Effect of Pressure on Dielectric and Frank Elastic Constants of a Material Exhibiting the Twist Bend Nematic Phase

We report the first investigation on the effect of applied pressure on the now well-known dimer α,ω bis­(4,4′-cyanobiphenyl)­heptane (CB7CB) that exhibits two types of nematic: the regular uniaxial nematic (N) and the recently discovered twist-bend nematic (N_TB) phase. At atmospheric pressure, the thermal behavior of ε_⊥, the permittivity normal to the director in the N phase decreases on entering the N_TB wherein the value represents permittivity orthogonal to the helical axis. Application of pressure initially decreases the magnitude of the change in ε_⊥ and with further increase in pressure exhibits an increase in the value. Such a change in the feature of ε_⊥ is similar to that obtained at room pressure when the monomeric heptyloxy cyanobiphenyl (7OCB) is doped to CB7CB at a high concentration of 50%. The dielectric anisotropy exhibits a trend reversal with temperature, the extent of which is affected at high pressures. Another salient feature of the study is the effect that pressure has on the Frank bend elastic constant K₃₃. Over the pressure range studied K₃₃ enhances by a large factor of 5. In contrast, the splay elastic constant exhibits a much smaller change of only 70%. The pressure–temperature phase boundary has a much smaller slope for the N–N_TB transformation than for the isotropic-N transition. We propose that all these features can be understood in terms of the relative population of the more energetic horseshoe and lower energy extended conformer adopted by the CB7CB molecule. The extended conformer is favored at lower temperatures or at higher pressures. This argument is validated by X-ray diffraction experiments at atmospheric pressure on the binary mixture of CB7CB and 7OCB, mentioned above

Binary System Exhibiting the Nematic to Twist-Bend Nematic Transition: Behavior of Permittivity and Elastic Constants

We describe measurements of the permittivity and Frank elastic constant in the nematic phase of a binary system displaying a transition between the nematic (N) and the recently discovered twist-bend nematic (N_TB) phase. Among the salient features observed are (i) the existence of the N_TB phase even when the system is loaded with a high concentration (∼64 mol %) of a rodlike component; (ii) a clear signature in permittivity of the N–N_TB transition; and (iii) a lower value of the bend elastic constant compared to the splay over a large phase space, with the difference between the two becoming a maximum for an intermediate mixture. These studies further support the surprising idea that the elastic features associated with bent molecules can be further augmented by suitable rodlike additives

Interactions of Surfactants with the Bacterial Cell Wall and Inner Membrane: Revealing the Link between Aggregation and Antimicrobial Activity

Surfactants with their intrinsic ability to solubilize lipid membranes are widely used as antibacterial agents, and their interactions with the bacterial cell envelope are complicated by their differential aggregation tendencies. We present a combined experimental and molecular dynamics investigation to unravel the molecular basis for the superior antimicrobial activity and faster kill kinetics of shorter-chain fatty acid surfactant, laurate, when compared with the longer-chain surfactants studied in contact time assays with live Escherichia coli (E. coli). From all-atom molecular dynamics simulations, translocation events across peptidoglycan were the highest for laurate followed by sodium dodecyl sulfate, myristate, palmitate, oleate, and stearate. The translocation kinetics were positively correlated with the critical micellar concentration, which determined the free monomer surfactant concentration available for translocation across peptidoglycan. Interestingly, aggregates showed a lower propensity to translocate across the peptidoglycan layer and longer translocation times were observed for oleate, thereby revealing an intrinsic sieving property of the bacterial cell wall. Molecular dynamics simulations with surfactant-incorporated bacterial inner membranes revealed the greatest hydrophobic mismatch and membrane thinning in the presence of laurate when compared with the other surfactants. The enhanced antimicrobial efficacy of laurate over oleate was further verified by experiments with giant unilamellar vesicles, and electroporation molecular dynamics simulations revealed greater inner membrane poration tendency in the presence of laurate when compared with the longer-chain surfactants. Our study provides molecular insights into surfactant translocation across peptidoglycan and chain length-induced structural disruption of the inner membrane, which correlate with contact time kill efficacies observed as a function of chain length with E. coli. The insights gained from our study uncover unexplored barrier properties of the bacterial cell envelope to rationalize the development of antimicrobial formulations and therapeutics