Interactions between Hydrolysable Tannins and Lipid Vesicles from Escherichia coli with Isothermal Titration Calorimetry

Abstract

Isothermal titration calorimetry (ITC) was used to study the interactions between hydrolysable tannins (HTs) and lipid vesicles prepared from a phospholipid extract of Escherichia coli (E. coli). A group of 24 structurally different HTs was selected, and structural differences affecting their affinities to interact with lipid vesicles in aqueous buffered media were identified. In general, the interactions between HTs and lipid vesicles were exothermic in nature, and ITC as a technique functioned well in the screening of HTs for their affinity for lipids. Most notably, the galloyl moiety, the structural flexibility of the entire tannin structure, the hydrophobicity of the tannin, and higher molecular weight were observed to be important for the stronger interactions with the lipids. The strongest interactions with lipids were observed for rugosins D and G. It was also observed that some HTs with moderate hydrophobicities, such as geraniin, chebulagic acid, and chebulinic acid, did not have any detectable interactions with the lipid vesicles, suggesting that a hydrophobic structure alone does not guarantee an affinity for lipids.

Insights into Polyphenol–Lipid Interactions: Chemical Methods, Molecular Aspects and Their Effects on Membrane Structures

“…ITC has been quite intensively applied to study the interactions of peptides or proteins with model and natural membrane phospholipids [ 56 , 57 , 58 , 59 , 60 , 61 , 62 ]. However, so far, there seems to be only one ITC study on polyphenol–lipid interactions focusing on the interactions between HTs and lipid vesicles prepared from the total phospholipid extract of Escherichia coli [ 29 ]. In this study, the interactions between tannins and lipids were found to be exothermic (heat released), and the strongest interaction was observed for rugosin G up to – 160 kJ/mol when 0.1 mM tannin solution was titrated into 2 mM lipid solution as shown in Figure 3 B [ 29 ].…”

“…In a typical ITC measurement, the lipid solution is placed in the sample cell of the calorimeter, and the polyphenol solution is titrated into the lipid incrementally [ 29 , 54 , 64 ]. The control measurements are important for excluding the other heats present.…”

“…Usually, the heat of the dilution of the polyphenol is significant. Tannins especially tend to self-associate into aggregates in concentrated aqueous solution and then undergo an endothermic process of deaggregation when injected from the syringe into buffer [ 29 , 66 , 67 , 68 , 69 , 70 , 71 ]. Therefore, these control data need to be subtracted from the sample data.…”

“…Therefore, these control data need to be subtracted from the sample data. The other control measurements, buffer into macromolecule and buffer into buffer, typically give negligible heats [ 29 , 60 , 71 ]. In general, ITC is easy to use, and reasonable heats of injections and sigmoidal thermograms enable fluent fitting and the determination of n , K a and Δ H .…”

“…The size of vesicles can be confirmed by dynamic light scattering (DLS) analysis [ 20 , 29 ] or by microscopic techniques such as transmission electron microscopy (TEM) or scanning electron microscopy (SEM) [ 30 , 31 ]. Model lipids include various sizes and different degrees of saturation in lipid tails, different charges on lipid heads (although in biological membranes, most of the lipids are zwitterionic and exist without an overall charge [ 32 ]) and mixtures of different lipids.…”