ESR spectroscopic evidence for hydration- and temperature-dependent spatial perturbations of a higher plant cell wall paramagnetic ion lattice

To study the influence of cell wall polyuronide structure on bound paramagnetic ion interactions, spin-spin coupling measurements were made on intact cell walls exchanged with a wide range fo Mn²⁺ and Cu²⁺ concentrations. These experiments were performed so that dimer-only intercationic nearest neighbor distances (d) and lattice constants (κ) could be calculated from the linewidth-concentration dependency. d values were estimated to be 12 and 14 Å for Cu²⁺ and Mn²⁺, respectively. At the maximal bound ion concentration, κ was 2.3–2.6, indicating that about 5–7 paramagnetic ion nearest neighbor spin-spin interactions occur per dipole in the nearly filled lattice. This latter observation strongly argues for the egg-box model of the cell wall-polyuronide lattice structure. Mn²⁺ linewidths of hydrated cell wall-bound paramagnetic ions displayed an unusual temperature dependency, whereby linewidths increased between 20°C and the temperature at which maximal linewidths were observed (T_max). T_max was inversely proportional to the degree of lattice hydration, indicating that the temperature dependency was not associated with the freezing of bound water. The relative change in Mn²⁺ linewidths, between 20°C and T_max, was affected by binding site-associated ¹H spin-lattice relaxation times, indicating that the temperature dependency is at least partially controlled by cell wall polyuronide structure.