IDENTIFICATION OF SLOW MOTIONS IN THE REDUCED RECOMBINANT HIGH-POTENTIAL-IRON-SULFUR PROTEIN-I (HIPIP-I) FROM ECTOTHIORHODOSPIRA-HALOPHILA VIA N-15 ROTATING-FRAME NMR RELAXATION MEASUREMENTS

Rotating-frame N-15 relaxation rate (R-1 rho) NMR experiments have bee n performed in order to study the dynamic behavior of the reduced reco mbinant high-potential iron-sulfur protein iso I (HiPIP I) from Ectoth iorhodospira halophila, in the mu s to ms time range. Measurements of R-1 rho were perfomed as a function of the effective spin lock magneti c field amplitude by using both on and off-resonance radio frequency i rradiation. The two data sets provided consistent results and were fit globally in order to identify possible exchange processes in an exter nal loop of the reduced HiPIP I. The loop consists of residues 43-45 a nd the correlation time of the exchange process was determined to be 5 0 +/- 8 mu s for the backbone nitrogen of Gln 44.