13C-n.m.r. studies of monomeric composition and sequence in alginate

Abstract

High-resolution, 13C-n.m.r. spectra of slightly depolymerised alginates have been interpreted. The sequence of monomer units, l-guluronate (G) and d-mannuronate (M), markedly influenced the chemical shifts. At 50 MHz, some of the individual carbon resonances of both units were resolved into four lines, in evident dependence upon the identities of the units immediately preceding and following them in the chains. The relative intensifies of the signals permitted rapid computation of ( 1) monomeric composition (M/G ratio), ( 2) monomeric sequence in terms of a complete set of four diad and eight triad frequencies, and ( 3) the composition (M/G ratio) of end units and of the units adjacent to M-residues at the non-reducing end. The diad frequencies indicated that alginate was a block co-polymer containing number-average, co-monomer block-lengths of ∼2−8. The triad frequencies indicated average lengths of ∼4−8 for blocks containing two or more units, these being somewhat longer for G- than for M-blocks. Regions of the chains having a strictly alternating sequence of M- and G-residues were short. The relative occurrence of G-centred triads deviated significantly from those predicted by first-order Markovian statistics.