Synthesis and Electron Spin Relaxation of Tetracarboxylate Pyrroline Nitroxides

Abstract

We report the design, synthesis, and electron spin relaxation properties of hydrophilic tetracarboxylate ester pyrroline nitroxides 1 and 2, which serve as models in the search for new spin labels for DEER distance measurement at room temperature. The nitroxides are designed to have the methyl groups further away from the N–O spin site to decrease the inequivalent couplings of the unpaired electron to the methyl protons that shorten Tm at T > 70 K in currently used labels. The key step in the synthesis of 1 and 2 is the reaction of the dianion of pyrrole-1,2,5-tricarboxylic acid tert-butyl ester dimethyl ester with electrophiles such as methyl chloroformate and methyl bromoacetate. Structures of 1 and 2 are confirmed by X-ray crystallography. Studies of electron spin relaxation rates in rigid trehalose/sucrose matrices reveal approximately temperature independent values of 1/Tm for 1 and 2 up to about 160 K and modest temperature dependence up to 295 K, demonstrating that increasing the distance between the nitroxide moiety and methyl groups is effective in lengthening Tm at T > 70 K.

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

“…The Hamiltonian parameters obtained by simulation of the spectra are in Table 2, along with the values reported previously for 1 and 2 in 9:1 trehalose:sucrose [5]. These g values are similar to those for a wide range of nitroxides [8, 23, 24, 25].…”

“…The preparation of 9:1 trehalose:sucrose glasses was performed as reported previously [5] with a 2000:1 molar ratio of sugars to 3 . The addition of sucrose to the trehalose enhances glass formation.…”

“…Spirocyclohexyl groups are relatively bulky and hydrophobic so they are not optimal for attachment to biomolecules. Although there are methyl groups in the substituents on the alpha carbons of spin labels 1 and 2 , the hyperfine couplings to these remote protons are much smaller than for the directly-bound methyls of MTSL so rotation of the methyl groups in 1 or 2 have much less impact on spin echo dephasing than the methyls of MTSL [5]. As a further step in characterizing spin labels without methyl groups, the azaadamantyl spin label 3 has been prepared.…”

Validation of Structural Grounds for Anomalous Molecular Mobility in Ionic Liquid Glasses

“…Since deuteron has a ca. 6.5 times smaller gyromagnetic ratio than the proton, replacement of CH 3 groups by CD 3 groups should significantly suppress the magnetic relaxation induced by rotations of these groups [ 68 , 69 ].…”

Reduction kinetics and electrochemistry of tetracarboxylate nitroxides

“…We found that, when immobilized in trehalose/sucrose glass, these nitroxides possessed relatively long electron spin phase memory times ( T m ) and short spin–lattice relaxation times ( T 1 ) at room temperature [18]. Therefore, spin labels derived from 4 and 5 should provide highly promising tools for determination of distances by DEER (or DQC) in doubly labelled biomolecules at room temperature [19-21]. …”