Characterization of the five-fold Ca 2+ binding site of satellite tobacco necrosis virus using Eu 3+ luminescence spectroscopy: A marked size-selectivity among rare earth ions

Abstract

Satellite tobacco necrosis virus (STNV) is an icosahedral virus which contains three classes of Ca 2+ binding site. One of these classes, a five-fold carbonyl site which is believed to exist in a Ca 2+ channel, has been investigated using laser-induced Eu 3+ luminescence spectroscopy. These twelve identical sites are rather rigid, as evidenced by the single narrow (full width at half-maximum is 6.5 cm −1) band observed at 579.58 nm in the 7 F 0 → 5 D 0 excitation spectrum of the EU 3+−STNV complex. Lifetimes of 270 μs in H 2O and 1620 μs in D 2O indicate that there are three water molecules bound to the Eu 3+ at this site. Ligand field splittings of the 7 F 0 → 5 D 1 and 7 F 0 → 5 D 2 excitation spectra show that this site possesses fairly high symmetry (≤ C 5V). The Eu 3+ complex of nitrilotriacetic acid was determined via titration to have a dissociation constant, K d, of 20 ± 2 n M; this value has been used in competition experiments to deduce that the virus site class binds Eu 3+ with a K d of 1.1 ± 0.3 n M. This putative ion channel demonstrates remarkable size selectivity, with lanthanide affinities varying by more than one order of magnitude.