Relaxation of water proton spins by paramagnetic spin-label molecules in porous glass

A water proton spin-lattice relaxation study of the spin-label molecules 2,2,6,6-tetramethyl-4-acetamido-piperidine-1-oxyl (TMAPO) solution in glass pores of 127 nm diam reveals that when the porous glass is hydrated with a 1 mM TMAPO solution, the TMAPO concentration in pore water is maintained constant during dehydration. However, it was observed that if the pore diameter is 49.1 nm or smaller, the Langmuir isotherm is not applicable, although the surface-to-volume ratio is larger. The observed proton relaxation's dependence on pore volume-to-water volume ratio V₀/V, is in excellent accord with the results of the electron paramagnetic resonance experiments and supports a model based on the fast exchange of water between nonequivalent environments.