Anhydro sugar and linkage contributions to circular dichroism of agarose and carrageenan, with conformational implications.

The geometry-dependent linkage contributions to the circular dichroism (CD) of agarose and carrageenan were determined by subtracting the monomeric CD from the CD of the polymers. For this purpose, the CD of methyl 3,6-anhydro-alpha-D-galactopyranoside, and of other anhydro sugars, was measured. Application of empirical quadrant rules indicates the observed CD to be that expected for the helical conformations of agarose and carrageenan as derived from diffraction data and modeling calculations. The difference in CD sign in the two polymers arises from a translocation of the beta-D-galactose O-2 atom from one quadrant to the neighboring quadrant of the C-5-O-5-C-1 ether chromophore of the preceding anhydro sugar residue, demonstrating the unusually high spatial resolution of conformational analysis that can be achieved with CD under favorable circumstances.