Alginate with long strictly alternating sequences of mannuronic (M) and guluronic (G) acid residues, F-G = 0.47 and F-GG = 0.0, was prepared by incubating mannuronan with the recombinant C-5 epimerase AlgE4. By partial acid hydrolysis of this PolyMG alginate at pH values from 2.8 to 4.5 at 95 degrees C, alpha-L-GulpA-(1 -> 4)-ss-D-ManpA (G-M) linkages were hydrolyzed far faster than ss-D-ManpA-(1 -> 4)-alpha-L-GulpA (M-G) linkages in the polymer chain. The ratio of the rates (k(G-M)/k(M-G)) decreased with increasing pH. The dominant mechanism for hydrolysis of (1 -> 4)- linked PolyMG in weak acid was thus proved to be an intramolecular catalysis of glycosidic cleavage of the linkages at C-4 by the undissociated carboxyl groups at C-5 in the respective units. The higher degradation rate of G-M than M-G glycosidic linkages in the polymer chain of MG-alginate at pH 3.5 and 95 degrees C was exploited to make oligomers mainly consisting of M on the nonreducing and G on the reducing end and, thus, a majority of oligomers with an even number of residues. The ratio of the rate constants k(G-M)/k(M-G) at this pH was 10.7. The MG- hydrolysate was separated by size exclusion chromatography and the MG oligosaccharide fractions analyzed by electrospray ionization-mass spectrometry together with H-1 and C-13 NMR spectroscopy. Chemical shifts of MG-oligomers (DP2-DP5) were elucidated by 2D H-1 and C-13d NMR.