Vol 50, No 7 (2019)

Our electron paramagnetic resonance (EPR) studies have demonstrated that at 293 K and 77 K, the spin–lattice relaxation time, T₁, of paramagnetic mononuclear dinitrosyl iron complexes (M-DNICs) with glutathione and hydroxyl ligands containing isotopes ⁵⁷Fe and ⁵⁶Fe notably exceeds the halflife of the Mössbauer transition, i.e., the lifetime of the ⁵⁷Fe nucleus in the first excited state (10⁻⁷ s). The Mössbauer spectra of M-DNIC with hydroxyl ligands, binuclear DNIC with glutathione (B-DNIC) and sodium dithionite-treated solution of B-DNIC with glutathione did not display the presence of the magnetic hyperfine structure (MHFS) characteristic of M-DNIC with glutathione. The Mössbauer spectra of all these DNICs were characterized by quadrupole splitting. The results of a comprehensive comparative analysis of MHFS of M-DNIC with glutathione and that in DMF reduced sodium nitroprusside suggest that M-DNIC with glutathione have a low-spin (S = ½) d⁷ electronic configuration with the predominant localization of the unpaired electron on the d_z² orbital of iron. This conclusion is fully consistent with the results of our previous studies of M-DNIC using the EPR method.

A number of Ir–N-heterocyclic carbene (Ir–NHC) complexes with asymmetric N-heterocyclic carbene (NHC) ligands have been prepared and examined for signal amplification by reversible exchange (SABRE). Pyridine was chosen as model compound for hyperpolarization experiments. This substrate was examined in a solvent mixture using several Ir–NHC complexes, which differ in their NHC ligands. The SABRE polarization was created at 6 mT and the ¹H nuclear magnetic resonance signals were detected at 7 T. We show that asymmetric NHC ligands, because of their favorable chemistry, can adapt the SABRE active complexes to different chemical scenarios.

The stretched exponential function (SEF) was used to analyze and interpret saturation recovery (SR) electron paramagnetic resonance (EPR) data obtained from spin-labeled porcine eye-lens membranes. This function has two fitting parameters: the characteristic spin–lattice relaxation rate (T_1str⁻¹) and the stretching parameter (β), which ranges between zero and one. When β =1, the function is a single exponential. It is assumed that the SEF arises from a distribution of single exponential functions, each described by a T₁ value. Because T₁⁻¹s are determined primarily by the rotational diffusion of spin labels, they are a measure of membrane fluidity. Since β describes the distribution of T₁⁻¹s, it can be interpreted as a measure of membrane heterogeneity. The SEF was used to analyze SR data obtained from intact cortical and nuclear fiber cell plasma membranes extracted from the eye lenses of 2-year-old animals and spin labeled with phospholipid and cholesterol analogs. The lipid environment sensed by these probe molecules was found to be less fluid and more heterogeneous in nuclear membranes than in cortical membranes. Parameters T_1str⁻¹ and β were also used for a multivariate K-means cluster analysis of stretched exponential data. This analysis indicates that SEF data can be assigned accurately to clusters in nuclear or cortical membranes. In future work, the SEF will be applied to analyze data from human eye lenses of donors with differing health histories.