The chain length dependence of the conformation for oligomers of L-lysine in aqueous solution: Optical rotatory dispersion studies

The optical rotatory dispersion of L -lysine oligopeptides (Lys_n, n = 2–22) in solution was measured in water and in 50% methanol. A gradual change with increasing chain length in the ORD curves of the oligomers was observed at pH 4. 3. Not even a chain of 22 residues had ORD identical with that of high molecular weight poly-L -lysine. A plot of the average molar residue rotation at 233 nm versus 1/n (where n is the chain length) resulted in a straight line with an intercept of ?1900, representing the internal residue rotation of a lysine residue in the random conformation, and a slope of +6200 representing the large end effect. At pH 11.9 a stright line is obtained up to n = 12 after which it deviates from the initial slope indicating onset of helicity. Extrapolation of the initially straight line to tire higher n's provided the necessary zero-helicity values for calculation of helicity. The highest oligolysine (n = 22) showed at pH 11. 9 13% helicity, which on adding methanol to 50% increased to about 50% helicity. It is shown that helix-coil data which are usually obtained from the temperature dependence of helicity can be obtained from the dependence of helicity on chain length applying the statistical theory. For the methanol-water system the cooperativity parameter v was calculated to be in the range 0.024–0.060, with corresponding equilibrium constants w of 1.32–1.43. The helical structure was calculated to be less stable in water than in methanol-water by about 250 calories per residue.     

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.     

Conformation of amylose–iodine–iodide complex in aqueous solution

The conformational characteristics of the amylose–iodine–iodide complex in aqueous solution, particularly for a rapidly mixed system, were studied by resonance polarized scattering measurements using a He-Ne laser at low concentrations of the complex and by viscosity measurements at high concentrations of the complex. For the scattering measurements, the following results were obtained: the depolarization ratios ρ_u and ρ_v showed a pronounced increase with the degree of saturation of the bound iodine (q) in amylose, depending on KI concentration. At q ? 0.7, the increase in these values appeared to be suppressed. However, the ρ_h value was approximately 1, irrespective of q. Additionally, the dissymmetry Z decreased appreciably with increasing q. The conformational change of the complex with q was characterized by the changes in the contour and persistence lengths of the chain and in the optical anisotropy of the scattering segments, which were obtained from numerical computations based on the polarized scattering equation for a wormlike-chain model with a restriction by the entropy force of the chain. The viscosity of the complex solution decreased with increasing q; above q ? 0.7 it increased strikingly. The conformational change of the complex with q was characterized by the change in exponent α in the Houwink-Mark-Sakurada equation [η] = KM^α. It was concluded that the iodine-saturated complex has the characteristics of a rod, regardless of the complex concentration.     

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.     

Molecular configuration of amylose and its complexes in aqueous solutions. Part IV. Determination of DP of amylose by measuring the concentration of free iodine in solution of amylose–iodine complex

In aqueous solutions of the amylase–iodine complex the concentration of free iodine [I_f]_v after reaching equilibrium (or closely approximating it) is determined by the following factors: temperature, pH, concentration of iodide ions and amylose, and DP of amylose. In the present paper the role of temperature, amylose concentration, and DP has been investigated. At half-saturation of amylose by iodine, the reciprocal value of free iodine defines the equilibrium constant: 1/[I_f]_v = K. The relation between [I_f]_v, in normality and temperature is the following: 5 + log [I_f]_v = ?(2.132/T) + 8.52, for DP _n = 1290, 0.4 mg. amylose in 100 ml. 0.1N HCl. The value of the energy of activation E_a between 2 and 52°C. is 9.72 kcal./mole. The influence of amylose concentration [Am] on photometrically determined [I_f]_v, at 20°C, in the range of 0.1–1.2 mg./100 ml. 0.1 N HCl for DP _n = 1290 is: 5 + log [I_f]_v = 0.209 ? 0.047 log [Am]. At [Am] = 0.6 mg. amylose/ 100 ml. 0.1 N HCl and 20°C, the value of [I_f]_v depends on DP _n as follows: 5 + log [I_f]_v = 0.085 = + 0.222 log (10⁴/DP _n). These above equations are summarized by the relation: [I_f]_v = exp {16.865 ? (E_a/RT)}[Am]^0.047(10⁴/DP _n)^0.222 ×10^?5 Considering that the determination of [I_f]_v by automatic photometric titration can be performed quickly and with appropriate reproducibility, this method is convenient for a rapid empirical and approximate determination of DP of amylose on a microscale. The iodine-binding capacity [IBC] as well as the value of λ_max, have been also investigated as functions of DP _n, by photometric and by amperometric titration.     

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.