Optical rotatory dispersion and circular dichroism of benzoyl polysaccharides

Measurements of optical rotatory dispersion (ORD) and circular dichroism (CD) were made in the range of 400–205 nm for polysaccharide tribenzoates such as 2,3,6-tri-O-benzoyl amylose (I), 2,3,4-tri-O-benzoyl dextran (II), tri-O-benzoyl pullulan (III), 2,3,6-tri-O-benzoyl cellulose (IV), 2,3,6-tri-O-benzoyl mannan (V), and polyglycan dibenzoates such as 2,3,-di-O-benzoyl amylose (VI), cellulose (VII), and mannan (VIII). All compounds exhibit Cotton effects in the region of their UV absorption bands (206–285 nm). Comparison of the corresponding di- and tribenzoyl polysaccharides shows a qualitative agreement in number, position and sign of the CD bands but differences in ellipticity magnitude. The disubstituted derivatives exhibit smaller amplitudes than the trisubstituted ones. The contribution of the C(6) chromophore (linked by a CH₂-group to the asymmetric C(5) atom) was determined to be of the same sign as the combined contribution of the C(2) and C(3) substituents. The CD bonds of the individual polysaccharide derivatives, which differ in number, sign, and position, were discussed in terms of the steric position of the single chromophores and the steric arrangement and interaction caused by the configuration of the polysaccharides. The optical behavior of these polysaccharide derivatives was found to be not strongly influenced by a definite chain conformation in solution.     

Optical rotatory dispersion and circular dichroism of carbanilyl polysaccharides

Measurements 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.     

Optical rotatory dispersion and circular dichroism of silver(I):Polyribonucleotide complexes

Optical rotatory dispersion (ORD) and circular dichroism (CD) spectra of single- and multistranded polyribonucleotides undergo extensive changes on binding of the silver ion. These changes are consistent with the proposition that Ag(I) binds to the heterocyclic bases and not to the phosphate groups of polynucleotides. ORD and CD of silver complexes of poly(A)·poly(U) and double-helical rice dwarf viral RNA display negative Cotton effects when there is more than one Ag(I) per two nucleotide residues in solution. These observations suggest a significant distortion of the double-helical conformation as a result of Ag(I) binding. Silver(I) binding sites of pyrimidine polynucleotides are apparently saturated when there is one Ag(I) per two nucleotide residues and those of purine polynucleotides at one Ag(I) per nucleotide in solution. These data are consistent with the supposition that some Ag(I) binding sites exist on the pyrimidine ring and additional sites on the imidazole ring of polynucleotides. The sedimentation coefficient of poly(A) increases by severalfold when one Ag(I) is present per nucleotide residue. Silver(I) may introduce intra- and interstrand cross-links (through bidentate chelates) in single-stranded polynucleotides, resulting in structures with high sedimentation coefficients. Among the polynucleotides studied, poly(U) was an exception. Silver(I) did not affect the optical properties (absorbance, ORD, and CD) of poly(U) at neutral pH.