ISSN:0006-3525    

DOI:10.1002/bip.1968.360060902

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

年代:1968

数据来源: WILEY

摘要:

AbstractUsing potential energy formulas for variation of bond angles and for ω‐distortion, the conformation of minimum energy for thecisform of the petide unit has been worked out. This agrees very well with the corresponding set of atoms in the crystal structure of Leu‐Pro‐Gly and the dimensions of a standardcispeptide unit are given based on these. The conformational (ϕ,Ψ) map for acispeptide unit has been worked out from contact criteria, both for a pair of linked units as well as for‐helices having constant (ϕ,Ψ). The small allowed region of the helical map contains the conformation experimentally observed for pol

ISSN:0006-3525    

DOI:10.1002/bip.1968.360060903

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

年代:1968

数据来源: WILEY

摘要:

AbstractDielectric relaxation of poly(γ‐benzylL‐glutamate) in solution has been studied in the 5 kcps‐10 Mcps range for various values of the helix content. The results give first experimental evidence for three effects of major significance. (1) The system exhibits dielectric relaxation due to a chemical rate process (namely helix formation). This confirms recent theoretical predictions. (2) The mean relaxation time τ* of the helix–coil transition could be evaluated as a function of the degree of transition. The results are in excellent agreement with a previously developed theory. At the midpoint of transition it is found τ*max= 5 × 10−7sec. The elementary process of helical growth turns out to be practically diffusion‐controlled (with a rate constant of hydrogen bond formation of 1.3 × 1010sec−1). (3) There is a considerable electric field effect of the helix–coil transition. This indicates that conformation changes in biological systems could be potentially caused by direct action

ISSN:0006-3525    

DOI:10.1002/bip.1968.360060904

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

年代:1968

数据来源: WILEY

摘要:

AbstractPhosphorescence and fluorescence from the dye in complexes of DNA with 9‐amino‐acridine and acridine orange in a glycerol‐H2O glass have been measured at 77°K. The dependence of thep/fratio for 9‐aminoacridine on the exciting wavelength demonstrates triplet–triplet energy transfer from DNA to dye. The result provides evidence for π electron overlap between the dye and the bases of native DNA. The observation that the magnitude of the enhancement in ultraviolet‐excited dye phosphorescence increases with the base to dye ratio indicates triplet delocalization in the polymer. Preliminary flash experiments provide evidence that this delocalization is not limited by slow diffusion of the triplet exciton. The inability to detect transfer on denaturation of the DNA illustrates the sensitivity of triplet–triplet energy transfer to the conformation of the macromo

ISSN:0006-3525    

DOI:10.1002/bip.1968.360060905

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

年代:1968

数据来源: WILEY

摘要:

AbstractThe ratio of the intrinsic viscosities of the linear and circular forms of λ DNA, [η]L/[η]c, has been measured as a function of ionic strength in the range [Na+] = 0.6.M–0.03MCorrections were made for the presence of uncyclizable linear contaminant in circular preparations. By combining data in the literature on the ionic strength dependence of linear DNA of various molecular weights with that obtained here, it was possible to determine the expansion parameter εLas a function of [Na+]. εLis defined by the relation 〈L2〉 = b2N1+εL, where 〈L1〉 is the mean‐square end‐to‐end distance of a chain ofNsegments of lengthb. The empirical relation εL= 0.05 − 0.11 log [Na+] for native NaDNA at 25°C is found. When εL= 0, [η]L/[η]c extrapolates to 1.6, in good agreement with the theoretical prediction of 1.55. As εLincreases, [η]L/[η]c increases, in agreement with

ISSN:0006-3525    

DOI:10.1002/bip.1968.360060906

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

年代:1968

数据来源: WILEY

摘要:

