Abstract
Aqua Gd3+ and Gd-DOTA (gadolinium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacete) complexes were studied as a contrast agent in cellular NMR (nuclear magnetic resonance) spectroscopy for distinguishing between intracellular and extracellular spaces. The contrast agents for this purpose should provide strong paramagnetic relaxation enhancement and localize in the extracellular space without disturbing biological functions. Cell membrane permeability to Gd complexes was evaluated from the concentrations of gadolinium complexes in the inside and outside of E. coli cells measured by the 1H-NMR relaxation. The site-specific binding of the complexes to E. coli cells was also analyzed by high-resolution solid-state 13C-NMR. The aqua Gd3+ complex did not enhance T1 relaxation in proportion to the amount of added Gd3+. This Gd3+ concentration dependence and the 13C-NMR indicated that its strong cytotoxicity should be due to the binding of the paramagnetic ions to cellular components especially at the lipid membranes. In contrast, Gd-DOTA stayed in the solution states and enhanced relaxation in proportion to the added amount. This agent exhibited strong T1 contrast between the intra- and extracellular spaces by a factor of ten at high concentrations under which the cells were viable over a long experimental time of days. These properties make Gd-DOTA suitable for selectively contrasting the living cellular space in NMR spectroscopy primarily owing to its weak interaction with cellular components.
Highlights
Paramagnetic relaxation enhancement (PRE) is the acceleration of nuclear spin relaxation by magnetic interactions with unpaired electron spins
The 1H NMR spectra of glycerol-E. coli mixtures showed a strong peak of the hydroxyl proton of water and glycerol (Figure 1) as water accounts for about 70% of the sample (Supplementary Materials S1) [32]
The 1H-NMR relaxation for T1 provided the concentrations of gadolinium complexes in the inside and outside of E. coli cells, which revealed the cell membrane permeability to Gd complexes
Summary
Paramagnetic relaxation enhancement (PRE) is the acceleration of nuclear spin relaxation by magnetic interactions with unpaired electron spins. Since the magnetic interaction depends on the distance between nuclear and electron spins, this effect provides the structural information for proteins and intermolecular interactions [1,2,3,4]. The lanthanide metal ion Gd3+ has the largest number of unpaired electrons in all transition metal ions, so it should provide the strongest PRE in biological systems. The PRE effect of the Gd3+ ion is commonly utilized in MRI as a contrast agent for clinical analysis [9,10,11,12]. Some gadolinium complexes including Gd-DOTA complex have been proven to be safe for humans, it has been reported that these complexes could be internalized into human cells [19,20,21,22,23]
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