Molecular Conformation in Oligo(ethylene glycol)-Terminated Self-Assembled Monolayers on Gold and Silver Surfaces Determines Their Ability To Resist Protein Adsorption

Abstract

We report data from infrared absorption (FTIR) and X-ray photoelectron spectroscopies that correlate the molecular conformation of oligo(ethylene glycol) (OEG)-terminated self-assembled alkanethiolate monolayers (SAMs) with the ability of these films to resist protein adsorption. We studied three different SAMs of alkanethiolates on both evaporated Au and Ag surfaces. The SAMs were formed from substituted 1-undecanethiols with either a hydroxyl-terminated hexa(ethylene glycol) (EG6-OH) or a methoxy-terminated tri(ethylene glycol) (EG3-OMe) end group, or a substituted 1-tridecanethiol chain with a methoxy-terminated tri(ethylene glycol) end group and a -CH 2 OCH 3 side chain at the C-12 atom (EG[3,1]-OMe). The infrared data of EG6-OH-terminated SAMs on both Au and Ag surfaces reveal the presence of a crystalline helical OEG phase, coexisting with amorphous OEG moieties; the EG[3,1]-OMe-terminated alkanethiolates on Au and Ag show a lower absolute coverage and greater disorder than the two other compounds. The molecular conformation of the methoxy-terminated tri(ethylene glycol) (EG3-OMe) is different on Au and Ag surfaces due to the different lateral densities of SAMs on these substrates: on Au we find a conformation similar to that of EG6-OH alkanethiolates, whereas on Ag the infrared spectra indicate a densely packed film with trans conformation around the C-C bonds of the glycol units. The resistance of these OEG-functionalized alkanethiolate SAMs to adsorption of fibrinogen from a buffered solution correlates with the molecular conformation of the OEG moieties. The predominantly crystalline helical and the amorphous forms of OEG on gold substrates are resistant to adsorption of proteins, while a densely packed "all-trans" form of EG3-OMe present on silver surfaces adsorbs protein. The experimental observations are compatible with the hypothesis that binding of interfacial water by the OEG moieties is important in their ability to resist protein adsorption.

XPS, TOF-SIMS, NEXAFS, and SPR Characterization of Nitrilotriacetic Acid-Terminated Self-Assembled Monolayers for Controllable Immobilization of Proteins

“…Since the thiol groups are located at the gold-monolayer interface, the lower sulfur concentrations are due to the attenuation of sulfur photoelectrons by overlying organic layers. 15,18,47 NTA a and NTA b were exposed to OEG diluent thiol for 0.5 hr to 48 hr. Composition trends observed suggests that the OEG thiol diluent produced different results for NTA a and NTA b.…”

“…The NTA concentration in the NTA b monolayer is calculated by the degree of attenuation of the substrate Au 4f XPS signal. 18,29 Surface NTA concentrations were significantly affected by the self-assembly conditions of the NTA and OEG thiols. Maximum NTA concentration was observed to be 1.9 headgroup/ nm 2 in NTA a assembled for 24 hr, while a lower concentration was observed in NTA b assembled for 5 min.…”

Protein Adsorption on Supported Phospholipid Bilayers

“…It is, however, clear through recent systematic experimental work and earlier experimental documentation that certain hydrophilic surfaces are quite protein resistant (5,(9)(10)(11)(40)(41)(42). Consequently, the bonding and structure of water at the surfaces, with which proteins interact, are important.…”

“…Lipid films at interfaces have actually previously been shown to be quite efficient in preventing protein adsorption (4-7), and have also been shown to limit cell attachment (8) (a fact that can probably be directly related to the low affinity to protein adsorption on the membrane; if proteins do not bind, cells will not find attachment sites). The actual mechanisms behind the protein resistance are, however, still poorly understood, although recent important experimental data have addressed, e.g., the role of the polarity of functional groups (9) interacting with proteins, surface wettability (10), and the role of the hydration layer of the protein-resistant surfaces (11). Recently the influence of surface polarizability and surface hydration layer thickness on the protein adsorption energetics was treated theoretically (12).…”

Inhibition of adhesion of yeasts and bacteria by poly(ethylene oxide)-brushes on glass in a parallel plate flow chamber

“…For instance, hydrophobic proteins have been suggested to be able to adsorb to PEObrushes through their hydrophobic moieties (Furness et al, 1998), which was confirmed by the work of Sheth et al (2000), demonstrating attraction of PEO to non-polar surfaces. It has furthermore been shown that PEO can exist in a protein-repulsive, polar, helical or random structure, and a protein-attractive, apolar, all-trans structure (Harder et al, 1998;Currie et al, 2003). The work of Efremova et al (2001) showed that changing circumstances, such as temperature or compressive load, can change PEO from a protein-repellent to a protein-attractive state.…”