MECHANISMS INVOLVED IN KJELDAHL MICROWAVE DIGESTION OF AMINO-ACIDS

The high flexibility of open vessel microwave digestion gives one the opportunity to follow the stepwise degradation of amino acids in the p resence of H2SO4 and H2O2. Thus, it is possible to demonstrate, for se veral amino acids, that only a stoichiometric fraction of nitrogen (ca lled the easy fraction) is digested when sulfuric acid is used alone. The other difficult fraction is totally dissolved when hydrogen peroxi de is added. These first results confirm the use of two-step digestion procedures as described in earlier works. The first step consists in charring the matrix the second in oxidizing. When considering amino ac ids that contain an aromatic ring, such as tryptophan and histidine, i t is possible to use UV spectrometry to give indications on the interm ediate products formed during the charring step. It was assumed that, at least, three compounds are formed and their probable structure, for tryptophan, can be confirmed by H-1 and C-13 NMR. It is then possible to propose a general digestion mechanism involving sulfonation follow ed by the opening of the aromatic ring.