New Method to Measure Packing Densities of Self-Assembled Thiolipid Monolayers

Abstract

For a monolayer of 2,3-di-phytanyl-sn-glycerol-1-tetraethylene glycol-D,L-a-lipoic acid ester lipid (DPTL) self-assembled (SAM) at a gold electrode surface we propose a new method to determine the charge number per adsorbed molecule and the packing density (area per molecule) in the monolayer. The method relies on chronocoulometry to measure the charge density at the SAM covered gold electrode surface. Two series of measurements have to be performed. In the first series, charge densities are measured for a monolayer transferred from the air-solution to the metal-solution interface using the Langmuir-Blodgett (LB) technique. This series of measurements allows one to determine charge numbers per adsorbed DPTL molecule. The second series is performed using a gold electrode covered with a self-assembled monolayer. The charge densities obtained in this series are then used to calculate the packing density with the help of charge numbers per adsorbed DPTL determined in the first series. The area per adsorbed molecule determined by the new method was compared to the area per molecule determined by the popular reductive desorption method. The molecular area determined with the new method is about 20% larger than the area calculated from the van der Waals model, which is a physically reasonable result. In contrast, the popular reductive desorption method gives an area per molecule 20% lower than the minimum estimated based on a van der Waals model. This is a physically unreasonable result. It is also shown that the charge numbers per adsorbed molecule depend on the electrode potential and may assume values smaller than the number of electrons participating in the reductive desorption step. An explanation of the origin of the "partial charge numbers" is provided. We recommend the new method be used in future studies of thiol adsorption at metal surfaces.

Can gramicidin ion channel affect the dipole potential of neighboring phospholipid headgroups?

“…DOPC, DOPS and DOPA were purchased in chloroform solution from Avanti Polar Lipids (Birmingham, AL, USA). The 2,3,di-O-phytanyl-sn-glycerol-1-tetraethylene-glycol-D,L-α lipoic acid ester thiolipid (DPTL) was provided by Prof. Adrian Schwan (Department of Chemistry, University of Guelph, Canada) [15]; stock solutions of this thiolipid were stored at − 18°C. Deionized water was distilled once and then redistilled from alkaline permanganate before its use.…”

Peptide molecular junctions: Electron transmission through individual amino acid residues

“…This method is often used for the determination of the surface coverage for non-electroactive thiolate monolayers. However, as demonstrated by several groups this approach gives coverages overestimated by 15-20% [24][25][26][27][28][29]. This is related to the substantial charge which contributes to the desorption process when the monolayer is displaced by the electrolyte at the metal-solution interface.…”

Experimental and theoretical studies of l-cysteine adsorbed at Ag(111) electrodes

“…These limitations on the use of reductive peaks to calculate the thiols coverage has been pointed out by Schneider and Buttry [32]. Recently, a new method to measure packing densities of self-assembled monolayers of thiols has been proposed [33,34]. The method relies on chronocoulometry to measure the charge density at the SAM covered metal electrode surface.…”

“…The method relies on chronocoulometry to measure the charge density at the SAM covered metal electrode surface. It is also shown [33] that the charge numbers per adsorbed molecule depend on the electrode potential and may assume values smaller than the number of electron participating in the reductive desorption step. However, the application of this method to the l-cysteine system is more complicated.…”