Circular dichroism study of polyriboxanthylic acid.

We report in the present paper the circular dichroism spectra of poly(X) at different pH and temperature values. The spectra are characteristic of three stable forms of poly(x) in the pH range of protonation of xanthosine. An electrostatic barrier is proposed to account for the hysteresis and metastability observed in a certain pH range. Some results on oligo(X) at basic pH are also presented. Poly(X) at basic pH is investigated also by hydrodynamic techniques.     

Circular dichroism of oligosaccharides containing neuraminic acid.

In order to test the usefulness of circular dichroism in stereochemical and structural studies of oligosaccharides of glycoproteins, we measured the circular dichroism (CD) for N-acetylneuraminic acid (NAcNA) and several derivates. By acidic mathanolysis, we have prepared the deacetylated methyl ester, methyl glycoside of NAcNA, as well as a saponified product. Circular dichroism of these compounds allows us to assign the transition due to the amide chromophore. There is a carboxyl n-pi transition at about 220 nm which has a negative CD band associated with it for the beta-methocyneuraminic acid, but changes sign for the methyl ester (methyl (methyl beta-D-neuraminid)ate). We isolated the trisaccharides N-acetylneuraminyl-(2 leads to 3)-beta-D-galactopyranosyl-(1 leads to 4)-D-glucopyranose [(2leads to 3)NAcN-Lac] as well as (2 leads to 6)NAcN-Lac by paper chromatography and compared the CD for each. The two isomers show similar but distinguishable CD patterns, with a weak negative band due to the carboxyl group centered at 225 nm and a stronger positive band at 200 nm containing contributions from both the amide and carboxyl groups.     

Circular dichroism of the nucleic acid monomers.

Circular dichroism spectra of the nucleic acid monomers have been measured in aqueous solution and extended into the vacuum ultraviolet region to about 166 nm. Measurements were made on ribo and deoxyribo derivatives of adenine, guanine, hypoxanthine, cytosine, thymine, and uracil derivatives both with and without the 5′-phosphate (with the exception of ribosyl thymine 5′-phosphate). Absorption spectra of the deoxyribonucleotides measured to about 175 nm are also presented. The results demonstrate that both the circular dichroism and absorption spectra observed below 200 nm are no more complicated than the spectra normally recorded above 200 nm. In most cases, the circular dichroism spectra of the various derivatives of a given base are similar, indicating that the conformations are similar. On the other hand, the differences among the circular dichroism spectra of the various derivatives of a given base are sufficient to identify a particular derivative. The average circular dichroism for the deoxyribonucleotides is compared with the circular dichroism of native E. coli DNA. The comparison reveals that the circular dichroism of DNA below 200 nm is due principally to the interaction between the bases rather than the intrinsic circular dichroism of the monomers. The monomer transitions are discussed in relationship to the absorption and circular dichroism spectra presented.     

Circular dichroism studies on the polymerization of N-carboxy alpha-amino acid anhydride

The D and L copolymerizations of N-carboxy γ-benzyl glutamate anhydride (NCA) were carried out in a homogeneous solution with various D /L ratios, initiated by either n-butylamine or sodium methoxide, and were followed directly by circular dichroism (CD) to observe the behavior of the secondary structure of growing polymer molecules. In the n-butylamine system, the difference of the helical content between the righthanded and the lefthanded (Δα-helix) gradually increased as the polymerization proceeded, while in the sodium methoxide system, the Δα-helix had a tendency to decrease during the later stages of the polymerization. These results suggest a difference of the power of stereo-selection of monomer antipodes by the growing chain end between these systems, the stereoselectivity by the growing chain end in the sodium methoxide system being higher than that in the n-butylamine system.     

Circular dichroism of phytochrome intermediates

Phototransformation P_t to P_fr was investigated with 124-kDa phytochrome from etiolated oat seedlings ( Avena sativa L. cv. Pirol) using circular dichroism spectroscopy at -110°C to +30°C. Using absorption spectra of the intermediates formed at the respective temperatures, circular dichroism spectra (300–800 nm) of pure intermediates were calculated.
The sign of the circular dichroic absorption bands changed upon formation of lumi-R, the primary photoproduct of P_r. This would be compatible with a Z→E isomerization taking place at this reaction step. The subsequent intermediates (meta-R_a and meta-R_c) as well as P_fr showed only small circular dichroism. Their absorption spectra were drastically shifted, but had similar spectral shapes. The results are discussed in terms of conformational changes of the phytochrome chromophore presumably taking place at the early steps of phototransformation P_r to P_fr.     

Circular dichroism of superhelical DNA

The circular dichroism (CD) spectra of a number of superhelical DNA's have been measured. The introduction of negative superhelical turns causes an increase in magnitude of the positive band around 280 mμ, while the trough around 250mμ is little affected. For two samples of λb2b5c DNA (20 Mdalton) containing different number of negative superhelical turns, the magnitude of the positive band relative to that of the nicked control increases with increasing number of superhelical turns. In 2M NaCl, the small (1.45 Mdalton) superhelical DNA from E. coli 15 shows an unusually large difference in CD compared with that of the same DNA with a few single-chain scissions per molecule. This large difference is not observed in a medium containing p. 0.11M NaCl. These results indicate that the double helix in a superhelical DNA is perturbed somewhat due to the bending and torsional forces in such a molecule. The magnitude of such structural alteration seems to depend on the number of superhelical turns per unit length, the size of the DNA molecule, as well as the ionic medium.