Dynamics in the Mn2+ binding site in single crystals of concanavalin A revealed by high-field EPR spectroscopy.
EPR spectroscopy at 95 GHz was used to characterize the dynamics at the Mn(2+) binding site in single crystals of the saccharide-binding protein concanavalin A. The zero-field splitting (ZFS) tensor of the Mn(2+) was determined from rotation patterns in the a-c and a-b crystallographic planes, acquired at room temperature and 4.5 K. The analysis of the rotation patterns showed that while at room temperature there is only one type of Mn(2+) site, at low temperatures two types of Mn(2+) sites, not related by any symmetry, are distinguished. The sites differ in the ZFS parameters D and E and in the orientation of the ZFS tensor with respect to the crystallographic axes. Temperature-dependent EPR measurements on a crystal oriented with its crystallographic a axis parallel to the magnetic field showed that as the temperature increases, the two well-resolved Mn(2+) sextets gradually coalesce into a single sextet at room temperature. The line shape changes are characteristic of a two-site exchange. This was confirmed by simulations which gave rates in the range of 10(7)-10(8) s(-1) for the temperature range of 200-266 K and an activation energy of 23.8 kJ/mol. This dynamic process was attributed to a conformational equilibrium within the Mn(2+) binding site which freezes into two conformations at low temperatures.
Carmieli
Raanan
R
Department of Chemical Physics, The Weizmann Institute of Science, Rehovot, Israel 76100.
Manikandan
Palanichamy
P
Epel
Boris
B
Kalb
A Joseph
AJ
Schnegg
Alexander
A
Savitsky
Anton
A
Möbius
Klaus
K
Goldfarb
Daniella
D
D001665:Binding Sites; D003208:Concanavalin A / Q000737:chemistry / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D008345:Manganese / Q000378:metabolism*
D003208:Concanavalin A; D008345:Manganese