Synthesis of a series of oligo(ethylene glycol)-terminated alkanethiol amides designed to address structure and stability of biosensing interfaces

A strategy for the synthesis of a series of closely related oligo(ethylene glycol)-terminated alkanethiol amides (principally HS(CH2)(m)CONH(CH2CH2O)(n)H; 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) dials as their mesylates with the use of silver(I) oxide was performed. The synthetic approach was based an carbodiimide couplings of various oligo(ethylene glycol) derivatives to omega-(acetylthio) carboxylic acids via a terminal amino or hydroxyl function. SAM: structures on gold were studied with respect to thickness, wettability (water contact angles similar to 30 degrees), 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 (Angstrom). Interestingly, the corresponding infrared spectroscopy analysis showed a dramatic change in conformation of the oligomeric chains from all-trans (n = 4) to helical in (n greater than or equal to 6) conformation. A crystalline helical structure was observed in the SAMs for n > 6.