Yan Yeung Luk

Applications of functional surfactants

Recent studies have reported the introduction of a range of new chemical and biochemical functionalities into the structures of amphiphilic molecules. Assemblies spontaneously formed by these amphiphiles are in many cases highly complex and possess properties not found in systems formed from amphiphiles with simpler structures. In particular, the incorporation of peptides and oligopeptides into the hydrophilic domains of amphiphiles has led to new classes of surfactants that self-assemble into structures that mimic a variety of the functions of natural materials (including organic scaffolds of bone; inhibitors of proteins involved in viral infection; chiral polymeric amphiphiles; materials that promote adhesion of cells to surfaces). Amphiphiles functionalized with a range of carbohydrates have also been reported. These amphiphiles assemble into aggregate morphologies that depend strongly on the stereochemistry of the carbohydrate. These assemblies offer the basis of promising approaches for the design of polyvalent (potent) carbohydrate-based drugs. Finally, the introduction of redox-active and polymerizable functional groups into the structures of amphiphiles has provided the basis of a class of novel tunable solvents with potential applications in separations technologies as well as new materials that exhibit controlled release and catalytic properties.

Inhibition of Escherichia coli Biofilm Formation by Self-Assembled Monolayers of Functional Alkanethiols on Gold

ABSTRACT Bacterial biofilms cause serious problems, such as antibiotic resistance and medical device-related infections. To further understand bacterium-surface interactions and to develop efficient control strategies, self-assembled monolayers (SAMs) of alkanethiols presenting different functional groups on gold films were analyzed to determine their resistance to biofilm formation. Escherichia coli was labeled with green florescence protein, and its biofilm formation on SAM-modified surfaces was monitored by confocal laser scanning microscopy. The three-dimensional structures of biofilms were analyzed with the COMSTAT software to obtain information about biofilm thickness and surface coverage. SAMs presenting methyl, l-gulonamide (a sugar alcohol tethered with an amide bond), and tri(ethylene glycol) (TEG) groups were tested. Among these, the TEG-terminated SAM was the most resistant to E. coli biofilm formation; e.g., it repressed biofilm formation by E. coli DH5α by 99.5% ± 0.1% for 1 day compared to the biofilm formation on a bare gold surface. When surfaces were patterned with regions consisting of methyl-terminated SAMs surrounded by TEG-terminated SAMs, E. coli formed biofilms only on methyl-terminated patterns. Addition of TEG as a free molecule to growth medium at concentrations of 0.1 and 1.0% also inhibited biofilm formation, while TEG at concentrations up to 1.5% did not have any noticeable effects on cell growth. The results of this study suggest that the reduction in biofilm formation on surfaces modified with TEG-terminated SAMs is a result of multiple factors, including the solvent structure at the interface, the chemorepellent nature of TEG, and the inhibitory effect of TEG on cell motility.

Identifying the important structural elements of brominated furanones for inhibiting biofilm formation by Escherichia coli

A collection of structurally closely related furanones was synthesized to identify the most important structural elements in brominated furanones for inhibiting the formation of bacterial biofilms. The results suggest that a conjugated exocyclic vinyl bromide on the furanone ring is the most important structural element for the non-toxic but inhibition activity for Escherichia coli biofilm formation. Furanones bearing monosubstituted bromide groups on saturated carbons were found to have a toxic effect that attenuates the bacterial growth.

Anti-fouling chemistry of chiral monolayers: Enhancing biofilm resistance on racemic surface

This work reports the resistance to protein adsorption and bacterial biofilm formation by chiral monolayers of polyol-terminated alkanethiols surrounding micrometer-sized patterns of methyl-terminated alkanethiols on gold films. We discover that patterned surfaces surrounded by chiral polyol monolayers can distinguish different stages of biofilm formation. After inoculation on the surfaces, bacteria first reversibly attached on the chiral polyol monolayers. Over time, the bacteria detached from the polyol surfaces, and attached on the hydrophobic micropatterns to form biofilms. Interestingly, while both enantiomers of gulitol- and mannonamide-terminated monolayer resisted adsorption of proteins (bovine serum albumin, lysozyme, and fibrinogen) and confined biofilms formed on the micropatterns, the monolayers formed by the racemic mixture of either pair of enantiomers exhibited stronger antifouling chemistry against both protein adsorption and biofilm formation than monolayers formed by one enantiomer alone. These results reveal the different chemistries that separate the different stages of biofilm formation, and the stereochemical influence on resisting biofoulings at a molecular-level.

