Rotational correlation times of 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy spin label with respect to heme and nonheme proteins

Noncovalent spin labeled proteins (ovalbumin, bovine serum albumin, hemoglobin, and cytochrome c) were investigated in order to follow the different type of interactions between the nitroxide radical of 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy spin label and functional groups of heme and nonheme proteins as well as the pH influence on molecular motion of the label with respect to these proteins. EPR spectra were recorded at room temperature and the computer simulation analysis of spectra was made in order to obtain the magnetic parameters. Noncovalent labeling of proteins can give valuable information on the magnetic interaction between the label molecule and the paramagnetic center of the proteins. The relevance of this interaction can be obtained from line shape analysis: computer simulations for nonheme proteins assume a Gaussian line shape, whereas for heme proteins, a weighted sum of Lorentzian and Gaussian components is assumed. In the framework of the "moderate jump diffusion" model for rotational diffusion, the rotational correlation time is strongly influenced by pH, because of the electrostatic interactions and hydrogen bonding.