The reactions of radioactive chlorine atoms recoiling from two nuclear processes, "Cl(n, 2n)"'"a and "Cl(n, 7)"CI have been studied in different liquid halobenzenes. The ratio "Q-for-Cl vs.^'Cl-for-H yield for C^HjCl is dose-dependent, indicative for radiation-enhanced ^' Cl-for-Cl exchange. The Cl-for-Cl replacement yield is lowered on the addition of Ij, but much more for "CI than for It is suggested that the precursors to the exchange is a charge-transfer 7r-complex, rather than a a-complex. The fraction of the * Cl-for-Cl replacement yield not affected by Ij is ascribed to direct hot one-step processes. The behaviour of m-Cj H4 Clj is different to the extent that -as measured for 34 mQ _ thermal exchange seems to take place. Also, isomeric dichlorobenzenes irradiated in a mixture show differences among each other in exchange behaviour. Part of the unidentified products is attributed to an excited jr-complex subject to ring-opening and/or fragmentation. The ""'Cl vs."Cl isotope effect is not due to external radiation and is discussed in terms of differences in the recoil energy distribution and the life-time of lower levels of nuclear excitation.

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Methanol extracts of the skin of the African amphibian Kassina senegalensis contain a dodecapeptide, kassinin, belonging to the family of tachykinins or physalaemin-like peptides. Kassinin, like all other natural tachykinins, possesses the characteristic C-terminal tripeptide Gly-Leu-Met-NH2 and a phenylalanine residue in position 5 from the C-terminus. However, the amino acid sequence in the N-moiety of the molecule differs sharply from that of the other tachykinins.

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does not appear to be of a type which can account for enzymic catalysis. ConclusionSince most enzymic reactions in which thiamine pyrophosphate is a cofactor are mechanistically similar to the pyruvate decarboxylase reaction,47 our results and the above discussion are relevant to many thiamine pyrophosphate-dependent enzymic reactions.