Non-toxic thermotropic liquid crystals for use with mammalian cells

This paper reports the results of a study that aimed to identify thermotropic liquid crystals that are not toxic to mammalian cells. Mesogenic compounds were mixed to create eight liquid crystalline phases, each with a unique set of functional groups. We investigated the toxicity of each liquid crystalline phase using two mammalian cell lines—3T3 fibroblast and SV-40 transformed human corneal epithelial (HCEC) cells. Using dual fluorescent staining assays based on calcein acetoxymethylester (Calcein-AM) and ethidium homodimer, we measured correspondingly the number of viable and dead cells following immersion of the cells in the liquid crystals. It was found that most of the liquid crystals, such as commonly used 5CB and E7, caused cell death after contact with cells for four hours. However, we identified a class of liquid crystals containing fluorophenyl groups that possess minimal or no toxicity (as indicated by results of assays based on Calcein-AM and ethidium homodimer) to cells. Following immersion in fluorophenyl liquid crystals for four hours, the cells were observed to proliferate in culture medium at rates similar to control cells exposed to phosphate-buffered saline (PBS) for four hours. In contrast, treatment of cells for 24 hours with either PBS or liquid crystals (media containing no nutrients and growth factors) resulted in cell death.

Molecular Gradients of Bioinertness Reveal a Mechanistic Difference between Mammalian Cell Adhesion and Bacterial Biofilm Formation

Chemical gradients play an important role in guiding the activities of both eukaryotic and prokaryotic cells. Here, we used molecularly well-defined chemical gradients formed by self-assembled monolayers (SAMs) on gold films to reveal that mammalian cell adhesion and bacterial biofilm formation respond differently to a gradient of surface chemistry that resists cell attachment. Gradient self-assembled monolayers (SAMs) consisting of two mixed alkanethiols were fabricated by differential exposure of the gold film to one alkanethiol, followed by soaking in another alkanethiol solution. A gradient in bioinertness that resisted cell attachment was created on SAMs from a gradient in the surface density of HS(CH2)11(OCH2CH2)3OH, backfilled with either HS(CH2)11OH or HS(CH2)11CH3. Measurements of the amounts of mammalian cells and bacterial biofilms on these gradient surfaces reveal that, for mammalian cells, a critical density of adhesion ligands from absorbed proteins on surfaces exists for supporting maximum adhesion and proliferation, whereas for the bacterium Escherichia coli , the amount of biofilm formed on surfaces increased linearly with the surface density of adhesive groups (methyl or hydroxyl groups) in different media. These results are consistent with mammalian cell adhesion requiring an anchorage via specific molecular recognitions and suggest that biofilms can form by immobilization of bacteria via nonspecific interaction between bacteria and surfaces.

Prolonged control of patterned biofilm formation by bio-inert surface chemistry.

A bio-inert surface chemistry was developed that can confine biofilm formation in designed patterns for at least 26 days.

Stereochemical effects of chiral monolayers on enhancing the resistance to mammalian cell adhesion

This work describes the different durations of surface confinement of adhered mammalian cells by monolayers comprised of enantiomers of bio-inert polyol-terminated alkanethiols. Enhanced resistance to protein adsorption and cell adhesion is obtained on monolayers formed by a racemic mixture of the enantiomeric alkanethiols.

Structures and biofilm inhibition activities of brominated furanones for Escherichia coli and Pseudomonas aeruginosa

A series of brominated furanones (BFs) with closely related structures was synthesized and evaluated for biofilm inhibition activity against Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa). The structure–activity relationships (SARs) indicated that methyl substituent on the furanone ring and on the exocyclic vinyl double bond is important for relieving the inhibitory effect of these molecules on the growth of both E. coli and P. aeruginosa. To explore the mechanism of BFs to inhibit E. coli biofilm, the biofilm inhibition activity of BFs in wild type E. coli and its sdiA knockout mutant (ΔsdiA) was compared, which is proposed to encode the receptor for BFs. We found that ΔsdiA caused ∼2 fold reduction in biofilm inhibition activity of BFs compared with that of the wild type E. coli. To explore the effect of BFs on quorum sensing of P. aeruginosa, we use reporter gene assays that examine the effect of BFs on the las and rhl quorum sensing systems. Interestingly, while BFs exhibited antagonistic activities to LasR protein in the las system, these molecules showed agonistic activity to RhlR protein in the rhl system. Furthermore, one BF molecule, BF15, inhibited the production of the virulence factor elastase B without significant inhibition of biofilm formation. As the growth and biofilm inhibition activity of BFs are closely related to their structural details, this class of molecules bears potential for further design of non-microbicidal agents to control bacterial biofilm formation.

Dipole-induced structure in aromatic-terminated self-assembled monolayers: A study by near edge x-ray absorption fine structure spectroscopy

The structure of self-assembled monolayers presenting aromatic rings at a surface is studied by near edge x-ray absorption fine structure spectroscopy (NEXAFS). Fluorine substitution at asymmetric positions in the aromatic rings is used to generate a layer of dipoles at the surface of the monolayer. We find that fluorine substituted aromatic rings are more ordered than unsubstituted aromatic rings by a factor of two based on the polarization dependence of the lowest C 1s to pi* transition, which is associated with transitions from phenyl carbons attached to hydrogens. This result is consistent with the influence of dipole-dipole interactions and quadrupolar interactions between the aromatic groups due to the substitution of fluorine atoms. The work also serves to illustrate how subtle variations in the orientation of an end group of a self-assembled monolayer can be determined by using NEXAFS.