Trityl radicals as spin labels

During the last 30 years, tetrathiatriarylmethyl (TAM) radicals have been widely used as spin probes for oxymetry in electron paramagnetic resonance (EPR) spectroscopy and EPR tomography because of the narrow EPR linewidth and high stability in living systems. Recently, some researchers proposed to use TAMs as spin labels for studies on the structure of proteins and nucleic acids using site-directed spin labeling (SDSL) and pulsed dipolar EPR spectroscopy. In this chapter, the peculiarities of applying TAMs as spin labels, including the synthesis of TAMs, approaches to TAM spin labeling of proteins and nucleic acids, the methods for distance measurement using TAM spin labels, electron spin relaxation time, room temperature measurements, and advantages of orthogonal spin labeling are reviewed. Examples of applications of TAMs to research on the structure and functions of biopolymers are presented from recently published papers.