Paramagnetic relaxation of protons in rotationally immobilized proteins

The proton magnetic relaxation dispersion profiles are reported over the proton Larmor frequency range from 0.01 to 30 MHz for cross-linked gels and for the dry lyophilized bovine serum albumin covalently labeled at lysine with diethylenetriaminepentaacetic acid chelates of either Gd(III) or Mn(II) ions. The proton spin-lattice relaxation dispersion for the cross-linked paramagnetic protein gel is accurately represented as a sum of two major relaxation contributions. The diamagnetic term is a power law from the magnetic field dependence of the protein protons. The paramagnetic term is approximately described by the Solomon-Bloembergen-Morgan class of models. However, the paramagnetic relaxation mechanism in the dry lyophilized protein is fundamentally different and we develop a new quantitative description of the dispersion profile. In the dry case, no peak in the proton relaxation dispersion profile is detected from the field dependence of the electron spin relaxation times. The high-field paramagnetic relaxation dispersion is a power law in the Larmor frequency with an exponent of -0.8, which results from modulation of the electron-nuclear coupling by the intramolecular dynamics of the protein which primarily propagates along the primary structure of the protein. The low-field plateau is caused by the interruption of the electron-nuclear spin correlation by electron spin relaxation. This new quantitative description provides a simple approach to the measurement of electron spin-lattice relaxation times in paramagnetic protein systems at room temperature based on the magnetic field dependence of the proton spin-lattice relaxation rate constant.