Metal-nitroxyl interactions. 25. Exchange interactions via saturated linkages in spin-labeled porphyrins

Abstract

A series of spin-labeled copper porphyrins has been prepared in which nitroxyl radicals are attached toa porphyrin pyrrole ring by propionamide and propionate ester linkages. The solution epr spectra at room temperature indicate that the values of the electron ..spin-electron spin-coipling constant, J , for these compounds are in the range 4 to 18 G, dependingon the solvent and the size of the nitroxyl ring. Below -45°C two isomers with different values of J were observed in the epr spectra of two of the compounds. Variable temperature epr spectra were also analyzed for analogous spin-labeled copper porphyrins which have cis and trans acrylic ester and acrylamide linkages between the porphyrin and the nitroxyl ring.

Cross polarization and magic angle sample spinning NMR spectra of model organic compounds. 2. Molecules of low or remote protonation

“…Preliminary investigations (6000 acquisitions) with 1.00 < rcp < 4.50 ms also showed that shorter contact times resulted in the carbonyl atom contributing an even smaller percentage of the total signal intensity. As in diphenylcyclopropenone (3), the multitude of resonances for the phenyl groups (Figure 5) indicates that the crystalline environment is removing the nominal spatial equivalence found for chemically equivalent carbon atoms related to each other by unimolecular symmetry elements.…”

Metal-nitroxyl interactions. 46. EPR spectra of low-spin iron(III) complexes of spin-labeled tetraphenylporphyrins and their implications for the interpretation of EPR spectra of spin-labeled cytochrome P450

“…It has been shown previously that "Wplan" conformations of saturated linkages between metals and nitroxyls give rise to substantially larger exchange interactions than other conformations. 15 The size of the nitroxyl ring and the presence or absence of a methyl group on the imidazole could influence the steric interactions that determine the conformation of the CH2 group and thereby cause changes in the values of J.…”

Metal-nitroxyl interactions. 35. Conformational effects on spin-spin interaction in spin-labeled copper salicylaldimines

“…The maximum spin delocalization at = 180°is similar to the "W plan", which has been observed to favor long-range spin-spin interactions. 3,17 Inspection of molecular models indicated that for both the Cu(salen-NO) and Cu(salpn-NO) derivatives, steric constraints favor an equatorial position for the bulky spin-labeled amide groups. However, in both cases the hydrocarbon backbone is substantially distorted from that of a simple ethylene linkage due to the constraints of the salicylaldimine coordination.…”

“…We have recently observed that metal-nitroxyl spin-spin coupling constants (J), which can be obtained from fluid solution EPR spectra, are sensitive probes of conformational changes. [1][2][3] We are therefore exploring the extent to which changes in the value of J can be used to monitor structural changes in solution. The information obtained from J complements that which can be obtained from ENDOR and ESEEM studies.…”

“…are given in units of gauss in the following discussion. The conversion between hertz and gauss is given by eq [2][3][4]. gt and g2 are the copper and nitroxyl g values, respectively.…”