IDENTIFICATION OF THE ELUSIVE AMINO ACID 7, 
RCH(NH,)CO,H + O, + H,O -> RCOCO,H + NH, + H,O, 
A limitation of the method arises from the fact that not all a-amino acids which 
possess the required optical configuration are necessarily susceptible to the enzymat- 
ically-induced oxidation, so that the inability of a given amino acid to undergo 
oxidative deamination by an L- or p-amino acid oxidase does not imply that its con- 
figuration becomes known by default. Although amino acid decarboxylases have also 
been employed in configurational analysis, their utility 1s somewhat limited in that 
their action is generally confined either to a single substrate or a small number of 
very Closely related substrates. It should further be noted that both L- and p-decar- 
boxylases are known to occur in the plant kingdom, so that their optical specificity 
with respect to substrates of known configuration must be established with certainty 
before any measure of confidence can be reposed in their use. 
The bland or bitter taste revealed by nearly all amino acids of the L-configuration 
as contrasted with the sweet taste revealed by D-amino acids®*: °® may also be em- 
ployed to indicate the configuration of an a-amino acid®’, but as the rule is not invariable 
and as the method is of a highly subjective nature, it cannot be employed with any 
degree of confidence. As the intestinal absorption of amino acids is presumably mediated 
by an active transport mechanism that is L-directed®’, and as toxicity studies with 
rats have indicated that the D-isomers of nearly all amino acids tested were less toxic 
than their corresponding L-forms**: °°, these phenomena too may form a theoretical, 
although admittedly impractical basis for configurational analysis®’. 
Although physical methods, such as X-ray analysis, kinetic reactions, and the 
melting-point curves of quasi-racemic compounds have also been utilized to determine 
the configuration of amino acids, the bulk of such physical methods, by far, have been 
concerned with the optical behavior of amino acids or their derivatives’. Thus, the 
L- or D-configuration of an amino acid can be reliably determined, in a single measure- 
ment, by the direction of rotation exhibited by the hydantoin®: 6, N- or C-terminal 
glycine®: § aminofluorene® ©, aminobiphenyl®™: © or benzidine®™: ® derivative. As 
a marked relationship exists between the configuration of an amino acid and its rotatory 
dispersion curve, the measurement of the optical rotation of an amino acid at various 
wavelengths will also serve to indicate its configuration®®—®8, This method, in conjunc- 
tion with kinetic studies, was successfully employed in our laboratory, in 1957, to 
establish the configuration of natural octopine, after attempts at several other methods 
had failed®: 7, Probably the most convenient, and one of the most reliable of the 
optical methods is that which involves the shift in optical rotation induced by ioni- 
zation’!. The principle upon which this method is based is contained in the CLOUGH- 
Lutz-JIRGENSONS rule. This rule states that: if the molecular rotation of an optically 
active amino acid is shifted toward a more positive direction upon the addition of acid to 
its aqueous solution, the amino acid is of the L-configuration; a negative direction of 
shift, however, is characteristic of a D-amino acid. Its use is exemplified in the case of 
alanine as follows: 

5 N HCl Water Difference 
L-Alanine + 13.0 minus +1.6 + 11.4 
p-Alanine —13.0° minus —1.6 —II.4 
References p. 22/24 
