ELECTRON SPIN RESONANCE INVESTIGATIONS 63 



nearly all amino acids, a number of di- and tripeptides and various 

 proteins and tissues, some of them biologically active, and some of them 

 destroyed beforehand by boiling. 



The source of irradiation was cobalt-(JO. The samples were irradiated 

 with a dose of 10^ to IC^ roentgens. The amino acids were investigated in 

 the crystal state, while proteins and tissues were in the form of lyophi- 

 lized specimens retaining a maximum of their native properties. Quanti- 

 tative measurements were carried out by comparing the areas of a 

 standard signal with that of the signal from the sample being tested. 



The structure of the free radicals fox^med by irradiating amino acids 

 especially were not investigated in detail. However, in a number of 

 cases we were able to reveal their structure sufficiently correctly and 

 completely. A more detailed study of the structure of free radicals, 

 formed by irradiating complex amino acids would require a special in- 

 vestigation with additional modification of the amino acids, for instance 

 the introduction of isotopic atoms such as deuterium, which has a 

 different nuclear spin from that of a proton and which can be introduced 

 into certain groups in amino acids. We feel, however, that such 

 additional complications would probably not help us much to solve the 

 main problem of ascertaining the peculiarities of the macrostructure 

 of the most important biopolymers, proteins and nucleic acids. 



The main experimental results may be considered: 



(i) Dry specimens of amino acids as a rule produce intensive ESR 

 spectra with a width of tens or hundi'eds of oersteds and with a clearly 

 defined hypei'fine structure, due to the interaction of unpaired electrons 

 with protons and nitrogen nuclei, which are part of the free radicals 

 produced. With a dose of about 10'^ r the yield of free radicals is about 

 1019 paramagnetic particles per gram. It is possible to assert that every 

 ionization event in the amino acid leads to the production of about one 

 free radical. As a rule, most amino acids evolve ammonia and CO2 

 following irradiation, and sulphur-containing amino acids evolve hydro- 

 gen sulphide. The spectra of some simple irradiated amino acids can be 

 interpreted as spectra of amino acid fragments which remain after 

 such decomposition products have been removed. However, the un- 

 paired electron of a sulphur-containing compound can be considered 

 to be localized near the sulphur atom; this results from the value of 

 the gr-factor for such compounds. 



It is worthwhile to note that in irradiated dry crystalline preparations 

 of amino acids ESR spectra, and that means free radicals also, have 

 been ke^Jt unchanged at room temperature (and in the presence of air) 

 for more than four years already ; however, if the ciystals are dissolved 

 in water the radicals vanish immediately. 



