Racemization of Isovaline by γ-Radiation. Cosmological Implications

Isovaline (a-amino-a-methylbutyric acid) is of considerable cosmological interest since (I) it is one of the 12 nonprotein amino acids isolated and identified from the Murchison carbonaceous chondrite¹ and (2) it has no a hydrogen at its asymmetric center and has been shown² to be incapable of racemization by the mechanisms accounting for the racemization of ordinary α-amino acids. It had previously been suggested³ that the enantiomeric composition of the Murchison isovaline should represent that prevailing at the time of its original synthesis in the meteorite, thus giving a clue as to the primordial enantiomeric composition of the other amino acids in the meteorite regardless of their present composition. Recently we reported^4-6 that ionizing radiation could cause —radioracemization– of α-amino acids both in the solid state and in solution, along with their well-known radiolysis.⁷ We now have found that such radiation is capable of racemizing isovaline as well.