Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/75

 the expressed juice is mixed with a measured solution of an optically active polypeptide of known composition. The mixture is poured into a polariscope tube and the rotation for the solution is ascertained as quickly as possible. If one then determines the rotation from time to time, an insight into the nature of the decomposition is acquired. Instead of optically active polypeptides we can employ racemic bodies. The latter are optically inactive, because they consist of two halves equally strong as regards their respective rotations in opposite directions. The peptolytic ferments generally decompose only such polypeptides as are built up out of the optically active amino-acids as they are found in nature. If we have to deal with a racemic polypeptide, of which one-half complies with this condition, then this part is reduced to its component parts, and we are left with the other half of the racemic body, which consists of amino-acids not found in nature. We recognize this asymmetric splitting through the fact that the original optically inactive mixture becomes optically active.

An example may convey a clear idea of these conditions. In nature we meet the amino-acids l-leucin and d-alanin, while d-leucin and l-alanin have never yet been found amongst the products of reduction of the proteins. If we allow peptolytic ferments to act on the racemic bodies d-alanyl—