252 Walter Gordy 



be established by variation of the modulation amphtude and observation 

 frequency. The outermost bend points will in general correspond to g,^ and g^^. 



III. FREE RADICALS IN IRRADIATED AMINO 

 ACIDS AND SIMPLE PEPTIDES 



The work of our group at Duke University has revealed that the isotropic 

 ^orbital contributions of hydrogen atoms in aliphatic hydrocarbon radicals are 

 very significant and that they give rise to hyperfine structure in the spin resonance 

 of these radicals v/hich is frequently of the order of 100, and sometimes as 

 much as 200, gauss. This couphng is an order of magnitude greater than that 

 generally found for the aromatic ringed radicals (14, 15, 20) which can be 

 prepared chemically and observed in solution. Furthermore, the first measure- 

 ments indicated, and later work on single crystals (21) confirmed, that the 

 isotropic j-orbital coupling to the hydrogen nuclei in aliphatic hydrocarbon 

 radicals is generally much greater than the orientation-dependent, dipole-dipole 

 component. This very fortunate circumstance makes possible detection and 

 often identification of the aliphatic hydrocarbon radicals made by irradiation 

 of solid matter in the polycrystalline powder and even in impure biological 

 solids. In other words, it seems possible with microwave spectroscopy to 

 'fingerprint' many of the common radicals produced within soUd matter by 

 irradiation. I need not emphasize the usefulness of such a set of fingerprints 

 for the study of radiation damage. 



There are two important factors which we beheve to be mainly responsible 

 for the reduction of the anisotropic nuclear coupling in hydrocarbon radicals. 

 One of these is the spreading of the odd electron density over a large molecular 

 orbital so that there is no appreciable fraction of the total density near a parti- 

 cular nucleus. The other is the twisting, turning, tunneUng, tumbling, or other 

 motion of the radicals, or their parts, within the solid cages in wliich they are 

 trapped. The first is generally more important for large radicals than for 

 small ones, and the latter is generally more important for room temperature 

 and elevated temperatures than for lower ones. 



These properties of aliphatic free radicals and their remarkably long Hfetime 

 within solids were not predicted by theory. The conclusions were forced upon 

 us from the experimental evidence for them. Furthermore, this pronounced 

 isotropic interaction through the 5-orbitals immediately gives much information 

 about the electronic wave functions and structure of hydrocarbon radicals. The 

 large coupling to the H nuclei in the CH3 radical (total spread of quartet 70 

 gauss) indicates that this radical is not planar. Amazingly, the characteristic 

 pattern of the ethyl free radical, C2H5, is a symmetrical sextet (or approximately 

 so) of about 130 gauss spread. This indicates equivalent, or nearly equivalent, 

 coupling to the electron spin of all five protons. 



Fig. 5 illustrates some characteristic hyperfine patterns of hydrocarbon 

 radicals produced by x-irradiation of some simple peptides. Compare these 

 with the theoretical patterns for different numbers of equally coupling protons 

 in Fig. 2. Similar patterns have been obtained by irradiation of amino acids (4) 

 and other compounds (6, 22, 23) with x-rays and with ultraviolet light (24). 



Figs. 6 and 7 illustrate somewhat more complex resonances. 



