R. 0. C. NORMAN AND B. C.
14
sibility of testing some recent theoretical predictions in the field of molecular biology.
Acknowledgments. This research has benefited from facilities made available by the Advanced Research Projects Agency through the Center for Materials Research at Stanford University.
GILBERT
protein structure. Minor distortions in the immediate vicinity of the label are of course inevitable. Work in progress by L. R. Berliner, J. C. A. Boeyens, and H. M. McConnell show that similar conclusions apply to a-chymotrypsin crystals spin labeled a t the active site.
Discussion
M. C . R. SYMONS(University of Leicester, Leicester). What is the distance between the nitroxide radical and the iron atom? Would one not expect to see some manifestation of the proximity of electron spins in some of the systems studied?
R. L. WALKER(Haverford College, Haverford, Pa.). Can the bulky label group interfere with normal folding of the protein chain to which it is attached? H. P*Z. ICZCCONNELL.The spin-labeled hemoglobin crystals were isomorphous with crystals of the native protein. From this we can conclude that there is no large-scale distortion of the
H. M. MCCONNELL.The distance between the radical and the heme iron is estimated to be in the 15-20-A range, and thus far we have not yet observed any effects to indicate an interaction between them. However, there is a reasonable chance that a spinspin interaction could be detected at low temperatures in methemoglobin.
Electron Spin Resonance Studies of Oxidation. IX.1
Some Interesting
Properties of Iminoxy Radicals
by R. 0.C. Norman and B. C. Gilbert Department of Chemistry, The University of Ymk,York, England
(Received September 67, 1966)
Analysis of the esr spectra of a wide range of iminoxy radicals, RR'C=NO., has enabled us t o elucidate the variations with structure of the hyperfine splitting constants of these u radicals. The results are in marked contrast with those for ?r radicals: in particular, coupling is maximal for substituents in the molecular plane, long-range transmission of spin density occurs through the u bonds, and in certain radicals with the appropriate geometry a direct orbital overlap is postulated. The direct overlap seems particularly effective for fluorine? chlorine, and bromine, for which remarkably large splittings are observed. Studies of the radicals from ortho-substituted benzaldoximes and acetophenone oximes, including the temperature and solvent dependencies of aF in the anti-iminoxy radical from o-fluoroacetophenone oxime, lead to conclusions about the conformational preferences of these radicals.
Introduction There have now been a, number of electron spin resonance studies of the iminoxy radicals, RR'C=NO., formed by the abstraction of a hydrogen atom from oximes. The radicals have been generated from the oximes by irradiation in the solid state2p3and by oxidation in solution using ceric ion4or lead tetraa~etate.~-' The J O U Tof~Physical Chemistry
Iminoxy radicals have been characterized as u radicals. The unpaired electron is contained in an or(1) Part VIII: A. L. Buley, R. 0. C. Norman, and R. J. Pritchett, J. Chem. SOC.,Sect. B , 849 (1966). (2) I. Miyagawa and W. Gordy, J. Chem. Phys., 30, 1590 (1959). (3) y. Kurita, M. Kashiwagi, and H. Saisho, 86, 578 (1965).
J. Chem, sot, Japan,
