Study of slow molecular motions in α-Crystallin by proton magnetic spin-lattice relaxation in the doubly rotating frame

Abstract

Proton magnetic spin-lattice relaxation in the effective field H2 acting in the doubly rotating frame (DRF) was first applied to the study of slow internal protein dynamics in the submillisecond range of correlation times in the solid state. In this method the local dipolar magnetic field is reduced by the magic-angle rotating-frame method so that the resonance frequency of the relaxation experiment may be set below the value of the local field. As a result, unachievable by the standard nuclear magnetic resonance (NMR) relaxation techniques, slow molecular motions become experimentally accessible. The second effective field H2 is produced by the shallow sine-wave phase modulation of the H1 pulse. The registration of the DRF spin-lattice relaxation signal takes place directly during the continuous H1 pulse by means of an additional low-frequency radio-frequency coil oriented along the H0 field and operating at the rotating-frame NMR frequency of 100 kHz. The measurements of the spin-lattice relaxation time in the DRF within a wide temperature range have been performed in dry and hydrated α-crystallin powders. This is the major protein in the eye lens, which prevents the uncontrolled aggregation of proteins and keeps the lens transparent. The results demonstrate that the protein hydration does not change the amplitude of slow side-chain motions but significantly shortens its correlation time: from about 50 to about 0.5 μs in dry and hydrated samples, respectively. The hydration also decreases the activation energy and restricts the distribution of the correlation times.