Abstract
In order to develop a simple, reliable and low cost enzymatic method for the determination of phenolic compounds we studied polyphenol oxidase activity of crude eggplant (S. melongena) extract using 13 phenolic compounds. Catechol, caffeic and chlorogenic acids, and l-DOPA have been rapidly oxidized with the formation of colored products. Monophenolic compounds have been oxidized at a much slower speed. Ferulic acid, quercetin, rutin, and dihydroquercetin have been found to inhibit polyphenol oxidase activity of crude eggplant extract. The influence of pH, temperature, crude eggplant extract amount, and 3-methyl-2-benzothiazolinone hydrazone (MBTH) concentration on the oxidation of catechol, caffeic acid, chlorogenic acid, and l-DOPA has been investigated spectrophotometrically. Michaelis constants values decrease by a factor of 2 to 3 in the presence of MBTH. Spectrophotometric (cuvette and microplate variants) and smartphone-assisted procedures for phenolic compounds determination have been proposed. Average saturation values (HSV color model) of the images of the microplate wells have been chosen as the analytical signal for smartphone-assisted procedure. LOD values for catechol, caffeic acid, chlorogenic acid, and l-DOPA equaled 5.1, 6.3, 5.8 and 30.0 µM (cuvette procedure), 12.2, 13.2, 13.2 and 80.4 µM (microplate procedure), and 23.5, 26.4, 20.8 and 120.6 µM (smartphone procedure). All the variants have been successfully applied for fast (4-5 min) and simple TPC determination in plant derived products and l-DOPA determination in model biological fluids. The values found with smartphone procedure are in good agreement with both spectrophotometric procedures values and reference values. Using crude eggplant extract- mediated reactions combined with smartphone camera detection has allowed creating low-cost, reliable and environmentally friendly analytical method for the determination of phenolic compounds.
Highlights
Phenolic compounds are a group of molecules that play an important role in human health due to their high biological activity
The substrate specificity of purified eggplant Polyphenol oxidase (PPO) showed that it belonged to the catechol oxidase family (EC 1.10.3.1) as the maximum activity was observed with catechol or its derivative substrates [18]
We aimed to study eggplant extract (EE) activity towards broad spectrum of different poly- and monophenolic compounds oxidation to determine the possible analytes and propose analytical procedures
Summary
Phenolic compounds are a group of molecules that play an important role in human health due to their high biological activity Some endogenous molecules, such as catecholamine neurotransmitters, and many compounds from consumed foods with antioxidant activity are phenolic compounds [1,2]. Different phenolic compounds can be chosen as a reference: caffeic [1,2], gallic [4,5,6,7], and chlorogenic acids [8], catechol [9], guaiacol [10], etc. Due to their role in human health
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