Improved 1H NMR and ATR-FTIR approaches for structural characterization of high-G-block and high-M-block alginic acids isolated from sugar kelp by a two-step gelation method

Abstract

Two low-molecular-weight alginic acid fractions, G-block-rich SKP-G1 and M-block-rich SKP-G2, were sequentially isolated from a hot-water extract of sugar kelp (Saccharina latissima) alcohol-insoluble residue by acetic acid-induced and subsequent freeze-thaw-induced gelation. High-field (700 MHz) 2D 1H-1H COSY and TOCSY spectroscopy enabled comprehensive cross-peak assignments for selected alginate diad and triad sequences, supporting interpretation of 1D 1H NMR spectra. Sequence frequencies were combined with previously established inequalities [1] to calculate constrained frequency ranges for homoblocks beyond the triad level. For selected homoblock lengths up to a degree of polymerization of 18, comparison of the experimentally derived frequency ranges with frequencies predicted by chain-growth statistical models ruled out the Bernoulli model for both fractions and the first-order Markov model for SKP-G1. Applying the recently developed logarithmic cosine transformation (LCT) algorithm [2] to first- and second-derivative ATR-FTIR spectra enhanced the visualization of subtle spectral features and produced distinct fingerprints for the two fractions. As a proof of concept, LCT-transformed derivative ATR-FTIR spectra yielded distinct score distributions for the fractions in principal component analysis, suggesting their potential for chemometric differentiation of alginates with different block structures. In conclusion, extending 1H NMR-based sequence analysis to calculate constrained frequency ranges for homoblock polyads beyond the triad level, together with LCT-transformed derivative ATR-FTIR fingerprinting, expands the analytical toolkit for alginate block structure characterization.