboxyl group of α- and β -alanine leads to a change in the Gibbs free energy values, which is favorable for the passage of ionization processes, and is unfavorable for their amino group. When the temperature changes from 278.2 to 318.2 K, the Gibbs free energy for the carboxyl group α- Ala ∆(ΔG) = 5.6 and for β – Ala ∆(ΔG) = 3.5 kJ/mol decreases. The opposite situation can be observed for the amino group: for α– Ala ∆(ΔG ) = 2.5 and for β – Ala ∆(ΔG ) = 1.1 kJ/mol increases.

REVIEWER: Mabatkadamzoda K.S.,

Doctor of Chemical and Mathematical Sciences

 

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THE TEMPERATURE INFLUENCE ON THE IONIZATION CONSTANTS OF α- AND β-ALANINE IN AQUEOUS SOLUTIONS

The acid-base properties of α- and β-alanine in aqueous solutions in the temperature range 278.2 - 318.2 K and I = 0.1 mol/l KCl were studied by the pH-metric method. The ionisation constants (pK_i) of α-alanine and β-alanine at different temperatures were calculated using the equation of titration of weak bidentate acids with strong base. Also, the Berrum formation function was used to estimate the values of ionisation constants. The experimental results at different temperatures showed that increasing temperature increases the acid property of α- and β-alanine from 18.6 to 12.6 times and decreases their basic property from 7.1 to 13.8 times. The acidity of α- and β-alanine is lower than that of glycine (propionic acid (pK₁ = 4.89) < β-aminopropionic acid (pK₁ = 3.60) < α-aminopropionic acid (pK₁ = 2.28)) and their basicity is higher. The transition of -NH₂ from the α-position to the β-position in the alanine molecule increases the stability of the proton bond in the carboxyl group, which is due to the influence of the negative inductive effect of the amino group. Thermodynamic parameters (ΔH⁰, ΔS⁰ and ΔG) of amino acid ionisation processes were calculated by the temperature coefficient method using the values of ionisation constants. The enthalpy and entropy values are unfavourable for all processes in the system, and the Gibbs free energy value shows the non-self-perpetuity of the ionisation process at the investigated temperatures.

Keywords: pH-meter, α-alanine, β-alanine, acid-base properties, ionisation constant, funksiyai Bjerrum, thermodynamic functions.

Information about the authors: Hakimov Jumakhon Nemonovich – Tajik National University, third-year Ph.D student at the Faculty of Chemistry. Address: 734025, Dushanbe, Tajikistan, Rudaki Avenue, 17. Phone: (+992)939-60-41-81. Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

                Rahimova Muboshirkhan - Tajik National University, doctor of chemical sciences, professor of the department of physical and colloidal chemistry. Address: 734025,   Dushanbe,Tajikistan, Rudaki Avenue, 17. Phone: (+992) 900-06-33-00. Email: muboshira09@ mail.ru

                Samadov Abdurasul Saidovich - Tajik National University, Candidate of Chemical Sciences, Senior Lecturer of the Department of Physical and Colloidal Chemistry. Address: 734025, Dushanbe, Tajikistan, Rudaki Avenue, 17. Phone: (+992) 111-50-38-38. Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

                Faizullozoda Erkin Fathullo - Tajik National University, candidate of chemical sciences, associate professor of the department of physical and colloidal chemistry. Address: 734025, Dushanbe, Tajikistan, Rudaki Avenue, 17. Phone: (+992) 935-56-96-69. Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

 

Article received 12.02.2024

Approved after review 20.05.2024