摘要:

AbstractThe nonbonded interaction energy was computed for xylobiose and xylan as a function of the dihedral angles (ϕ,ψ). The energy maps indicate that interactions higher than the second neighbor are negligible. Of the four possibilities, the left‐handed helical conformation with (ϕ,ψ) = (63°,25°) is of the lowest energy. The hydrogen bond search and the energy maps reveal that the xylan helix is stabilized mainly by van der Waals forces. The allowed region map shows that the freedom of rotation of the monomer units in cellulose is more restricted than that of the monomer units in xylan, because of the presence of the CH2OH group in the former. The intramolecular hydrogen bond of the O5⃛O3′ type is stronger

ISSN:0006-3525    

DOI:10.1002/bip.1969.360080302

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractThe nonbonded energy (van der Waals) was computed for isolated helical amylose chains as a function of the dihedral angles (ϕ,ψ), i.e., the relative orientations of the glucose residues in the polysaccharide chain. In conformity with x‐ray data, different helical conformations have been proposed for different crystalline modifications of amylose. Right‐handed helical conformations were preferred for 71.13, 61.33, and 42.65‐type helices from minimum energy considerations. The hydrogen bond search reveals the possibility of formation of strong OH ⃛O type hydrogen bonds between contiguous residues in the polymer chain for 61.33and 71.13helices. Smooth transition from a 71.13to a 61.13helix is possible with small changes in the values of (ϕ,ψ) and without the loss of intramolecular hydrogen bonds, whereas all the intramolecular hydrogen bonds will be broken during the conversion from 61.33

ISSN:0006-3525    

DOI:10.1002/bip.1969.360080303

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractThe conformational energy of amylose molecule was computed as a function of rotations ϕ and ψ about interunit glycosidic bonds. The characteristic ratio 〈r2〉0/Nl 2Vof the mean‐square end‐to‐end distance of the unperturbed chain to the product ofn, the number of glucose residues in the chain and the square of the virtual bond lengthlV= 4.2 Å calculated for amylose chain is about 6.9, which is in good agreement with the experimental value (6.6 ± 0.2). The value of this ratio calculated for free rotation about the inter unit glycosidic bond is 1.1. The conformational map reveals that the rotations of pyranose units about the interunit glycosidic bonds are highly restricted; this tends to lock these units in a preferred orientation. From the probability of occurrence of the glucose residue conformations it is concluded that amylose exists as a random coil with short helical sequences in neutral aq

ISSN:0006-3525    

DOI:10.1002/bip.1969.360080304

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractThe effect of pressure on the temperature‐dependent association of β‐casein was studied by the method of light scattering. Between 1 and 1500 kg/cm2, β‐casein is reversibly depolymerized, indicating an increase in partial specific volume of the protein with association. Possible mechanisms for association are discussed. At still higher pressures reversible reassociation is observed, which is ascribed to conformational changes in the β‐casei

ISSN:0006-3525    

DOI:10.1002/bip.1969.360080305

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractThe circular dichroism (CD) spectrum of an unordered polypeptide chain does not correspond, as has been assumed heretofore, to that of a charged chain such as poly‐L‐glutamic acid or poly‐L‐lysine. The latter have been shown to have locally ordered structures with characteristic CD spectra. We have now obtained CD spectra of the unordered forms of the above synthetic, polypeptides, as well as of two fibrous proteins (collagen and feather keratin) and a globular protein (myoglobin). These spectra are all similar to that of unordered polyproline, having a negative band in the vicinity of 2000 mμ and no additional bands at longer wavelengths. The lack of structural uniqueness of the unordered polypeptide chain is emphasized by these

ISSN:0006-3525    

DOI:10.1002/bip.1969.360080306

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractThe interaction between poly (I) and poly (C) in acid medium has been studied by potentiometric titration, mixing curves and thermal denaturation. Phase diagramms as a function of ionic strength, pH, and temperature have been established. From these data it is shown that the acid titration of the complex poly (I) · poly (C) passes through a triple‐stranded intermediate poly (I) · poly (C) · poly (C+) to yield finally the protonated double‐helical complex poly (I) · poly (C+). The mixing curves indicate the sole presence of the three‐stranded complex in the intermediate zone. On the basis of the pK's the coexistence between the three‐stranded complex with the neighboring double‐stranded structure is demonstrated in a narrow rang of pH and ionic strength. The geometry of the base arrangements, their conformation and the sense of the strands are discussed in the light of the data presented. A Hoogsteen‐type pairing between the bases for poly (I) · poly (C+) is favored, although the reverse Hoogsteen pair ca

