Polarization Modulation Infrared Absorption Spectroscopy Research Articles

Synthesis and electrochemical characterization of 4-thio pseudo-glycolipids as candidate tethers for lipid bilayer models

Two 4-thio pseudo-glycolipids, a disulfide and a thiol, were synthesized as candidates to build tethered bilayer lipid membranes (tBLMs) on gold (111) surface. Monolayers of the disulfide and thiol were built on gold surfaces using the self-assembly and Langmuir-Blodgett (LB) transfer methods. Monolayers were prepared in several solvents and at various temperatures; their quality was assessed by differential capacitance. Best monolayers were achieved when the disulfide was self-assembled in ethanol. The quality of such monolayers was further improved by allowing the assembly to proceed under stirring of the solution at increased temperatures. The charge number per adsorbed molecule and surface concentration of disulfide on gold (111) surface were determined by chronocoulometry. A DPhPC/disulfide tBLM was then built by vesicle fusion of 1,2-diphytanyl-sn-glycero-3-phosphocholine (DPhPC) on top of the disulfide monolayer. The minimum capacitance of the gold electrode with tBLM (1.3 μF cm−2) was close to the value of a real cell membrane and the AFM force spectroscopy measurements showed that the DPhPC/disulfide tBLM had a thickness of 6.2 ± 0.6 nm consistent with the thickness expected for a bilayer. Finally, the potential of the tBLM to study transmembrane proteins was assessed by investigating the reconstitution of gramicidin A into the membrane by polarization modulation infrared absorption spectroscopy (PM-IRRAS). These results demonstrate that the disulfide is a good candidate to construct tBLMs on gold surface and to study transmembrane proteins.

Adsorption of functionalised thiols on gold surfaces: How to build a sensitive and selective sensor for a nitroaromatic compound?

Adsorption of thiols of varying structures bearing amine, acid or aromatic functionalities was characterised by Polarisation Modulation Infrared Absorption Spectroscopy (PM-IRRAS). The so functionalised surfaces were then submitted to atmospheres containing traces of 2,4-dinitrotrifluoromethoxybenzene and gas capture was evaluated by Quartz Crystal Microbalance (QCM). The selectivity of these new sensors was also evaluated thanks to exposures to solvents: ethanol, methylethylketone, toluene and dichloromethane. The QCM response appeared to be the best one, high and specific to the nitroaromatic, for aromatic amine functions.

Chemical selectivity of self-assembled monolayers of calix[4]resorcinarene

Selective molecular interactions at an interface formed by self-assembly of a macrocyclic synthetic host, calix[4]resorcinarene with four thiol groups (R4SH), are investigated. The recognition of guest adsorbates from aqueous solutions is monitored using surface plasmon resonance (SPR) and the orientation of the guest-molecule is probed using polarization modulation infrared absorption spectroscopy (PM-IRRAS). The experiments reported here demonstrate that the chemical selectivity of self-assembled monolayers (SAMs) of host molecules such as calix[4]resorcinarenes extends to isomers of several different guest molecules. By using structural isomers of guest molecules such as bipyridine and nitrophenol that are multidentate hydrogen bond acceptors, it is shown that geometric match between guest and host molecules is an integral aspect of the recognition phenomena. Results from SPR and PM-IRRAS experiments reported here highlight the interplay between steric size and forces such as hydrogen bonding and hydrophobic interactions. Competitive and sequential adsorption of guest molecules such as α-hydroxy-γ-butyrolactone and 4,4′-bipyridine shows that these guests compete for the same binding sites on the surface and that the interplay between steric size and molecular forces underlies the preferential selectivity of one guest molecule over another.

Structure and formation of self-assembled monolayers of helical poly ( γ-benzyl l-glutamate) by surface plasmon resonance and infrared spectroscopy

The self-assembly of a rigid, rod-like, α-helical polypeptide was investigated on gold substrates. A disulfide labeled poly( γ-benzyl- l-glutamate) (PBLG) was prepared by a novel synthetic route wherein the polymerization of the N-carboxyanhydride was initiated with an amine terminated disulfide. Two different SSPBLG materials with molecular weight of 11 and 28 kDa were prepared and characterized using dielectric spectroscopy. The kinetics of self-assembly of the two polypeptides was studied using surface plasmon resonance (SPR) and the organization of the self-assembled monolayers (SAMs) was characterized by polarization-modulation infrared absorption spectroscopy (PM-IRRAS). SPR revealed that self-assembly of the high molecular weight polypeptides on gold substrates proceeds rapidly on a time-scale of minutes and is comparable to the time of assembly of simple n-alkanethiols. The increase in length of the polypeptide led to an increase in the dipole moment from 156 to 363 D. The enhanced electrostatic interactions in the SAMs were manifested as an increase in the average tilt of the helix axis from the normal to the surface and monolayers with smaller thickness.

Self-Assembly of a Rodlike Polypeptide on Solid Surfaces: Role of Solvent, Molecular Weight, and Time of Assembly

The interplay between preparative conditions and molecular characteristics during the self-assembly of a rigid, rodlike, α-helical polypeptide was investigated. Poly(γ-benzyl-l-glutamate) was prepared by a novel synthetic route wherein the polymerization of the N-carboxyanhydride was initiated with an amine terminated disulfide, and the SSPBLG was characterized using light scattering, viscometry, and size-exclusion chromatography. Self-assembly of SSPBLG on gold substrates was found to proceed rapidly and high molecular weight (∼27 kDa) polypeptides formed self-assembled monolayers (SAMs) on a time scale of minutes. Characterization of the SAMs by polarization modulation infrared absorption spectroscopy (PM-IRRAS) was used to explore the effects of the molecular weight of the polypeptide, the time of assembly, and the solvent. An increase in the length of the polypeptide from 66 to 156 Å caused an increase in the average tilt of the helix axis from the normal to the surface. The use of a nonpolar solvent (benzene), where antiparallel aggregation of helices is known to occur, did not prove to be an effective strategy for improving orientation of the longer polypeptides. The time of assembly was found to play a key role, and long times of assembly led to a decrease in surface coverage and increase in tilt angle. This effect was more pronounced for the short polypeptide than the long polypeptide. Self-assembly of a debenzylated SSPBLG showed that physisorption due to hydrophobic interactions was important in the preparation of SAMs formed from long polypeptides. Reduction of physisorption by use of the debenzylated SSPBLG led to an enhancement in surface coverage and decrease in the tilt angle of the higher molecular weight SSPBLG.