Synthesis and monolayer characterization of phosphorylated amino acid analogs

The synthesis of a series of thiols containing phosphorylated and non-phosphorylated serine, threonine, and tyrosine amino acid residues is described. The synthesized molecules, based on 3-mercaptopropionic acid, were assembled onto gold and subsequently characterized using infrared reflection-absorption spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, and contact angle goniometry. The ellipsometric analysis indicates that they form densely packed and well-oriented monolayers on gold, with thicknesses that are in good agreement with estimated values from space-filling models. The bulky and space-demanding phosphorylated threonine analog was, however, found to be an exception. The increase in layer thickness when adding a phosphate group to the threonine is only 35% of that observed for the two other analogs. A detailed infrared examination of the influence of cation coordination to the phosphorylated serine analog using calcium and magnesium reveals structural similarities to those of the inorganic phosphate compound calcium hydroxy apatite. We furthermore discuss the application of these monolayers as soft templates for biomineralization.