摘要:

AbstractThe thermal transitions of single‐stranded polynucleotides are noncooperative. In contrast, Cu(II) cooperatively disorders the single‐stranded helical structures of poly(A) and poly(C), as demonstrated by ORD and UV spectral changes as a function of the Cu2+activity, and by a dramatic chain‐length dependence of the spectral changes. Equilibrium dialysis binding studies indicate that the cooperative disordering is paralleled by a somewhat less cooperative binding process.The difference between the thermal‐ and Cu(II)‐induced transition is explained by the following mechanism. (1) Cu(II) initially binds in a noncooperative fashion to phosphate. (2) The Cu(II) so bound forms a second bond to a nonadjacent base site on the same polymer strand or another strand. These intramolecular and intermolecular crosslinks to the bases are responsible for the disordering. (3) The initial crosslinks formed provide nuclei for the cooperative formation of additional crosslinks, producing cooperative spectral changes paralleled by cooperative binding.A comparison of the spectral and binding transitions indicates that there is appreciable noncooperative binding of copper to phosphate, which produces no spectral changes in the presence of added electrolyte. This comparison also indicates that each copper crosslink disorders several bases. The formation of intermolecular crosslinks is demonstrated by a polymer concentration dependence of the disordering. The formation of intramolecular crosslinks can be deduced from the fact that the “cooperative unit” required to explain the differences between the hexamer, which does not readily form intramolecular crosslinks, and the polymer is considerably larger than the cooperative unit determined from the polymer results.The poly(A) disordering transition is less symmetrical than that of poly(C), particularly at low polymer concentrations. These results, together with other phenomena, are explained by a greater flexibility of poly(A), which favors the formation of small intramo

ISSN:0006-3525    

DOI:10.1002/bip.1976.360151002

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

年代:1976

数据来源: WILEY

摘要:

AbstractThe crystal structure of native cellulose (Valonia) has been analyzed by electron diffraction. Possible models for the structure were refined by rigid‐body least squares methods, which incorporated parameters defining the preferred orientation of the fibrils around their long axes in the cell wall lamellae. The structure was found to consist of an array of chains having the same sense (i.e., parallel), with packing parameters similar to those recently determined by X‐ray diffraction. The eight‐chain unit cell could be approximated adequately by a two‐chain monoclinic unit cell with dimensionsa= 8.18 Å,b= 7.84 Å,c= 10.38 Å (fiber axis), and γ = 97.04°; the space

ISSN:0006-3525    

DOI:10.1002/bip.1976.360151003

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

年代:1976

数据来源: WILEY

摘要:

AbstractSingle‐stranded polynucleotides are used as model systems for the investigation of conformational changes induced by electric fields. It is demonstrated that the single‐strand helix–coil transition in poly(A), poly(dA), and poly(C) can be induced by application of high electric fields. The transition is measured by UV absorbance using polarized light at an angle of 54.8° with respect to the vector of the electric field and by electrodichroism. A linear increase of the absorbance, reflecting the helix‐to‐coil transition, is observed at increasing field strength. When ions are added to the polymer, electric fields do not induce conformation changes, unless a threshold value of the electric field strengthE0is exceeded. At field strengths above this threshold, the degree of transition is a linear function of the increase in field strength. The threshold valuesE0show a linear increase with the logarithm of the ion concentration. Bivalent ions cause thresholds at much lower ion concentrations than mo‐novalent ions. The shielding efficiency of ions is correlated to the binding affinity of these ions to the polymer.The conformation changes induced by the field and the existence of thresholds can be explained on the basis of dissociation field effects. Similar threshold effects may be expected for other macromolecules as well as for membrane structures and may be important in the regulation of bi

ISSN:0006-3525    

DOI:10.1002/bip.1976.360151004

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

年代:1976

数据来源: WILEY

摘要:

