Driving forces of biomolecular organisation specific to nanometer scale: protein orientation on the surface, controlled by pH level

Control over chemical functionality of interfacial structures based on biological subunits is the prime focus of the modern nanotechnology. Supramolecular units realising definite functions (recognition, conversion, transport, etc.) are very suitable for integration with physical transducers to develop novel smart sensors. One of them is an optoelectronic device based on surface plasmon resonance (SPR) occurring in thin metal films. Use of the SPR method enables one to immediately observe intermolecular interactions at sensor surface without any radioactive or fluorescent labels, as well as to get information on the process kinetic parameters. It is known that behaviour of composite protein molecules in the native conditions essentially depends on their charge state. Therefore, one can change a position of the molecule under consideration relative to the charged sensor surface by changing charge of definite amino-acid groups via changing pH of the ambience. Here we propose a technique for modification of the gold surface using thiocyanat (CNS-). This prevents from protein denaturation and, in addition, gives an extra negative charge to the surface. The latter fact, in its turn, favors more reliable fixing of positively charged proteins. Here we demonstrate that the state and orientation of molecular subunits on the surface are very essential for overall reactivity and/or catalytic properties of the interface. Simple procedures for control of protein orientation on both untreated and chemically modified gold surfaces are proposed and tested for immunological (protein A-antigenantibody) and enzymatic (trypsin-soybean trypsin inhibitor) systems.