Azaadamantyl nitroxide spin label: complexation with β-cyclodextrin and electron spin relaxation.

An iodoacetamide azaadamantyl spin label was studied in fluid solution and in 9:1 trehalose:sucrose glass. In 9:1 toluene:CHCl solution at 293 K, the isotropic nitrogen hyperfine coupling is 19.2 G, T is 0.37 µs and T is 0.30-0.35 µs. Between about 80 and 150 K 1/T in 9:1 trehalose:sucrose is approximately independent of temperature demonstrating that the absence of methyl groups decreases 1/T relative to that which is observed in spin labels with methyl groups on the alpha carbons. Spin lattice relaxation rates between about 80 and 293 K in 9:1 trehalose:sucrose are similar to those observed for other nitroxide spin labels, consistent with the expectation that relaxation is dominated by Raman and local mode processes. Although complexation of the azaadamantyl spin label with β-cyclodextrin slows tumbling in aqueous solution by about a factor of 10, it has little impact on 1/T or 1/T in 9:1 trehalose:sucrose between 80 and 293 K.