Optical rotatory dispersion of mucopolysaccharides and mucopolysaccharide--dye complexes. II. Ultraviolet cotton effects in the amide transition bands

Ultraviolet optical rotatory dispersion curves of mucopolysaccharides exhibit particular Cotton effects in the spectral region of the n–π and π–π amide transitions. Two general patterns emerge: (1) enhancement of negative rotation and of the first negative Cotton effect (troughs 217–220 mμ) and (2) relative dominance of the positive Cotton effect in the π–π transition region (peak ～198 mμ). Groups (1) and (2) can be correlated with a structural difference in the linkages of the amino sugars: (1) occurs with polymers containing 3-1-linked glycosamino sugars and (2) with glycosamino moieties linked 4–1 by either α- or β-glycosidie bonds. Measurements of the circular dichroic absorption bands support the qualitative conclusions from optical rotation. All mucopolysaccharides exhibit a first, negative band centered at 208–211 mμ, while only those in group (2) show, in addition, a positive band centered at 189–192 mμ. A suggested unifying model considers that difference in kind and/or degree of preferred geometry of the amide groups obtains from two forms of secondary order: (1) having a linear hydrogen bond from the N (acceptor) to the (C₂)O—H of the preceding uronic acid and (2) having a linear hydrogen bond from the N (acceptor) to the (C₂ or ₃) O—H of the following sugar. The hydrogen bonds would have similar strength but opposite directions in two systems towards the nonreducing end (1) or towards the reducing end (2)], closing eight-membered rings