Paper BI-ThM12
Seriatim Operando STM and FTIR Study of Phospholipid Membrane Phase Transition Driven by Electrochemical Potential Control

Thursday, October 25, 2018, 11:40 am, Room 101B

Phospholipid (1,2-dialkanoyl-sn-glycero-3-phosphocholine, DHPC for alkyl=C₆H₁₃, DDPC for C₁₀H₂₁) monolayers were prepared on 1-octanethiol-terminated gold surface, as a model of biological cell membrane, and nano-scopically observed by electrochemical scanning tunneling microscopy (EC-STM) and internal multiple reflection Fourier-transformed infrared absorption spectroscopy (FTIR) within aqueous electrolytic solutions. This dual-technique in situ operando observation revealed structural changeover of the phospholipid membrane according to the applied electrode potential. In 0.05 M NH₄ClO₄ solution (pH 7.0) at 0 V vs RHE, DHPC monolayer was a fluidic monolayer along the underlying thiol monolayer, with mobile chasms through which underlying vacancy islands of the substrate were frequently seen.by application of -0.2 V, the monolayer was slowly converted into solid striped structure. This is designated as “hemimicellar aggregation” [1] with a periodicity of 4.3 nm, and observed also for DDPC. By returning the substrate potential, the lipid monolayers restored fluidic phase. This transition was reversible and repeatable. By application of +0.2 V the fluidic feature was maintained despite a slight increase of monolayer height. When the potential was swept from +0.2 V to -0.2 V, elliptic agglomerates with an average diameter of 13 nm was observed. After this, the hemimicellar aggregation was never observed by any kind of potential cycling. This irreversible change of phase coincided with the seriatim FTIR observation, using deuterium-labelled DHPC molecules. The initial change of fluidic to hemimicellar did not exhibit drastic change in IR spectra, except a reversible splitting of the P-O stretching in the region of 1200-1300 cm-1. After application of +0.2V DHPC with the head-group choline part (-C₂D₄N⁺(CD₃)₃) was lost as a surface IR signal. This is an evidence for irreversible dissociation of DHPC into choline and phosphatidyl acid. The elliptic grains correspond to the phosphaticyl acid, differently agglomerated from that of intact DHPC hemimicelles. The potential shift of this amplitude is similar to the membrane potential of real cells. It is seen that phospholipid molecules, the robust solid component of cell membrane, can be easily involved chemical reactions under such membrane potential by the aid of membrane proteins for example. This series of experiment also demonstrates the applicability of seriatim surface observation techniques such as IR spectroscopy in addition to STM, which does not always distinguish the molecular species and detect chemical reactions.

[1] J. Am. Chem. Soc. 126 (2004) 12276.