AbstractOptically active poly‐O‐acetylthreonine and poly‐O‐acetyl‐allothreonine exist in the β form in pyridine, in 2‐chloruethanol–dichloromethane mixed solvent., and in the solid state. The optical rotatory dispersion of the β form of poly‐O‐acetyl‐L‐threonine in a 2‐ehloroethanol–dichlomethane mixture showed a Cotton effect in the 230 mμ region. The ORD data in the visible spectral region fit, the Moffitt. equation. The absolute values ofb0, as measured in pyridine for polypeptides with a molecular weight above 17000, varied in the range 100–200°, the sign depending on the absolute

ISSN:0006-3525    

DOI:10.1002/bip.1968.360060907

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

年代:1968

数据来源: WILEY

摘要:

AbstractThe hydration of DNA is an important factor in the stability of its secondary structure. Methods for measuring the hydration of DNA in solution and the results of various techniques are compared and discussed critically. The buoyant density of native and denatured T‐7 bacteriophage DNA in potassium trifluoroacetate (KTFA) solution has been measured as a function of temperature between 5 and 50°C. The buoyant density of native DNA increased linearly with temperature, with a dependence of (2.3 ± 0.5) × 10−4g/cc‐°C. DNA which has been heat denatured and quenched at 0°C in the salt solution shows a similar dependence of buoyant density on temperature at temperatures far below theTm, and above theTm. However, there is an inflection region in the buoyant density versusTcurve over a wide range of temperatures below theTm. Optical density versus temperature studies showed that this is due to the. inhibition by KTFA of recovery of secondary structure on quenching. If the partial specific volume is assumed to be the same for native and denatured DNA, the loss of water of hydration on denaturation is calculated to be about 20% in KTFA at a water activity of 0.7 at 25°C. By treating the denaturation of DNA as a phase transition, an equation has immmi derived relating the destabilizing effect of trifluoroacetate to the loss of hydration on denaturation. The hydration of native DNA is abnormally high in the presence of this anion, and the loss of hydration on denaturation is greater than in CsCl. In addition, trifluoroacetate appears to decrease the ΔHof

ISSN:0006-3525    

DOI:10.1002/bip.1968.360060908

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

年代:1968

数据来源: WILEY

摘要:

AbstractThe dependence of the net hydration of DNA on its base composition has been measured by density gradient ultracentrifugation of three DNA's in a series of cesium and lithium salt solutions of different water activities. Extrapolation to zero water activity showed the dependence of the partial specific volume on base composition to be very small for CsDNA and aero for LiDNA. At least 99% of the dependence of buoyant density on base composition can be accounted for on the basis of a differential hydration, with a mole of adenine–thymine pairs binding about 2 moles more water than a mole of guanine–cytosine pairs in C

ISSN:0006-3525    

DOI:10.1002/bip.1968.360060909

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

年代:1968

数据来源: WILEY

摘要:

AbstractMeasurements of specific heat of wool‐water systems were made at approximately 5°C intervals over the temperature range −70 to 100°C. Ten different, samples were used, each with a different amount of absorbed water in the range from dry ness to saturation at 0°C. The graph of specific heat against temperature for dry wool is precisely linear over the complete temperature range, suggesting that thermal motion is entirely vibrational. When absorbed water is present the data can be conveniently discussed in terms of behavior below and above an amount of absorbed water of 22.7 g in 100 g of wool (22.7% of absorbed water). Below 22.7% there is only one temperature range in which the results indicate an appreciable transition in heat absorbing properties. The temperature of transition depends on water content but is higher than 0°C. Above 22.7% a second transition appears in the range −30 to 0°C and grows rapidly larger with increase of water content. The first transition is tentatively ascribed to a slightly cooperative breakdown of polar bonds in wool, and the second to a process analogous to melting in the absorbed water. The results are discussed in these terms as well as with reference to specific heat theories, the heat absorption of the wool component and the water component, and enthalpy differences between the vario

ISSN:0006-3525    

DOI:10.1002/bip.1968.360060910

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

年代:1968

数据来源: WILEY

ISSN:0006-3525    

DOI:10.1002/bip.1968.360060911

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

年代:1968

数据来源: WILEY