A series of glucopyranose (α-D-glucose, β-D-glucose, D-gluconic acid and 6-amino-6-deoxy-D-glocuse) terminated alkanethiol acetals have been successfully synthesized. α and β-D-glucose were configured in controlled ways coupling with alkanethiol, in which bromosugar and trichloroacetirnidates were selected as activated glycosyl donors using different promoters tetraalkylammonium bromide (Et4NBr) and trimethylsilanetriflate (TMSOTf) respectively.
The glycosylation of glucuronic acid and 6-amino-6-deoxy-β-D-glucose to ω-mercaptohexadecanol depends on powerful donors and stronger promoters. The corresponding trichloroacetimidates of the methyl D-glucuronic ester and 6-azido-6-deoxy-β-D-glucose were used as active donors, and strong Lewis acid, trimethylsilyl triflate (TMSOTf) was also involved in this study.
These structures were mixed in different proportions with HO(CH2)16SH, and the corresponding SAMs on gold were characterized by ellipsometry, contact angle goniometry, and infrared reflection-absorption spectroscopy. Infrared reflectionabsorption spectroscopy (IRAS) indicated well-ordered SAMs with a high degree of crystallinity on the hydrocarbon layer. Different orientation between α and β-D-glucose self-assembled monolayers, and the functionality of carboxyl acid and amine were explored on surface for further immobilization of lipid bilayers.
New model systems were developed, in which amphiphilic hexadecly OEG derivative or glucose derivatives were designed to present self-assembled monolayers on gold. HS(CH2)15CONH-EG6-CH2CONH(CH2)15CH3 displayed particularly sharp features in IRAS spectra at room temperature. Carbohydrate unit instead of OEG moiety could be more natural and it possesses three-dimensional water-filled space.
A strategy for the synthesis of a series of closely related oligo(ethylene glycol)-terminated alkanethiol amides (principally HS(CH2)mCONH(CH2CH2O)nH; m = 2, 5, 11, 15, n = 1, 2, 4, 6, 8, 10, 12) and analogous esters has been developed. These compounds were made to study the structure and stability of self-assembled monolayers (SAMs) on gold in the prospect of designing new biosensing interfaces. For this purpose, monodisperse heterofunctional oligo(ethylene glycols) with up to 12 units were prepared. Selective monoacylation of the symmetrical tetra- and hexa(ethylene glycol) diols as their mesylates with the use of silver(I) oxide was performed. The synthetic approach was based on carbodiimide couplings of various oligo(ethylene glycol) derivatives to ω-(acetylthio) carboxylic acids via a terminal amino or hydroxyl function. SAM structures on gold were studied with respect to thickness, wettability (water contact angles ∼30°), and conformation. A good fit was obtained for the relation between monolayer thickness (d) and the number of units in the oligo(ethylene glycol) chain (n): d = 2.8n + 21.8 (Å). Interestingly, the corresponding infrared spectroscopy analysis showed a dramatic change in conformation of the oligomeric chains from all-trans (n = 4) to helical (n ≥ 6) conformation. A crystalline helical structure was observed in the SAMs for n > 6.
The formation of highly ordered self-assembled monolayers (SAMs) on goldfrom an unusually long and linear compound HS(CH2)15CONH(CH2CH2O)6CH2CONH(CH2)15CH3 is investigated by contact angle goniometry, ex situ null ellipsometry, cyclic voltammetry and infrared reflection-absorption spectroscopy. The molecules are found to assemble in an upright position as a complete monolayer within 60 min. The overall structure of the SAM reaches equilibrium within 24 h as evidenced by infrared spectroscopy, although a slight improvement in water contact angles is observed over a period of a few weeks. The resulting SAM is 60 Å thick and it displays an advancing water contact angle of 112° and excellent electrochemicalblocking characteristics with typical current densities about 20 times lower as compared to those observed for HS(CH2)15CH3 SAMs. The dominating crystalline phases of the supporting HS(CH2)15 and terminal (CH2)15CH3 alkyl portions, as well as the sealed oligo(ethylene glycol) (OEG) “core,” appear as unusually sharp features in the infrared spectra at room temperature. For example, the splitting seen for the CH3 stretching and CH2 scissoring peaks is normally only observed for conformationally trapped alkylthiolate SAMs at low temperatures and for highly crystalline polymethylenes. Temperature-programmed infrared spectroscopy in ultrahigh vacuum reveals a significantly improved thermal stability of the SAM under investigation, as compared to two analogous OEG derivatives without the extended alkyl chain. Our study points out the advantages of adopting a “modular approach” in designing novel SAM-forming compounds with precisely positioned in plane stabilizing groups. We demonstrate also the potential of using the above set of compounds in the fabrication of “hydrogel-like” arrays with controlled wetting properties for application in the ever-growing fields of protein and cell analysis, as well as for bioanalytical applications.