Measurement of proton relaxation rates in proteins

Summary

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 ρ=1/T₁, transverse relaxation rates ρ=1/T₂ measured under conditions of free precession, transverse relaxation rates ρ¹ _LOCK=1/T_{1 ρ} measured under conditions of spin-locking, and transverse relaxation rates ρ^DQC=1/T₂ ^DQC and ρ^ZQC=1/T₂ ^ZQC of double- and zero-quantum coherences. The surprisingly large discrepancy between the transverse rates ρ^t and ρ^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^ε and H^δ of Tyr²³, to H^α, H^β and H^β′ of Cys³⁰, and to H^α and H^β of Ala²⁴ in bovine pancreatic trypsin inhibitor (BPTI).