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

Abstract

S>Serum albumin, ribonuclease, tryptophan, tyrosine, and some other amino acids in dilute aqueous solution were irradiated with x (or beta ) 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 radiations differed, and, although the protein could be protected by the addition of cysteine against x radiation, there was little protection against alpha radiation. No oxygen effect was noted for either radiation. The protein was little affected by hydrogen peroxide but was attacked by ozone to produce changes closely similar to those after alpha irradiation, although 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 alpha irradiation of tryptophan closely matched those appearing on the alpha irradiation of serum albumin. The 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 radicalsmore » 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. Consideration is given to the role of the molecular product of alpha radiation in determining the relative biological efficiencies of densely and sparsely ionizing radiations.« less