Functional in vivo EPR Spectroscopy and Imaging Using Nitroxide and Trityl Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 540

16.3 Triarylmethyl (trityl) radicals

Trityl radicals have an even longer history than NRs, since the initial report by Gomberg in 1900 of the
synthesis of triphenylmethyl radical. 37 Nevertheless, only recently have these compounds, with sterically
protected trivalent carbon, regained attention as the basic structural fragment for the synthesis of stable
organic radicals. By the late 1990s, Nycomed Innovation AB further refined Gomberg's original trityl
radical in order to avoid hydrogen hyperfine coupling and enhance its stability and water solubility. 21,38
The placement of heteroatoms in these systems eliminates hydrogen atoms in close proximity to the radical
center, and consequently the numerous hyperfine splittings and broadening of the EPR signal caused by
coupling of the radical electron with the hydrogen nuclear spins. A new family of trityl spin probes was
synthesized, also known as tetrathiatriarylmethyl (TAM), bearing four sulfur atoms on the phenyl ring. The
most representative members are TAM derivatives containing carboxyl groups, namely cTAM, deuterated
cTAM, and the more hydrophilic Oxo63 and Oxo31 derivatives (Scheme 16.2). Recently, creative efforts
have been employed for improved synthesis of these complex molecules. 39,40 cTAM can now be synthesized
on a large scale in an efficient way. 40
TAMs display extraordinary stability toward tissue redox processes with life time up to 24 hours in human
blood, 21 surpassing the behavior of NRs in this regard. The EPR spectra of these TAM derivatives 21,39,40
display a very narrow single line which is generally not broadened by interaction with proteins and
other biological molecules, making these molecules particularly attractive for imaging applications using
EPRI and proton electron double resonance imaging (PEDRI) techniques. 41,42 In the latter case, the long
relaxation time of TAMs makes them easily saturatable by radio frequency (RF) irradiation and provides
an advantage over NRs for PEDRI applications allowing for enhancement of sensitivity and resolution
with less RF heating of the sample. 21,33,43 Applications of TAM radicals include EPR oximetry 21,33,43 and

recently a reported sensitivity to the superoxide anion 44,45 and pH. 46,47
In this chapter, functional applications of EPR-based spectroscopy and imaging of the NR and TAM
probes are reviewed, with particular emphasis on in vivo EPR measurement of oxygen, pH, tissue redox,
and intracellular GSH content.

16.4 In vivo EPR oximetry using nitroxyl and trityl probes

The measurements of molecular oxygen concentration in living tissues are of crucial importance for
monitoring the energetic metabolism from a physiological and pathological point of view. In certain stress
conditions, for example, high exercise levels, interruption of normal blood supply or biochemical shock, the

C

C C

HO OH

3

3 3

HO

S S

X 3 C CX 3 S S S S OH

O O

CX 3

X 3 C S S O OH

S S S S

O

HO

OH

HO

COOH

COOH

COOH

cTAM (X=H) Oxo63 Oxo31

Deuterated cTAM (X=D)

Scheme 16.2 Representative chemical structures of triarylmethyl radicals (TAMs)