The topography of membrane-bound proteins at atomic resolution is known only in rare cases. [1] Although the primary amino-acid sequence of glycophorin A (GPA), the major sialoglycoprotein of the human erythrocytes, has been known for more than twenty years and was the first membrane protein sequence elucidated, a three-dimensional picture of the protein is still missing. We have previously developed the photoactivable membrane probe 1. This is a phospholipid with two distal, polar heads (a bola-amphiphile) and carries a photosensitive group (benzophenone) in the middle of a transmembrane chain. It is easily incorporated into DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocoline) vesicles, where it spans the bilayer at least in the presence of physiological concentrations of cholesterol. We demonstrated recently that the tandem use of the probe 1 a and cholesterol (for its ordering effect) in photolabeling experiments on DMPC vesicles led to a remarkable regioselective functionalization and we are presently conducting such studies. With this information, we expect that the present strategy using Au/ Ag(upd) electrodes for halide detection will provide the notable abilities to measure the concentrations of multiple halides simultaneously and to provide specific signals for confirming their identification.
Experimental Section
Materials: Au (99.99 %) shot and Cr-coated tungsten filaments were obtained from Americana Precious Metals (East Rutherford, NJ) and R. D. Mathis (Long, Beach, CA), respectively. Sulfuric acid (double distilled, 98 %) and silver sulfate (c(Cl À ) 0.02 %) were obtained from Aldrich and used as received. KCl, KBr, and KI were obtained from Mallinckrodt and used as received. Au(111) samples were prepared by sequential evaporation of Cr (2 ± 3 nm) and Au (150 nm) onto glass slides and a post-evaporation annealing in a hydrogen flame. [12] Electrochemical measurements: Cyclic voltammetry was conducted with a computer-controlled PAR Model 263A potentiostat. A solution of 0.6 mm Ag 2 SO 4 and 0.1m H 2 SO 4 in deionized water (Millipore, 18.2 MW) was used to deposit the Ag upd adlayer on gold. Modified electrodes were prepared by cycling once in this solution and being removed at a 300 mV versus Ag / Ag 0 under potentiostatic control. [13] The Au(111)/Ag(upd) electrodes were rinsed with deionized water and immersed into a halide test solution for varying amounts of time. The samples were rinsed with water and characterized electrochemically in a 0.6 mm Ag 2 SO 4 /0.1m H 2 SO 4 (aq) solution. A standard three-electrode configuration was used to obtain all CVs, and all potentials are quoted relative to silver wire (Ag /Ag 0 ).