AbstractEffects of deuteration on the Raman spectrum of a tryptophan residue have been examined. The 1386 cm−1line of deuterated tryptophan residue has been found to be useful for tracing the hydrogen‐deuterium exchange reaction of this residue in a protein. An examination on bovine α‐lactalbumin at pH 6.4 and at 20°C indicates that two of the four tryptophan residues exchange with a rate constant much greater than 9 × 10−4sec−1, while the other two exchange with a rate constant of 4 × 10−5sec−1. The latter two have been assigned to Trp 28 and Trp 108 of this protein. The kinetics of hydrogen‐deuterium exchange reaction of completely “free” tryptophan residue have been examined by a proton magnetic resonance study on tryptophan itself. By taking the result of this examination into account, the chance of exposure to the solvent for Trp 28 or Trp 108 has been estimated to be 3 × 1

ISSN:0006-3525    

DOI:10.1002/bip.1976.360151005

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

年代:1976

数据来源: WILEY

摘要:

AbstractThe coil‐to‐helix transition temperatures of hydrogen bearing and deuterated poly(γ‐benzyl‐L‐glutamate) in 1,3‐dichlorotetrafluoroacetone/H2O and/D2O mixtures, respectively, have been determined. Together with previously obtained data for the conformational transition of this polypeptide in normal and deuterated dichloroacetic acid, these results have been used in an analysis of the effect of deuterium substitution on the intrinsic stability of the α‐helical form of poly(γ‐benzyl‐L‐glutamate). The findings, consistent for both solvent systems, showed that the deuterated polypeptide is some 5% more stable than the normal protonated poly(γ‐benzyl‐L‐glutamate), while the polypeptide‐active solvent interaction enthalpy is also slightly increased by deuterium substitution in the respective molecules. A consideration of available data for poly(β‐benzyl‐L‐aspartate) reveals an anomal

ISSN:0006-3525    

DOI:10.1002/bip.1976.360151006

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

年代:1976

数据来源: WILEY

摘要:

AbstractPotential energy calculations were employed to examine the effect of ribose 2′‐O‐methylation on the conformation of GpC. Minimum energy conformations and allowed conformational regions were calculated for 2′MeGpC and Gp2′MeC. The two lowest energy conformations of 2′MeGpC and Gp2′MeC are similar to those of GpC itself. The helical RNA conformation (sugar pucker‐C(3′)‐endo, ω′ and ω,g−g−, bases‐anti) is the global minimum, and a helix‐reversing conformation with ω′, ω in the vicinity of 20°, 80° is next in energy. However, subtle differences between the three molecules are noted. When the substitution is on the 5′ ribose (Gp2′MeC), the energy of the helical conformation is less than that of GpC, due to favorable interactions of the added methyl group. When the substitution is at the 3′ ribose (2′MeGpC) these stabilizing interactions are outweighed by steric restrictions, and the helical conformation is of higher energy than for GpC. Furthermore, the statistical weight of the 2′MeGpCg−g−helical region is substantially less than the corresponding weight for Gp2′MeC. In addition, 2′MeGpC′s methoxy group is conformationally restricted to a narrow range centered at 76°. This group has a broadly allowed region between 50 and 175° in Gp2′MeC. These differences occur because the appended methyl group in 2′MeGpC is located in the interior of the helix cylinder, as it would be in polynucleotide, while it hangs unimpeded in Gp2′MeC. These findings suggest that 2′‐O‐methylation has both stabilizing and destabilizing influences on the helical conformation of RNA. For 2′MeGpC the destabili

ISSN:0006-3525    

DOI:10.1002/bip.1976.360151007

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

年代:1976

数据来源: WILEY

摘要:

AbstractCircular dichroism (CD) spectroscopy has been used to investigate the effects of changes in salt concentration and pH on the interactions between basic polypeptides and connective tissue glycosaminoglycans in dilute aqueous solution. The polypeptides undergo conformation‐directing interactions in the presence of glycosaminoglycans, which are subject to transitions as the ionic strength and pH are varied. For poly(L‐lysine), the conformational change due to interaction breaks down as the ionic strength (monovalent ions) is increased. Based on the ionic strength at which disruption occurs, the glycosaminoglycans can be placed in order of increasing strength of interaction: chondroitin 6‐sulfate, hyaluronic acid, chondroitin 4‐sulfate, heparin, and dermatan sulfate. Prior to the conformational transition, scattering effects are observed, indicating the development of larger aggregates. Each glycosaminoglycan induces α‐helicity for poly(L‐arginine), which does not break down as the ionic strength is increased, indicating a stronger interaction for this polypeptide. The pH‐induced transitions are in the pH range 2.5–3.8 and are probably related to deionization of carboxyl groups. For poly(L‐lysine) the conformational effect is disrupted at low pH. For poly(L‐arginine), the transitions are not complete, but appear to correspond to an in

