The Effects of Paramagnetic Relaxation Reagents on 15N Spin Relaxation and the Use of Gd(dpm)3 as a Nitrogen-15 Nuclear Magnetic Resonance Spin Label

摘要

Electron-nuclear relaxation times (T(1) sup e's) for (15)N and (13)C in natural abundance are measured for a series of amines of a wide range of pK(a)s using four paramagnetic relaxation reagents that are soluable in organic solutions. Cr(acac)3 and Cr(dpm)3 are seen to affect the nuclear spin predominantly via an outer sphere mechanism, or through normal translational diffusion when no interactions occur. Gd(dpm)3 and Gd(acac)3 are observed to be specific for the basic sites in substrate molecules where relaxation rate enhancement is seen to be strongly dependent on the availability of the lone electron pair. Variable concentration and temperature studies are used to separate the various contributions to T(1) sup e. (15)N T(1) sup e's are used as one measure of spin labeling; changes in the negative nuclear Overhauser effect (NOE) of proton-decoupled (15)N spectra can also be used in some cases. Examples are presented illustrating spin labeling due to both basicity and steric effects.