Expansion of crystals resulting from X irradiation was studied in the case of a peptide, N-acetyl-D, L-alanine. This expansion was discovered by examining several irradiated organic solids for possible expansion, which was suggested by the recent observation of crystal imperfections in another peptide. The expansion, which occurred anisotropically along the c axis, was found to depend on dose, the maximum being as large as 20% when assisted by heat treatment.

ESR and ENDOR techniques were used to study the crystals of N-acetyl-D, L-alanine which were irradiated by X rays at room temperature. One of the unstable radicals was found to be <tex-math>${\rm CH}_{3}\dot{{\rm C}}{\rm HCOOH}$</tex-math>.

The hydrogen exchange reaction in X-irradiated crystals of imidazole was studied with the use of electron spin resonance technique. This exchange was found to occur between the deuterium atom bonded to the C(2) carbon atom of the free radical and a C-H proton of the radical or that of a neighboring molecule. The activation energy was determined to be 17.4 ± 2 kcal/mole.

The hydrogen-exchange reaction in X-ray-irradiated crystals of glycine was studied with electron spin resonance techniques. An intraradical exchange was detected which took place between the NH 3 hydrogens and the α hydrogen of the radical <tex-math>${\rm NH}_{3}{}^{+}\dot{{\rm C}}{\rm HCO}_{2}{}^{-}$</tex-math>. The exchange rate was very high so that the change in the absorption could not be followed by the available spectrometer. Another exchange between the hydrogen of the radical and those of the host crystal was also observed. This relatively slow exchange was essentially similar to the exchange reactions observed in X-ray-irradiated crystals of some other amino acids.