JAMESW. SINCLAIR AND MELVIN W. HANNA
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determined by Simpson and Marsh,I2 and the direction cosines which the a-carbon-hydrogen bond makes with the three crystallographic axes are 0.059,0.96, and 0.21, re~pective1y.l~If, as is usually the case, the direction of greatest positive anisotropic coupling lies along the C-H bond, then a consideration of the data in Table I1 shows that in the intermediate fragment, the C-H bond has the same orientation as it does in the undamaged molecule. The differences in the a-proton hyperfine couplings between the intermediate and roomtemperature fragments also suggest that the hybridization around the a-carbon atom is different in the intermediate fragment than in the room-temperature fragment. The smaller anisotropic couplings would be consistent with the odd electron orbital having some sp3 hybrid character since that would decrease the value of ( l / r 3 ) between the odd electron and the proton. Thus, in making the transition between the intermediate and room temperature fragments, the alanine radical both changes its orientation in the crystal and its hybridization about the a-carbon atom. Many of the orientation and hybridization questions will be resolved by a CY3 study which is currently in progress. There is a convenient mechanism which can be written to rationalize the formation of the intermediate fragment (111) from the low-temperature fragment (11) that does not involve the formation of an additional paramagnetic species. This mechanism involves loss of free ammonia as shown in (B).
Thus, the general mechanism for radiation damage in amino acids that is suggested by this work is the following.
(1) Ionization of an electron from one alanine molecule leaves a positive hole and the negative ion I. (2) The negative ion decays according to reactions A and B. (3) The positive ion decays even at 77°K according to the well-known reaction +NE3
I
a"+
CHaCHCOz.
I +CHaCH.
+ COz
IV
The ethyl ammonium radical IV is not observed because of the fact that the epr intensity is spread among so many lines compared with radicals I and 11. It is significant that this mechanism predicts the formation of both of the principle fragments in glycine.'.S
Discussion C. THOMSON (University of St. Andrews, Scotland). Have you any data on Cla splittings in these species, both in the -COOH group and the CHaCH- group?
J. W. SINCLAIR.We are currently preparing C13-labeled materials and will have these data in the near future. M. C. R. SYMONS (Leicester University). For the radical
H
RC022- or RC
