Chemicals Gold shot was purchased from precious metal vendors (Canadian Maple Leaf, 99.99%) and was initially converted to HAuCl 4 ·3 H 2 O by boiling Au 0 in HCl/HNO 3 solution [27] . N-(2-Mercaptopropionyl)-glycine (tiopronin, reagent grade), bovine serum albumin (BSA, fraction V, 96%), and sodium phosphate (monobasic, reagent grade) were purchased from Sigma. Other chemicals were obtained as follows: protein G was purchased from Southern Biotech, Fmoc-dPEG-COOH from Quanta Biodesign, anti-FLAG mouse monoclonal antibody from Stratagene, and anti-HA mouse monoclonal antibody from the Vanderbilt Molecular Recognition Core facility. The following reagent was obtained through the NIH Biodefense and Emerging Infections Research Resources Repository, NIAID, NIH: H5 HA protein from influenza virus, A/Hong Kong/156/97 (H5N1), recombinant from baculovirus, and NR-652 (56 kDa) [28] . Analytical grade solvents for nuclear magnetic resonance (NMR) were obtained from Cambridge Isotope Laboratories, and water was purified using a Modulab Water Systems unit (∼18 MΩ/cm). Buffers were prepared according to standard laboratory procedure. Other chemicals were reagent grade and used as received.