2D HSQC-derived "dark forest" image with enhanced local resolution via first derivative processing-logarithmic cosine transformation (FDP-LCT): Demonstration on per-O-ethylated kappa- and iota-carrageenans
Solution-state two-dimensional (2D) 1H-13C HSQC NMR is a powerful tool for polysaccharide structure elucidation but often suffers from limited sensitivity and broad peaks due to the low natural abundance of 13C and poor digital resolution of the indirect dimension, respectively, as well as the typically low concentration and high viscosity of polysaccharide solutions. It is therefore pivotal to improve the resolution of 2D 1H-13C HSQC spectra for accurate peak picking and assignment, particularly in the indirect 13C dimension. In this study, we developed an algorithm that combines first derivative processing with a novel logarithmic cosine transformation (FDP-LCT) to convert 2D 1H-13C HSQC spectra into local-resolution-enhanced images resembling a dark forest of straight, densely standing trees. These images revealed sharpened spectral features and enabled extraction of precise 1H and 13C chemical shifts, as demonstrated using per-O-ethylated kappa-and iota-carrageenans, two sulfated galactans differing only by a single substitution at the O-2 position of anhydrogalactose. In conclusion, this approach provides an effective post-acquisition strategy for enhancing digital resolution in 2D HSQC spectra and improving the structural analysis of closely related complex polysaccharides. Graphic abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=77 SRC="FIGDIR/small/689839v1_ufig1.gif" ALT="Figure 1"> View larger version (29K): org.highwire.dtl.DTLVardef@c85f3dorg.highwire.dtl.DTLVardef@e4ded2org.highwire.dtl.DTLVardef@1e0b7b5org.highwire.dtl.DTLVardef@3cebcf_HPS_FORMAT_FIGEXP M_FIG C_FIG Highlights- An algorithm combining first derivative processing and logarithmic cosine transformation was developed for 2D HSQC. - The algorithm was used to boost the local resolution of 2D HSQC spectra of the per-O-ethylated carrageenans. - Structures were distinguished by complete interpretation of 2D NMR spectra. - A post-processing workflow was developed to facilitate chemical shift extraction from sharpened HSQC.