Acoustic coupling of transverse waves as a mechanism for the label-free detection of protein-small molecule interactions

An on-line acoustic transverse wave device has been used to study the binding interactions of human serum albumin with the small molecule drug, warfarin. Four linking systems for the covalent attachment of the protein to the surface of the gold electrode of the sensor were employed, namely thioctic acid, cysteamine, an N-hydroxysuccinimide ester and 11-mercaptoundecanoic acid. All the attachment protocols involve the ability of thiols to form gold-sulfur bonds at the metal surface. The functional group present at the distal end of each thiol was chemically activated in order to facilitate covalent attachment of the protein. On-line sensor measurements of acoustic parameters show that the binding of warfarin to the protein can be detected, and depending on the linking monolayer used three of four possible combinations of changes in series resonance frequency and motional resistance are observed. Calculations of possible mass and thickness viscoelastic effects demonstrate that these conventional notions are invalid in terms of an explanation of the acoustic signals observed for the warfarin-protein interaction. The responses are ascribed to acoustic coupling phenomena.