Optical rotatory dispersion and circular dichroism of carbanilyl polysaccharides

Abstract

AbstractMeasurements of optical rotatory dispersion (ORD) and circular dichroism (CD) have been made in the range of 600‐210 mμ for the β‐glycan carbanilates as for instance, 2,3,6‐tricarbanilylcellulose (I), 2,3,6‐tricarbanilylmannan (II), 2,3‐dicarbanilylcellulose (III), and octacarbanilylcellobiose (IV) and also for the α‐glycan carbanilates, such as 2,3,6‐tricarbanilylamylose (V), tricarbanilylpullulan (VI), 2,3‐dicarbanilylamylose (VII), and octacarbanilylmaltose (VIII). Furthermore, the 2,3,4,6‐tetracarbanilyl‐α‐methyl‐glucopyranoside (IX) and the 1,2,3,4,6‐pentacarbanilylglucose (X) have been measured in dioxane at 20°C. For the β‐glycans a small negative CD in the region of 238–240 mμ and nearly symmetrical ORD curve with a crossover point at 238–240 mμ are found; this indicates a simple negative Cotton effect. In the case of α‐glycosides, a strong negative CD with a maximum at 240–242 mμ and a strong positive CD with a maximum at 223–225 mμ were found; the ORD curves are asymmetrical and cross the abscissa in two places, at 241–243 and 220–222 mμ. With 2,3,4,6‐tetracarbanilyl‐α‐methylglucoside (IX) no CD and ORD in the ultraviolet region and with 1,2,3,4,6‐pentacarbanilyl‐glucopyranoside (X) the ORD, but not the CD, could be measured. The ORD curve is nearly symmetrical, like those of the β‐glycans but is of opposite sign. It seems impossible to discuss the striking difference of the CD and ORD spectra between the α‐and the β‐glycans in terms of contributions of single independant chromophores influenced by their individual different steric arrangements and their spatial relation to the glycosidic bond in C1. The exciton theory of Moffitt, which is suitable for explaining the ORD and CD spectra of helical polymers, has been applied to α‐ and β‐glycans. A structure with helical parts is proposed for the α‐glycans while a nearly planar arrangement is assumed for the β‐glycans.