MEASUREMENT OF PROTON RELAXATION RATES IN PROTEINS

Five different types of experiment are described which make it possible to measure various relaxation rates of selected protons in crowded spectra of macromolecules such as proteins: longitudinal spin-lattice relaxation rates rho = 1/T1, transverse relaxation rates rho(t) = 1/T2 measured under conditions of free precession, transverse relaxation rates rho(LOCK)t = 1/T1rho measured under conditions of spin-locking, and transverse relaxation rates rho(DQC) = 1/T2DQC and rho(ZQC) = 1/T2ZQC of double- and zero-quantum coherences. The surprisingly large discrepancy between the transverse rates rho(t) and rho(LOCK)t is discussed in detail. To separate overlapping proton signals, the experimental schemes involve one or several magnetization transfer steps. using a doubly selective homonuclear Hartmann-Hahn method. Numerous variants of the basic ideas can be conceived, depending on the extent of signal overlap and on the topology of the networks of scalar couplings. Applications are shown to H(epsilon) and H(delta) of Tyr23, to H(alpha), H(beta) and H(beta') of Cys30, and to H(alpha) and H(beta) of Ala24 in bovine pancreatic trypsin inhibitor (BPTI).