ISSN:0006-3525    

DOI:10.1002/bip.1976.360151008

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

年代:1976

数据来源: WILEY

摘要:

AbstractThe morphology of fibrin strongly depends on solvent medium, as shown by clotting experiments carried out in the presence of different salts. The clots were characterized by electron microscopy and spectrophotometric methods; the kinetics of gelation were determined.In the presence of electrolytes which strongly delay clotting, the strands are thin and few branching points are observed; opposite morphological changes are induced by salts which act as accelerating agents. On the basis of this correlation, and of previous data on the structure of fibrin, a kinetic model of the self‐assembly process is outlined. It accounts well for the observed solvent effects on the morphology of the network.An important result emerging from our experiments is that the fibers undergo branching prior to gelation. Branching points arise from the defective growth of the fibers; a simple explanation of the occurrence of branching may be obtained by our self‐assembly mo

ISSN:0006-3525    

DOI:10.1002/bip.1976.360151009

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

年代:1976

数据来源: WILEY

摘要:

AbstractThe low‐temperature heat capacity of collagen (in the hydrated and dehydrated states) and the large entropy of collagen in the coiled state relative to the same protein in the helical state were investigated. The heat capacity for collagen in the solid state in the temperature range 4°–50° K changes proportionally to the square of temperature (Cp∼T2). Above 50°K there is a linear dependence (Cp∼T). The differences in the character of temperature dependence of heat capacity for the hydrated and dehydrated collagen show the importance of the specific interaction of water molecules with polypeptide chains of this protein. The peculiarities of the temperature dependence of the heat capacity difference (ΔCp) of hydrated denatured (random coiled) and hydrated native (helical) collagen are observed at 15°, 120°, and 240°K. These differences are caused by the varying degree of ordering of the hydrate water molecules in native and denatured collagen macromolecules. At all temperatures (4°–300°K) the entropy of the random coiled state is higher than that of collagen in the native state and at 298°K ΔS 0298= ∫ 0298(

ISSN:0006-3525    

DOI:10.1002/bip.1976.360151010

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

年代:1976

数据来源: WILEY

摘要:

AbstractPoly(Lys(HBr)‐Gly‐Pro‐Pro‐Gly‐Pro) has been synthesized and studied by circular dichroism (CD) spectroscopy. It is apparently the first polyhexapeptide collagen model reported with an ionizable side chain. The monomer (ε‐(p‐nitrobenzyloxycarbonyl)‐Lys‐Gly‐Pro‐Pro‐Gly‐Pro‐p‐nitrophenyl‐ester) was prepared by a stepwise strategy employing active esters. Polymerization inN,N‐dimethyl formamide, followed by removal of the Lys side chain protection with HBr/acetic acid, gave a polydisperse product. Fractionation was accomplished by gel filtration chromatography. The polydisperse material had a molecular weight (Mr= 5–17,000). High molecular weight fractions from triple helices under concentrated conditions at 2°C. The triple helical structure gives a CD pattern very similar to that of collagen and its triple helical analogs. However, unlike collagen, the polyhexapeptide undergoes spontaneous dissociation at temperatures substantially below the melting temperature from a triple helical form to single chains. This process is promoted at low concentrations, high temperature, neutral pH, and low molecular weight, and is apparently due, in large part, to unfavorable ionic side‐chain interactions. In addition to this relatively slow “ionic” dissociation the triple helical polypeptide may be thermally dissociated in a manner similar to collagen. The thermal denaturation is a relatively fast process compared with ionic dissociation.A high molecular weight fraction (3 ×Mr= 48,000) was found to melt at 42°C at neutral pH but increased to 54°C at pH 12 where the lysyl side chains are predominantly deprotonated. Furthermore, reconstitution of triple helices appeared to be more readily achieved at high pH. Thus it is concluded that ionic repulsion between side chains causes destabilization

ISSN:0006-3525    

DOI:10.1002/bip.1976.360151011

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

年代:1976

数据来源: WILEY