Interplay of Hydrophobic and Electrostatic Interactions between Polyoxometalates and Lipid Molecules

Abstract

The Hofmeister series has long been of both scientific and technological importance because it allows systematic predictions of the interaction between ions and molecules to be made. In this work, three different polyoxometalate (POM) anions and three different lipid monolayers were used to investigate the interplay between electrostatic and hydrophobic interactions. We used three Keggin-type POMs with different charges ([PW 12 O 40 ] 3− , [SiW 12 O 40 ] 4− , and [H 2 W 12 O 40 ] 6− ) to eliminate any effects of morphology and to focus on the effects of the charge and hydrophobicity of the POM anions. The POMs adsorb onto cationic lipid monolayers via electrostatic interactions, while hydrophobic interaction is the dominant factor for adsorption onto anionic lipid monolayers when charge neutralization is provided by cationic counterions. In contrast, the dominant interactions experienced by zwitterionic lipid monolayers can switch between electrostatic interactions for POMs with higher charge density and hydrophobic interactions for POMs with lower charge density. Furthermore, the dominant interaction could also switch as a function of lipid density. In the gas phase, lipid monolayers mostly interact with POMs through electrostatic interactions, and the strength of the influence of the POM varies in the order [PW 12 O 40 ] 3− < [SiW 12 O 40 ] 4− < [H 2 W 12 O 40 ] 6− . In contrast, the order switches to [PW 12 O 40 ] 3− > [SiW 12 O 40 ] 4− > [H 2 W 12 O 40 ] 6−when the POM interacts with a compressed lipid monolayer (surface pressure = 40 mN/m), which reflects the Hofmeister series and the chaotropic nature of the POMs. Thus, hydrophobic interactions govern the penetration of the lipid monolayer for condensed cell membranes irrespective of the charge of the lipid. The observed switch implies the importance of the interplay between the electrostatic and hydrophobic interactions. Our findings provide new insight into the switchable binding modes of POMs for cell membranes with different lipid composition, density, and hydrophobicity, enabling the design of tailor-made POM-based materials for a new generation of antimicrobial agents.

Luminescent Carrageenan Hydrogels Containing Lanthanopolyoxometalates

“…A possible explanation for the above observations is that the K + ions will compensate the negative charge of the sulfate groups, and therefore other interactions between the biopolymer chains and the EuPOM anions can be determinant, namely hydrogen bonding between the octahedra WO 6 and hydroxyl groups of the galactopyranose units. This is in line with observations made by other authors that have pointed out the relevance of hydrophobic interactions between negatively charged POMs and uncharged molecular components, such as in lipids and cyclodextrins , . For example, the adsorption of POMs onto lipid monolayers depends on the interplay between electrostatic and hydrophobic interactions .…”

“…This is in line with observations made by other authors that have pointed out the relevance of hydrophobic interactions between negatively charged POMs and uncharged molecular components, such as in lipids and cyclodextrins , . For example, the adsorption of POMs onto lipid monolayers depends on the interplay between electrostatic and hydrophobic interactions . Hence, electrostatic interactions govern the adsorption of anionic POMs onto cationic lipid monolayers, while hydrophobic interactions are prevalent when POMs adsorb onto charge‐neutralized anionic lipid monolayers, as a result of the presence of positive counterions.…”

Antibacterial Activity of Polyoxometalates Against Moraxella catarrhalis

“…The increased susceptibility of these strains toward β-lactams is probably due to the higher permeability of the outer membrane toward these agents. POMs, as examples of super chaotropic anions, can adsorb onto lipid monolayers via electrostatic and/or hydrophobic interaction depending on the charge of the lipid layer (Kobayashi et al, 2017). The model experiments with three differently charged Keggin anions show that dominant interaction equally depends both on the charge density of POMs and on the lipid density (Kobayashi et al, 2017).…”

The Chaotropic Effect as an Assembly Motif in Chemistry

“…[1][2][3][4][5][6] Specific-ion solvation has additionally been related, since Hofmeister, to the kosmotropic (water-structure-forming) and chaotropic (water-structure-breaking) properties of ions. [7][8][9] Following up on scattered reports on the unusual behavior of large anions, [10][11][12][13][14][15][16][17][18] the propensity of these anions to associate with hydrophobic and neutral polar phases has recently been generalized and named the chaotropic effect, contrasting the hydrophobic effect as ad istinct assembly motif in chemistry. [3,19,20] Thee ffect becomes particularly pronounced for superchaotropic ions that reach beyond the classical Hofmeister scale.…”