Stability of 5-aminolevulinic acid in aqueous solution

The chemical stability of 5-aminolevulinic acid (ALA) was studied in aqueous solution as a function of concentration, pH, temperature and in the presence of ethylenediaminetetraacetic acid (EDTA). The degradation of ALA was followed by reversed-phase liquid chromatography using a pH where ALA is protonated (pK(a1) = 3.90; pK(a2) = 8.05, as determined potentiometrically). ALA was degraded by a reaction following second order kinetics. Stock solutions of 1% (60 mM) ALA were incubated at 50 degrees C. At pH 2.35, ALA was stable during the whole incubation period (37 days). The half-lives for the second-order decomposition of 1% ALA at pH 4.81 and 7.42 were 257 and 3.0 h, respectively. The degradation rate increased about 1.5 times with each 10 degrees C rise in temperature at pH 7.53 within the range studied (37-85 degrees C). The energy of activation, E-a, for the second-order decomposition of ALA was 43.7 kJ mol(-1). EDTA did not influence the degradation of ALA when a mixture of 1% ALA and 1% EDTA was incubated at pH 7.42. (C) 1998 Elsevier Science B.V. All rights reserved.