Measuring the longitudinal NMR relaxation rates of fast relaxing nuclei using a signal eliminating relaxation filter

A new experiment for selective determination of the relaxation rates of fas t relaxing NMR signals is presented. The experiment is derived from the con ventional inversion recovery experiment by substituting the 180 degrees inv ersion pulse of this experiment with a signal eliminating relaxation filter (SERF) consisting of three 180 degrees pulses separated by two variable de lays, Delta (1) and Delta (2). The SERF experiment allows a selective suppr ession of signals with relaxation rates below a given limit while monitorin g the relaxation of faster relaxing signals. The experiment was tested on a sample of 20% oxidized plastocyanin from Anabaena variabilis, where the fa st exchange of an electron between the reduced (diamagnetic) and the oxidiz ed (paramagnetic) form results in a series of average signals with widely d ifferent relaxation rates. To ensure an optimum extraction of information f rom the experimental data, the relaxation rates were obtained from the SERF experiment by a simultaneous analysis of all the FIDs of the experiment us ing a fast linear prediction model method developed previously. The reliabi lity of the relaxation rates obtained from the SERF experiment was confirme d by a comparison of the rates with the corresponding rates obtained from a conventional inversion recovery experiment. (C) 2001 Academic Press.