Bioactive adsorbates on gold surfaces : structural properties and bio-interaction studies

Abstract: This thesis investigates the properties of biomolecular, model adsorbates on gold such as amino acid derivatives, peptides and related organic molecules. Subsequent bin-interaction studies were also conducted. The physico-chemical and structural properties of the adsorbates were characterized using X-ray Photoelectron Spectroscopy (XPS), Infrared-Reflection Absmption Spectroscopy (IRAS) and Near-Edge X-ray Absmption Fine Structures spectroscopy (NEXAFS). Complementary techniques such as Null ellipsometry and Cyclic Voltammetry (CV) were also used. The interaction of the bioactive monolayers with biologically relevant molecules, such as proteins and metal ions, were investigated using Surface Plasmon Resonance (SPR) spectroscopy and Electrochendcallmpedance Spectroscopy (EIS).The first part of the thesis is directed towards the interaction of bovine-brain G-protein with adsorbates involving arginine residues and receptor-derived peptides mimicking the 2nd and 3rd intracellular (ic) loop of the α2A Adrenergic G-protein coupledreceptor (GPCR). The general aim is to find a peptide sequence that will selectively, with high affinity, interact with the G-protein. The specific aims were to examine the importance of the presence of positively charged arginine residues, to investigate the influence of molecular orientation of the adsorbates, and to verify which intracellular loop has the highest affinity to the G-protein. The investigation involved characterizing the chemical composition and the molecular orientation of Arginine-based dipeptide adsorbates (Arg-Cys and Arg-Cysteandne) and receptor-detived peptides (GPR1R also labeled GPRi3c, GPR1K, GPR1A, GPRi2c, GPRi3n) innnobilized on gold surfaces, followed by G~protein interaction studies. On all the adsorbates subjected to interact with G-proteins, the presence of arginine residues was proven to be of special importance in the affinity of G-proteins. A molecularly"oriented Arg-Cysteamine, with main molecular axis perpendicular to the gold surface, showing more available arginines, attracts more G-proteins as compared to Arg-Cys that has a compact configuration when adsorbed on gold. The peptide adsorbates derived from the third ic loop (GPRi3c and GPRi3n) have higher affinity than peptides derived from the second ic loop (GPRi2c). This shows that this arginine-rich area of the third ic loop has a major influence on the affinity and selectivity of G-proteins.