ISSN:0006-3525    

DOI:10.1002/bip.1969.360080307

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractThe relaxation of the flow dichorism signal of native DNA molecules after rapid (∼0.004 sec) stoppage of the flow has been studied in the narrow‐channel flow dichroism apparatus previously described. The flow dichroism signal decays with a spectrum of relaxation times. The slowest relaxation time accounts for about 0.40 of the total signal for shear gradients in the range of 100–300 sec−1and for about 0.25 of the total signal for shear gradients in the range of 5,000– 20,000 sec−1. This longest relaxation time in dilute aqueous salt solution at 25°C for T4‐DNA molecules is measured as 0.45 sec; in the molecular weight range from 125 × 106to 12 × 106, the relaxation time varies as.M(1.6±0.1). The relaxation time is proportional to η/Tfor large (sixfold) changes in solvent viscosity and for temperature changes between 25 and 0°C, showing that the molecular relaxation is hydrodynamically limited. An accurate analysis of the data at short times to determine the shorter relaxation times was not possible. A theoretical prediction based on the Zimm‐Rouse theory of polymer dynamics for the longest relaxation time is in good agreement with the observed value, but the theory predicts that 87%, of the total signal should decay with this relaxation time, whereas experimentally only a smaller fraction of the total signal does so. This discrepancy occurs because the theory is applicable to low hydrodynamic shear stress flows and the experiments were made at

ISSN:0006-3525    

DOI:10.1002/bip.1969.360080308

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractUltraviolet absorption and dichroism measurements on oriented films of calf thymus deoxyribonucleic acid (DNA) at varying degrees of hydration show a structural hysteresis which occurs within a relative humidity of 0 to 65% RH. This hysteresis is interpreted on the basis of a model which assumes the existence of structure‐stabilizing hydrogen water bridges in the DNA double heli

ISSN:0006-3525    

DOI:10.1002/bip.1969.360080309

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractSimple approximate expressions have been derived from the theory of Zimm and Bragg for use in the analysis of experimental data on the helix‐coil transition in polypeptide. On the basis of the resulting expressions practical procedures are proposed to determine two basic parameters characterizing a thermally induced transition, i.e., helix initiation parameter σ and enthalpy change for helix formation, ΔH. They have been applied to the data for poly(β‐benzylL‐aspartate) (PBLA) with the result: σ = 1.6 × 10−4and ΔH = −450 cal/mole for PBLA inm‐cresol; σ = 0.6 × 10−4and ΔH= 260 cal/mole for PBLA in chloroform containing 5.7 vol‐% of dichloroacetic acid. This result gives evidence that σ may change not only from one polypeptide to another but also for a given polypeptide in different solvents. The change in limiting viscosity number [η] accompanying the transition was measured in the same solvents. The curve of [η]versus helical content had a relatively monotonic shape for the chloroformdichloroacetic acid solutions as compared with that for them‐cresol solutions, indicating that [η] depended largely on σ. Provided that [η]is a direct measure of the mean‐square radius of gyration, 〈S2〉, the results are consistent with the theoretical predictions of

ISSN:0006-3525    

DOI:10.1002/bip.1969.360080310

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractThe d. c. conductivity of five samples of NaDNA in dry state was investigated. All specimens show similar behavior, with specific resistivity at 40°C of the order of 1013–1014ohm‐cm and an energy gap ofEc= 2.04–2.25 eV. Based on the Eley's model for conductivity mechanism, the possibility of conservation of an ordered structure in the dry state (temperature range 20–80°C) is discussed. This is ascribed to the interaction of electron systems of individual bases. From the activation energy values it may be estimated that 17–19 π‐electrons take part in the interaction. Photoconductivity measurements indicated activation energiesEfof 0.88–0.96 eV in th

ISSN:0006-3525    

DOI:10.1002/bip.1969.360080311

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY