Illustrating Interfacial Interaction between Honey Bee Venom Phospholipase A2 and Supported Negatively Charged Lipids with Sum Frequency Generation and Laser Scanning Confocal Microscopy

Triglyceride-Tethered Membrane Lipase Sensor

Sensors that can quickly measure the lipase activity from biological samples are useful in enzyme production and medical diagnostics. However, current lipase sensors have limitations such as requiring fluorescent labels, pH control of buffer vehicles, or lengthy assay preparation. We introduce a sparsely tethered triglyceride substrate anchored off of a gold electrode for the impedance sensing of real-time lipase activity. The tethered substrate is self-assembled using a rapid solvent exchange technique and can form an anchored bilayer 1 nm off the gold electrode. This allows for an aqueous reservoir region, providing access to ions transported through membrane defects caused by triglyceride enzymatic hydrolysis. Electrical impedance spectroscopy techniques can readily detect the decrease in resistance caused by enzymatically induced defects. This rapid and reliable lipase detection method can have potential applications in disease studies, monitoring of lipase production, and as point-of-care diagnostic devices.

Exploring Interfacial Hydrolysis of Artificial Neutral Lipid Monolayer and Bilayer Catalyzed by Phospholipase C

Phospholipase C (PLC) represents an important type of enzymes with the feature of hydrolyzing phospholipids at the position of the glycerophosphate bond, among which PLC extracted from Bacillus cereus (BC-PLC) has been extensively studied owing to its similarity to hitherto poorly characterized mammalian analogues. This study focuses on investigating the interfacial hydrolysis mechanism of phosphatidylcholine (PC) monolayer and bilayer membranes catalyzed by BC-PLC using sum frequency generation vibrational spectroscopy (SFG-VS) and laser scanning confocal microscopy (LSCM). We found that, upon interfacial hydrolysis, BC-PLC was adsorbed onto the lipid interface and catalyzed the lipolysis with no net orientation, as evidenced by the silent amide I band, indicating that ordered PLC alignment was not a prerequisite for the enzyme activity, which is very different from what we have reported for phospholipase A₁ (PLA₁) and phospholipase A₂ (PLA₂) [Kai, S. Phys. Chem. Chem. Phys. 2018, 20(1), 63-67; Wang, F. Langmuir 2019, 35(39), 12831-12838; Zhang, F. Langmuir 2020, 36(11), 2946-2953]. For the PC monolayer, one of the two hydrolysates, phosphocholine, desorbed from the interface into the aqueous phase, while the other one, diacylglycerol (DG), stayed well packed with high order at the interface. For the PC bilayer, phosphocholine dispersed into the aqueous phase too, similar to the monolayer case; however, DG, presumably formed clusters with the unreacted lipid substrates and desorbed from the interface. With respect to both the monolayer and bilayer cases, mechanistic schematics were presented to illustrate the different interfacial hydrolysis processes. Therefore, this model experimental study in vitro provides significant molecular-level insights and contributes necessary knowledge to reveal the lipolysis kinetics with respect to PLC and lipid membranes with monolayer and bilayer structures.

Probing Covalent Interactions at a Silicone Adhesive/Nylon Interface

Covalent bonding is one of the most robust forms of intramolecular interaction between adhesives and substrates. In contrast to most noncovalent interactions, covalent bonds can significantly enhance both the interfacial strength and durability. To utilize the advantages of covalent bonding, specific chemical reactions are designed to occur at interfaces. However, interfacial reactions are difficult to probe in situ, particularly at the buried interfaces found in well-bonded adhesive joints. In this work, sum frequency generational (SFG) vibrational spectroscopy was used to directly examine and analyze the interfacial chemical reactions and related molecular changes at buried nylon/silicone elastomer interfaces. For self-priming elastomeric silicone adhesives, silane coupling agents have been extensively used as adhesion promoters. Here with SFG, the interfacial chemical reactions between nylon and two alkoxysilane adhesion promoters with varied functionalities (maleic anhydride (MAH) and epoxy) formulated into the silicone were observed and investigated. Evidence of reactions between the organofunctional group of each silane and reactive groups on the polyamide was found at the buried interface between the cured silicone elastomer and nylon. The adhesion strength at the nylon/cured silicone interfaces was substantially enhanced with both silane additives. SFG results elucidated the mechanisms of organo-silane adhesion promotion for silicone at the molecular level. The ability to probe and analyze detailed interfacial reactions at buried nylon/silicone interfaces demonstrated that SFG is a powerful analytical technique to aid the design and optimization of materials with desired interfacial properties.

Label-Free, Real-Time Phospholipase-A Isoform Assay

Phospholipase-A (PLA) enzymes catalyze the hydrolysis of ester bonds in select glycerophospholipids. Sensors for rapidly measuring the PLA activity in biological samples have relevance in the study of venom compositions and in medical diagnostics for the diagnosis of diseases such as acute pancreatitis. Current PLA sensor technologies are often restricted by the time it takes to prepare an assay, the necessity of using fluorescent labels, or the fact they might require strict pH control of the buffer vehicles used. Here we present a tethered bilayer lipid membrane (tBLM) impedance sensor array for the rapid and real-time detection of PLA, which includes the ability to selectively detect phospholipase-A₂ (PLA₂) from phospholipase-A₁ (PLA₁) isoforms. Comparing the activity of PLA₁ and PLA₂ in an array of tBLMs composed of ether phospholipids, ester phospholipids or ether-ester phospholipids allows for the rapid and reliable distinction between the isoforms, as measured using swept-frequency electrical impedance spectroscopy. After testing the assay using pure enzymes, we demonstrate the capacity of the sensor to identify specific PLA₂-type, calcium-dependent activity from the venom of the South American bullet ant, Paraponera clavata, at a concentration of 1 μg/mL. The specificity of the phospholipase activity was corroborated using matrix-assisted laser-desorption/ionization time-of-flight mass spectrometry. As further validation, we tested the activities of a PLA₁ isoform in the presence of different buffers commonly used in biology and biochemistry experiments. Sensitivity testing shows that PLA₁ can be detected at an activity as low as 0.06 U/mL. The rapid and reliable detection of phospholipases presented in this study has potential applications in the study of animal venoms as well as in lipase bioreactors and point-of-care devices.