MEASUREMENT OF PROTON RELAXATION RATES IN PROTEINS

Five different types of experiment are described which make it possibl e to measure various relaxation rates of selected protons in crowded s pectra of macromolecules such as proteins: longitudinal spin-lattice r elaxation rates rho = 1/T1, transverse relaxation rates rho(t) = 1/T2 measured under conditions of free precession, transverse relaxation ra tes rho(LOCK)t = 1/T1rho measured under conditions of spin-locking, an d transverse relaxation rates rho(DQC) = 1/T2DQC and rho(ZQC) = 1/T2ZQ C of double- and zero-quantum coherences. The surprisingly large discr epancy between the transverse rates rho(t) and rho(LOCK)t is discussed in detail. To separate overlapping proton signals, the experimental s chemes involve one or several magnetization transfer steps. using a do ubly selective homonuclear Hartmann-Hahn method. Numerous variants of the basic ideas can be conceived, depending on the extent of signal ov erlap and on the topology of the networks of scalar couplings. Applica tions are shown to H(epsilon) and H(delta) of Tyr23, to H(alpha), H(be ta) and H(beta') of Cys30, and to H(alpha) and H(beta) of Ala24 in bov ine pancreatic trypsin inhibitor (BPTI).