A Comparison of the Effects of X-Rays and Alpha Rays on Some Proteins and Amino Acids in Dilute Aqueous Solution

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Abstract
Serum albumin, ribonuclease, tryptophan, tyrosine, and some other amino acids in dilute aqueous solution were irradiated with X- and [alpha]-rays, and changes were observed in sedimentation of serum albumin and in the absorption spectra of all the materials. For serum albumin the effects of X- and [alpha]-radia-tions differed, and although the protein could be protected by the addition of cysteine against X-radiation, there was little protection against [alpha]-radiation. Within experimental error, there was no oxygen effect (less than 25% in the case of X-irradiation and less than 5% in the case of [alpha]-irradiation). The protein was little affected by hydrogen peroxide, but was attacked by ozone to produce changes similar to those following [alpha]-irradiation; but experimental evidence showed that the effect of [alpha]-radiation was not brought about by ozone. [alpha]-radiation attacked the indole ring of tryptophan twice as readily as did X-radiation. The spectral changes found on the a-irradiation of tryptophan closely matched those appearing on the [alpha]-irradiation of serum albumin. Effects of X-radiation on tryptophan could be greatly reduced by adding protective compounds to the solution, but tryptophan was protected against [alpha]-radiation to a much lesser extent. It is proposed that the action of X-radiation on proteins is exerted by free radicals which attack all the common amino acids, whereas the action of [alpha]-radiation is by a short-lived molecular product which preferentially attacks the tryptophan residue or molecule. It is proposed that the molecular species concerned is a metastable excited state of hydrogen peroxide.