Aqueous relaxation reagents in nitrogen-15 NMR

Abstract

Electron-nuclear relaxation times ( T 1 2′ s) for 15N and 13C in natural abundance are measured for a series of amines in aqueous solution using Gd(III) complexes of a series of polyaminocarboxylate ligands as paramagnetic relaxation reagents (PARRs). The PARRs are classified by their predominant mode of interaction with the amine substrates (i.e., specific or non specific). The specific PARRs are evaluated qualitatively as NMR spin labels through their selectivity as measured by 15N T e 1′s toward subtrates of different Lewis base strength and through the degree of scalar line broadening induced in the substrate resonances. Additionally, the aqueous PARRs are compared with better characterized nonaqueous PARRs, Cr(acac) 3 and Gd(dpm) 3. It is concluded that the diethylenetriaminepentaacetic acid complex, Gd(DTPA) 2−, is a satisfactory specific PARR and NMR spin label. The triethylenetetraaminehexaacetic acid complex, Gd(TTHA) 3−, is shown to be a useful non specific PARR although it is a charged complex; the presence of charged groups in substrates results in weak spin label effects. Finally, the behaviour of these PARRs is contrasted with that of hydrated transition metal and lanthanide ions.