Abstract
Label-free detection methods such as the quartz crystal microbalance (QCM) are well suited to the analysis of molecular interactions in complex mixtures such as crude botanical extracts. In the present study, the binding characteristics of epigallocatechin gallate (EGCG) and crude green tea extract solutions to bovine serum albumin (BSA) have been investigated. The adsorbed mass levels onto BSA-functionalized surfaces were measured at various solution concentrations. Langmuir and Freundlich isotherms were used to model the adsorption data. The Langmuir isotherm better described the adsorption behavior with correlations of 0.68 and 0.70 for the EGCG and the crude extract solutions, respectively. The better fit of the Langmuir model indicates that adsorption occurs homogeneously and that aggregation is negligible. The mass saturation is estimated to be 58% higher for the crude green tea solution as compared to the pure EGCG solution (7.9 ng/cm2 for green tea and 5 ng/cm2 for EGCG). The increased adsorption for the crude extract indicates that the additional tea chemical constituents are binding to alternate sites on the protein molecule and that competitive binding is a nondominant effect. However, a reduced adsorption rate for the crude extract was also observed, indicating some presence of competitive mechanisms. The results demonstrate the utility of the QCM for the analysis of protein binding in crude mixtures as well as pure compounds.
Highlights
Natural products are an important source of therapeutic agents with more than 75% of the modern drugs used to treat infectious diseases and 60% of the drugs used to treat cancer comprised of either natural products or synthetic molecules inspired by the pharmacology of natural products [1], such as Etoposide, Teniposide, Paclitaxel (Taxol), morphine, digitoxin, quinine, and atropine [2,3,4,5]
Tea polyphenols are mainly composed of catechins, flavonoids, and phenolic acid. e catechins comprise more than 80% of the content, and the biological activity of tea is primarily attributed to these catechins [7]. e major catechins of green tea are epicatechin (EC), (− )-epigalocatechin (EGC), (− )-epicatechin gallate (ECg), and epigallocatechin gallate (EGCG)
Prior to each green tea or EGCG experiment, Bovine serum albumin (BSA) was immobilized onto the gold electrode of the quartz crystal microbalance (QCM) crystal to create a saturated monolayer. e average frequency change between the baseline and BSA saturation was 21 Hz, corresponding to 375 ng/cm2, which is consistent with previous studies [24]
Summary
Natural products are an important source of therapeutic agents with more than 75% of the modern drugs used to treat infectious diseases and 60% of the drugs used to treat cancer comprised of either natural products or synthetic molecules inspired by the pharmacology of natural products [1], such as Etoposide, Teniposide, Paclitaxel (Taxol), morphine, digitoxin, quinine, and atropine [2,3,4,5]. E major catechins of green tea are epicatechin (EC), (− )-epigalocatechin (EGC), (− )-epicatechin gallate (ECg), and epigallocatechin gallate (EGCG). A QCM is used to study the protein binding characteristics of a solution of pure EGCG and a crude green tea extract and compare the adsorption behavior and magnitude of both solutions to the same protein (i.e., BSA). E quartz crystal microbalance (QCM) is an instrument that enables label-free detection of molecular interactions through the measurement of changes in the resonant frequency of a piezoelectric crystal due to adsorbed mass Label-free techniques are advantageous in the development of arrays for characterizing multiple molecular interactions [22]. e quartz crystal microbalance (QCM) is an instrument that enables label-free detection of molecular interactions through the measurement of changes in the resonant frequency of a piezoelectric crystal due to adsorbed mass
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