Surfactant Charge Modulates Structure and Stability of Lipase-Embedded Monolayers at Marine-Relevant Aerosol Surfaces

Abstract

Lipases, as well as other enzymes, are present and active within the sea surface microlayer (SSML). Upon bubble bursting, lipases partition into sea spray aerosol (SSA) along with surface-active molecules such as lipids. Lipases are likely to be embedded in the lipid monolayer at the SSA surface and thus have the potential to influence SSA interfacial structure and chemistry. Elucidating the structure of the lipid monolayer at SSA interfaces and how this structure is altered upon interaction with a protein system like lipase is of interest, given the importance of how aerosols interact with sunlight, influence cloud formation, and provide surfaces for chemical reactions. Herein, we report an integrated experimental and computational study of Burkholderia cepacia lipase (BCL) embedded in a lipid monolayer and highlight the important role of electrostatic, rather than hydrophobic, interactions as a driver for monolayer stability. Specifically, we combine Langmuir film experiments and molecular dynamics (MD) simulations to examine the detailed interactions between the zwitterionic dipalmitoylphosphatidylcholine (DPPC) monolayer and BCL. Upon insertion of BCL from the underlying subphase into the lipid monolayer, it is shown that BCL permeates and largely disorders the monolayer while strongly interacting with zwitterionic DPPC molecules, as experimentally observed by Langmuir adsorption curves and infrared reflectance absorbance spectroscopy. Explicitly solvated, all-atom MD is then used to provide insights into inter- and intramolecular interactions that drive these observations, with specific attention to the formation of salt bridges or ionic-bonding interactions. We show that after insertion into the DPPC monolayer, lipase is maintained at high surface pressures and in large BCL concentrations by forming a salt-bridge-stabilized lipase–DPPC complex. In comparison, when embedded in an anionic monolayer at low surface pressures, BCL preferentially forms intramolecular salt bridges, reducing its total favorable interactions with the surfactant and partitioning out of the monolayer shortly after injection. Overall, this study shows that the structure and dynamics of lipase-embedded SSA surfaces vary based on surface charge and pressure and that these variations have the potential to differentially modulate the properties of marine aerosols.

Role of Sea Surface Microlayer Properties in Cloud Formation

“…Since HMW organic species tends to be surface active, their presence even in relatively small quantities may reduce the equilibrium vapor pressure over drops promoting the activation and growth of cloud droplets (Lohmann et al, 2016;Kubicki et al, 2019). It has been shown that after marine primary aerosol are formed they can become coated with organic surfactants, such as longchain hydrocarbons (Djikaev and Ruckenstein, 2014;Luo et al, 2019). In this study, SML samples were collected in the North Atlantic Ocean to quantify the relative importance of the salt and organic constituents of aerosols emitted from the surface waters.…”

Insights into the behavior of nonanoic acid and its conjugate base at the air/water interface through a combined experimental and theoretical approach

“…The Infrared Reflection Absorption Spectroscopy (IRRAS) setup has been described previously. 38 , 39 The IR beam from an infrared spectrometer (Bruker Tensor 37) is directed onto the aqueous solution surface in the Petri dish at an angle of 30° from the surface. We chose 30° because that is the optimal angle for the incident beam at an air–water interface for the unpolarized beam used in our setup.…”

The Ocean’s Elevator: Evolution of the Air–Seawater Interface during a Small-Scale Algal Bloom

“…3,27 By studying the individual role of each primary subset of algal exudates as demonstrated in the studies above, the fundamental interactions at the ocean's interface and consequent aerosol surfaces can be determined and include interfacial adsorption, 22,27 binding interactions, 28,29 and reaction mechanisms. 30 Biogenic surfactants have also been explored in more complex systems and have been shown to alter surface features, 31 enhance interactions, 24,25,32,33 and influence the overall film properties. 34 Ex situ analysis of the interfacial regime of native marine systems has been reported, with some studies extending to surface tension measurements, 31 Brewster angle microscopy (BAM), 35,36 and sum-frequency generation (SFG) spectroscopy.…”