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 stabil ity of self-assembled monolayers (SAMs) on gold in the prospect of designin g new biosensing interfaces. For this purpose, monodisperse heterofunctiona l oligo(ethylene glycols) with up to 12 units were prepared. Selective mono acylation of the symmetrical tetra- and hexa(ethylene glycol) dials as thei r mesylates with the use of silver(I) oxide was performed. The synthetic ap proach was based an carbodiimide couplings of various oligo(ethylene glycol ) derivatives to omega-(acetylthio) carboxylic acids via a terminal amino o r hydroxyl function. SAM: structures on gold were studied with respect to t hickness, wettability (water contact angles similar to 30 degrees), and con formation. A good fit was obtained for the relation between monolayer thick ness (d) and the number of units in the oligo(ethylene glycol) chain (n): d = 2.8n + 21.8 (Angstrom). Interestingly, the corresponding infrared spectr oscopy 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.