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, circular dichroism and far-ultraviolet spectra of avidin and streptavidin

1. The optical-rotatory-dispersion and circular-dichroism curves of avidin showed positive Cotton effects centred at 228mmu and 280mmu, close to the ultraviolet-absorption bands of tryptophan. These effects disappeared when avidin was dissociated into sub-units in guanidine hydrochloride. 2. Binding of biotin had only a small effect on the optical-rotatory-dispersion curve of avidin. 3. The absence of negative circular dichroism at wavelengths above 216mmu showed that there was little or no alpha-helix present in avidin. This interpretation was confirmed by Moffitt-Yang plots of the partial rotation due to the peptide bonds in the visible region of the spectrum. The calculated dispersion constants were remarkably similar to those of gamma-globulin and suggested the presence of peptide conformations other than alpha-helix and random coil. 4. The far-ultraviolet spectrum was also similar to that of gamma-globulin, the mean extinction coefficient of the peptide chromophore being much lower than the experimental value for a random-coil structure. 5. Streptavidin resembled avidin in showing two positive Cotton effects, but the negative dichroism below 220mmu suggested the presence of more alpha-helix than was found in avidin. Formation of the complex with biotin was accompanied by changes in rotation that were rather larger than those observed with avidin.     

Optical properties of hyaluronic acid. Ultraviolet circular dichroism and optical rotatory dispersion

The molar optical rotation at 220 nm and ellipticity values at 210 nm of both sodium hyaluronate and hyaluronic acid are greatly enhanced in comparison to the values for the monomeric units and oligosaccharides indicating a degree of preferred order. With increasing hydrogen ion concentration, there is no appreciable change in the 210 nm circular dichroic band, but the second circular dichroic band below pH 4 changes abruptly to the positive side and reaches a maximum value at pH 2·5. This positive circular dichroic band of hyaluronic acid is temperature and concentration dependent. The major change in sign and position of the second circular dichroic band of hyaluronic acid below pH 4 is attributed to the conformational change of a single polysaccharide chain or to a chain-chain interaction. The results indicate that increase in concentration or decrease in temperature and in the ionization of carboxyl group promotes the formation of ordered cross-link regions. The conformational changes found in solution have been interpreted as an order-disorder transition in the crosslink regions based on the interconversion of random coil and double helix.     

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.