Spectrophotometric Determination of Protonation Constants of L-Dopa in Dimethylformamide-Water Mixtu

GRD Journals- Global Research and Development Journal for Engineering | Volume 6 | Issue 3 | February 2021 ISSN- 2455-5703

Spectrophotometric Determination of Protonation Constants of L-Dopa in Dimethylformamide-Water Mixtures S. Raju Department of Chemistry Govt. Degree College, Chodavaram, Visakhapatnam, India

G. Nageswara Rao Department of Chemistry Andhra University, Visakhapatnam-530003, India

Abstract Solute-solvent interactions of L-Dopa have been studied in 0–60 % v/v DMF–water media using Spectrophotometric method.

The optical density of some solutions has been measured by UV-VIS Spectrophotometer, Model 108, (Systronics). The spectral range of the instrument is from 200 nm. - 800 nm. i.e., UV, Visible. Distributions of species, protonation equilibria and effect of influential parameters on the protonation constants have also been presented. The aim of the present study is to determine the protonation- deprotonation equilibria of L-Dopa in low dielectric media.

Keywords- L-Dopa, Spectrophotometry, Step-Wise Protonation Constants, DMF

I. INTRODUCTION L-Dopa is an important neurotransmitter that is found in the brain and as a hormone in the circulatory system. Besides its natural and essential biological role L-Dopa is a popular drug in the treatment of Manganese poisoning and Parkinson’s disease [12].Once L-Dopa enters the central nervous system (CNS) it is converted into dopamine by the enzyme aromatic L-amino acid decarboxylase, also known as dopa decarboxylase.Potentiometric titrations of L-Dopa with Al3+, Cr3+, Fe3+, Cu2+, and Zn2+ are studied and compared with UV-Vis-spectroscopy [3]. L-Dopa possesses four protonation constants (H4L). Out of these four protons, two of these will be phenolate (catecholate) protons. The first proton to coordinate (a phenolate proton) has a very high affinity for the LH3- ion. The next two protons to coordinate bond to the other phenolate oxygen and the amine nitrogen.

L-Dopa is also a popular drug in the treatment of dopamine-responsive dystonia and to increase dopamine concentration, since it is capable of crossing the blood brain barrier, where Dopamine itself cannot. L-Dopa, when oxidized, can form bonds with sulfur containing compounds (such as cysteine) to polymerize with other amino acids and lower bioavailability of protein when L-Dopa is consumed via foods [4]. L-dopa (3, 4-dihydroxy-L-phenylalanine) is a drug related compound, found in certain kinds of food and herbs and is made from L-tyrosine [5], which is an amino acid naturally occurring in the human body.

N, N-Dimethylformamide (DMF) was first prepared in 1893 by the French chemist Albert Verley. It is a clear, transparent, high-boiling point liquid with a light amine flavor and a relative density of 0.9445 (25°C). It is soluble in water and most organic solvents [6] that used as a common solvent for chemical reactions. In Petroleum Industry DMF can be used as a gas absorbent for separating and refining gases. In Pesticide and Pharmaceutical industries DMF finds application as an intermediate of organic synthesis. It is also used as a catalyst in carboxylation reactions, in organic synthesis, as a quench and cleaner combination for hot-dipped tin parts (e.g., for high-voltage capacitors), as an industrial paint stripper and in inks and dyes in printing and fiber-dyeing applications [7-8].

II. MATERIALS AND METHODS 0.05 mol L-1 solution of L-Dopa (Himedia, India) was prepared in deionised triple-distilled water by maintaining 0.05 mol L-1 concentration of hydrochloric acid to increase the solubility. Dimethylformamide (Merck, India) were used as Solvent. 2 mol L-1 Sodium Chloride (Merck, India) was prepared to maintain the ionic strength. While the concentration of hydrochloric acid was determined using standardized sodium hydroxide and the primary standard borax solutions. All the weighings were carried out using Shimadzu TX223L analytical balance, while spectrophotometric measurements were obtained on UV-Visible Spectrophotometer. The absorbance of each of the solution was taken at the wavelength of maximum absorbance of the complex which was initially determined by varying the wavelengths from 200–400–800 nm. The procedure was repeated for each of the mole fractions of the complex at various pH ranges and the respective absorbances were recorded at the point of mixing.

A. UV–Vis Measurements The UV-visible spectra were taken using a Shimadzu SP65 UV Visible spectrophotometer in 200 - 800 nm range using a 1.0 cm quartz cell path length at a controlled temperature of 25±0.1 ◦C with a Cole–Parmer bath.

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Spectrophotometric Determination of Protonation Constants of L-Dopa in Dimethylformamide-Water Mixtures (GRDJE/ Volume 6 / Issue 3 / 002)

III. PROTONATION EQUILIBRIA The stepwise protonation constants and number of equilibria can be determined from the secondary formation functions such as average number of protons bound per mole of ligand (𝑛H). The pH values at half integral of 𝑛H correspond to the protonation constants of the ligand and the number of half integrals in the pH range of the study corresponds to the number of equilibria.

