Spin Probes and Spin Labels

Abstract

Electron paramagnetic resonance (EPR) spectroscopy is a versatile and exceptionally sensitive technique for detecting and studying molecular systems possessing unpaired electronic spins. It finds substantial use in characterizing paramagnetic metal ions and organic free radicals. The scope of EPR applications in analytical chemistry can be expanded significantly by means of spin labels and spin probes — exogenous EPR‐active species that are introduced solely for the purpose of gaining information on the system under study. These labels and probes are persistent free radicals or paramagnetic metal ions designed to be partitioned or attached to specific molecular sites. To be suitable for analytical applications, spin labels and probes should readily report on properties of interest, exhibit distinguishable from background EPR signals, and leave the system only minimally perturbed by their presence. In this article, we describe a series of spin labels and probes as well as the associated EPR methods that are tailored to specific analytical applications of EPR spectroscopy — from assessing local oxygen and proton concentration to profiling electrostatic properties of biopolymers and lipid bilayers. The primary focus is bioanalytical applications of spin label/probe methods, with an emphasis on the synergy between the rational design of spin‐bearing molecules using the tools of organic chemistry and desired analytical capabilities.

Proton Activity in Nanochannels Revealed by Electron Paramagnetic Resonance of Ionizable Nitroxides: A Test of the Poisson–Boltzmann Double Layer Theory

“…Acid–base properties of organic and inorganic porous materials are of great interest from perspectives of fundamental understanding of molecular processes in heterogeneous catalysis and chemical adsorption as well as numerous practical applications including chromatography. − These processes occur at the channel surfaces and are known to be affected by both the chemical nature of solutions and surface properties of these materials. In the past, the classic method of acid strength measurements based on monitoring the color changes of adsorbed dye indicators (Hammett indicators) upon titration as well as microcalorimetry and thermal programmed desorption measurements of other basic and/or acidic probes of different strength absorbed on catalysts was employed to determine the concentration and strength of the acid sites. − A number of spectroscopic methods including Fourier-transform infrared spectroscopy (FTIR), magic angle spinning, NMR, IR, photoluminescence, Raman, UV–vis, X-ray photoelectron spectroscopy, and electron paramagnetic resonance (EPR) have been also successfully applied to some of catalytic mesoporous materials in order to identify and characterize acid–base properties of the surface sites. − While each of these spectroscopic methods has its own pros and cons, EPR spectroscopy of ionizable nitroxides − has the benefits of being applicable to nontransparent/turbid materials. Also, spin-probe EPR generally exhibits better concentration sensitivity than solid-state NMR.…”

“…The EPR method is based on a spectroscopic observation of reversible protonation of stable free radicals such as, for example, pH-sensitive nitroxide radicals (NRs) − or triarylmethyl radical derivatives . Such probes provide measurements of local pH (pH loc ) when either absorbed by the surfaces or being dissolved in a liquid phase trapped by the pores.…”

EPR Oximetry with Nitroxides: Effects of Molecular Structure, pH, and Electrolyte Concentration

“…( 3) when combined with leastsquares fitting provides efficient means of extracting Lorentzian broadening parameter ΔB p−p regardless of the spectral lineshape. This eliminates the needs of introducing empirical calibrations of relative peak intensities such as carried out by Subczynski and Hyde for nitroxide CTPO (3-carbamoyl-2,2,5,5-tetramethyl-3-pyrroline-1-oxyl) [10] and makes the method broadly applicable to all types of nitroxides and regimes of molecular tumbling [23,41]. Among all nitroxides studied here, the largest effect of molecular oxygen on the EPR spectrum is expected for the nitroxide with the narrowest overall lines-1 (DTBN).…”

“…The main goals of these continuing research efforts are improvements in sensitivity and resolution of the EPR method especially for studies in vivo (e.g., [15,16,[19][20][21]). The field was also reviewed extensively (e.g., [22][23][24]).…”

4-Dialkylamino-2,5-dihydroimidazol-1-oxyls with Functional Groups at the Position 2 and at the Exocyclic Nitrogen: The pH-Sensitive Spin Labels

“…EPR spectroscopy of ionizable nitroxides is a convenient method for the investigation of the above-mentioned phenomena [ 2 , 4 , 5 ], and is fully applicable to opaque or turbid materials [ 5 , 6 ]. Nitroxide spin probes are small enough to penetrate directly into the pores and to be adsorbed onto the surface of the material under study.…”