Measured electron spin relaxation rates in frozen solutions of azurin: Vitamin B12r and nitrosyl ferrous myoglobin
Measured electron spin-lattice relaxation rates in frozen glycerin/water solutions at temperatures from 1.4 K to 20 K are reported for a copper-containing protein, azurin, and a cobalt -containing bi omol ecul ar complex, vitamin B 12r, which is the paramagnetic product of the photolysis of coenzyme B 12. The results are interpreted in terms of a spectral dimensianality. Rates are also reported for nitrosyl ferrous myoglobin in frozen water solution, which ex hi bits a dominant one-phonon relaxation process at temperatures up to 20 K and thus provides no data from which to determine the spectral dimensionality. The aim of this study was to determine whether the anomolous variation of the relaxation rate with temperature observed in several iron-containing proteins is also present in proteins and large molecular complexes containing other paramagnetic ions. Within a model involving a two-phonon mechanism of relaxation, the anomalous temperature variations can be conveniently specified by a spectral dimensionality. The spectral dimensionality, m, is a parameter that indicates the variation of the vibrational density of states with frequency, i.e. p(v) ex: vm-1. It is named in analogy with the Debye vibrational density of states of crystalline solids in 1-, 2-, and 3-dimension a 1 space. At sufficiently high temperatures, a non-resonant, two-phonon relaxation mechanism, the Raman process, should dominate the relaxation of a paramagnetic ion unless low-lying (under 70 cm-1) electronic states are present. Low-lying excited states are common among rare earths and high spin ferric complexes, including many ferric proteins. The temperature dependence of a Raman relaxation rate for a Kremers ion (odd number of electrons) is T3+2m H the spectral dimensionality 1s m and the temperature is sufficiently low in comparison wHh E> = h Vmaxlk, the De bye temperature. The maxi mum values of m extracted from relaxation data on heme proteins, iron-sulfur proteins, and one copper-and-iron-containing protein, have ranged from about 1.3 to 1.8. The values of m obtained from frozen solutions of a given protein have sometimes been dependent upon sol vent conditions. The relaxation rates were measured by the pulse saturation/recovery technique. The temporal profiles of the recovery signals reported here for azurin and vitamin B 12r could not be characterized as exponential over their ent 1re recoveries. However, a system aU c estimate of the recovery rates could be developed by inspecting the semllogarithmic displays of the recovery signals. the temperature dependence of the relax at 1 on rates for azur1 n bet ween 1.5 K and 22 K can be fit quite n1cely with a spectral dimensionality of 3 and a rather 1 ow Debye temperature of approximately 69 K. Th1 s result is very different from previous relaxation data on 1ron proteins. Relaxation data from the vitam1 n B 12r samples varied substantially between samples, indicating that some crucial parameters in the photolysis and freezing were not being controlled. The Raman contribut1ons to these data were generally well fit by a s1mple power law in temperature, but the values of m var1 ed from 1. 14 to 1.48 with sample preparations.
electron spin relaxation, frozen solutions of azurin, vitamin B12, nitrosyl ferrous myoglobin
University of Illinois - Urbana-Champaign
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