Thus, three half integrals (0.5, 1.5, and 2.5) versus pH in the case of L-Dopa confirm the presence of three protonation deprotonation equilibria [9]. The typical distribution plots Figure. 2 [(a), (b), (c)] produced using protonation constants.

H3L H2L HL

H2L+H -----------------1 HL+H -----------------2 L+H ------------------3

IV. EFFECT OF SOLVENT The reaction medium is one of the most important influencing factors in determining the equilibrium constants. Many workers were of the opinion that both electrostatic and non-electrostatic effects should be considered even in the case of simple acidobasic equilibria, one dominates the other, depending upon the nature of solute and solvent [10-12]. The solvent effect on protonation constants could be explained on the basis of dielectric constant of the medium, solvent structure, preferential salvation and microscopic parameters. The variation of protonation constants or change in free energy with the organic solvent content depends upon two factors; electrostatic one, which can be estimated by Born's equation and non-electrostatic one, which includes specific solute solvent interactions. When the electrostatic effects dominate the equilibrium proceeds, according to above equation [13], the energy of electrostatic interaction is related inversely to dielectric constant [14]. Hence, the logarithm of stepwise protonation constants (log K) should vary linearly as a function of the reciprocal of the dielectric constant of the medium. It is observed that in both media the log K values of L-Dopa increase linearly with increase of organic content in pure water as a solvent [15-18] but small differences are possibly due to the different experimental procedures, temperature and different background electrolytes used. This linear increase can be attributed to ion-association reaction, solute-solvent interactions, proton-solvent interactions and solvent basicity (acidity) effects [19]. The presence of co-solvent in the medium influences the protonation - deprotonation equilibria in solution due to change in the dielectric constant of the medium.

The logarithm of step-wise protonation constants (log K) can be calculated by log K1 = log β3 – log β2 log K2 = log β2 – log β1 log K3 = log β1 Where logβs are overall protonation constants.

V. RESULT AND DISCUSSION A. Method of Protonation Constant Calculation All calculations were done by our computation program based on the following assumption. In each point of titration curve of weak base by strong acid the following equation must be satisfied

Where 𝑐 is the overall base concentration, 𝐾𝑖 is the successive protonation constants, 𝑎 is the titration fraction, 𝐾𝑠 is the ionic product of solvent, [H+] is the equilibrium concentration of hydrogen ions, and 𝑛 is the number of protons which can be attached to the base molecule.

The results of species and Step-wise protonation constants of L-Dopa in DMF-water mixtures along with some important statistical parameters are given in Table-1.

Table 1: Step-wise protonation constants of dopa %v/v LogK DMF LH3 LH2 LH 0.0 20.64 18.43 9.59 10 21.11 18.82 9.98 30 22.35 18.93 10.21 50 23.76 19.72 10.54

Typical spectrophotometric parameters are given in Table 2. The absorption spectra of ligand in 0–50% v/v DMF - water media is given in Fig.1.

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Spectrophotometric Determination of Protonation Constants of L-Dopa in Dimethylformamide-Water Mixtures (GRDJE/ Volume 6 / Issue 3 / 002)

Table 2: Parameters L-dopa in 0–50% v/v DMF – water L-Dopa v/v DMF 0 10 30 50 Λmax (nm) 245 245 245 245 Abs 0.613 0.658 0.698 0.761

Fig. 1: Absorption spectra of ligand in 0–50% v/v DMF – water

(a)

(b)

Fig. 2: Distribution diagrams of protonated and deprotonated L-Dopa species in (a) 10.0% v/v, (b) 30.0% v/v, (c) 50.0% v/v DMF-water mixtures respectively

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Spectrophotometric Determination of Protonation Constants of L-Dopa in Dimethylformamide-Water Mixtures (GRDJE/ Volume 6 / Issue 3 / 002)

VI. CONCLUSIONS 1) L-Dopa has three dissociable protons and one amino group which can associate with a proton. It exists as LH 4+ at low pH and gets deprotonated with the formation of LH3, LH2- and LH2- successively with increase in pH. 2) The log values of protonation constants of L-Dopa increase linearly with decreasing dielectric constant of DMF-water mixtures. This indicates the dominance of electrostatic forces in the protonation-deprotonation equilibria.

3) Secondary formation functions confirm the existence of three protonation equilibria for L-Dopa.

4) The effect of systematic errors in the influential parameters on the protonation constants shows that the errors in the concentrations of alkali and mineral acid affect the protonation constants more than those in the concentration of ligand solutions.

ACKNOWLEDGEMENT The authors thank the University Grants Commission, Government of India, New Delhi, for financial support under Minor Research Project.

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