摘要:

AbstractThe conformations of melanostatin have been studied experimentally using CD spectroscopy and via calculations. In aqueous solution and 2,2,2‐trifluoroethanol (TFE) there is no evidence that monomers of the tripeptide exist in an ordered (β‐bend) structure. In water and TFE solutions (3–6 × 10−4M) the neutral molecules aggregate very slowly, taking about 3 days to attain equilibrium at room temperature. At equivalent concentrations in TFE, although not in water, the cationic molecules also slowly aggregate, although to a lesser extent. Calculations using rotational isomeric state theory give the most probable unperturbed end‐to‐end distance of the molecule at 9.3 ± 0.1 Å and indicate that a vast majority of the molecules exist in some extended conformation, end‐to‐end distance ≥6 Å. Only 0.4% of the molecules are calculated to have O…︁H separations compatible with a β‐bend structure. An intramolecular hydrogen bond must have an energy at least 2 kcal/mol lower than that of an intermolecular hydrogen bond to solvent if a β‐bend is

ISSN:0006-3525    

DOI:10.1002/bip.1979.360180802

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

年代:1979

数据来源: WILEY

摘要:

AbstractThe “condensed” counterions which characterize high‐charge‐density polyelectrolyte solutions can be analyzed into two subpopulations: (1) site‐bound counterions and (2) atmospherically entrapped counterions. The distinction is achieved experimentally by combining the data from self‐diffusion coefficient or electrical mobility measurements, which give the amount of “condensed” ions, and those from nmr, chemical shift measurements, which indicate the amount of site‐bound ions. In the case of a solution of chondroitin sulfate with excess Co++counterions, it can be estimated that 20% of the structural charge of the polyion is neutralized by site‐bound, dehydrated, condensed counterions, while a further 30% is neutralized by atmospherically entrapped, h

ISSN:0006-3525    

DOI:10.1002/bip.1979.360180803

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

年代:1979

数据来源: WILEY

摘要:

AbstractRaman spectra were measured for poly(L‐histidine) in H2O, poly(L‐histidine‐d2and ‐d3) in D2O,L‐histidine in H2O,L‐histidine‐d3(andd4) in D2O, and 4‐methylimidazole in H2O with various pH (or pD) values. The Raman scattering peaks observed for these samples were ascribed to the neutral and positively charged imidazole groups on the basis of the spectral changes due to the pH variation and to the deuterium substitution of the imino protons. The vibrational modes of these peaks were deduced from the normal coordinate analysis made on the positively charged and neutral 4‐ethylimidazoles. The Raman scattering peaks from the imidazole groups in the neutral form clearly indicate that these imidazole groups exist in the equilibrium between the two tautomeric forms, the 1‐N protonated from (tautomer I) and the 3‐N protonated one (tautomer II). For example, the breathing vibration of the 1‐N protonated form is observed at 1282 cm−1forL‐histidine and at 1304 cm−1for 4‐methylimidazole, while the breathing vibration of the 3‐N protonated form is observed at 1260 cm−1forL‐histidine and 4‐methylimidazole. From the temperature dependence of the relative intensities of the tautomer I peak to that of the tautomer II, it was concluded that the tautomer I is energetically more stable than the tautomer II, and the ΔHvalue is 1.0 ± 0.3 kcal/mol forL‐histidine and 0.4 ± 0.1 kcal/mol for 4‐methylimidazole. Poly(L‐histidine) with the neutral imidazole side chains shows the amide I peak at 1672 cm−1, indicating that the sample assumes the antiparallel pleated‐sheet structure. Poly(L‐Ala75L‐His25) and poly(L‐Ala50L‐His50) were found

ISSN:0006-3525    

DOI:10.1002/bip.1979.360180804

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

年代:1979

数据来源: WILEY

摘要:

AbstractThe particular role of H1 in the structure of histone–DNA associations is shown by means of ir linear dichroism. H1–, H2A–, and H4–DNA complexes are studied for different histone: DNA input ratios and various relative humidities (r.h.). The measurement of the dichroic ratios allows one to determine the secondary structure of DNA in the complexes. It is shown that the progressive addition of histone H2A or H4 to DNA inhibits the structural B → A transition and DNA remains in a B‐type form at low r.h. It is found that the B → A transition is inhibited for 19 or 26 base pairs of DNA per molecule of H2A or H4. The stabilization of DNA in a B‐conformation by H2A and H4 has been also observed by H2B and H3 but with a different efficiency. In contrast, histone H1, which does not belong to the core of the nucleosomes in chromatin, leaves the DNA in H1–DNA complexes free to adopt an A conformation at low r.h. for H1/DNA ratios below 0.6/1. Thus a major difference in the structural role between histone H1 and histones belonging to the nucleosomal core with respect to the conformational flexibility of DNA in the histone–DNA comple

ISSN:0006-3525    

DOI:10.1002/bip.1979.360180805

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

年代:1979

数据来源: WILEY

摘要:

AbstractThe interactions between surfactants and block polypeptides were investigated by titration calorimetry and CD. The polypeptides exhibited signs of an interaction only with surfactants bearing a charge opposite to the charge on the polymer. The stoichiometry of the resulting complex was determined to be approximately equal to the polymer net charge. In general, a decrease in helical content accompanied the interaction between the block polypeptides and the surfactants. Both positive and negative enthalpy changes were noted, depending on the heat of micelle formation. None of the thermal effects noted were preceded by polymer unfolding, as is characteristic of the interaction between surfactants and typical globular proteins.

ISSN:0006-3525    

DOI:10.1002/bip.1979.360180806

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

年代:1979

数据来源: WILEY

摘要:

AbstractThe polymerization of various experimentally observed conformers of RNA from tRNA and some dinucleoside monophosphates have been examined with a program that computes the basic helix parameters directly from the six backbone torsion angles ω′, ϕ′, ψ′, ψ, ϕ, ω to given(= 360/θ), the number of residues per turn;h, the rise per residue; andr, the radius of the phosphate atoms from the helix axis. The single‐stranded regions of tRNA that have A‐form residues have a notably lower value ofnthan the double‐stranded regions. The G‐U “wobble” base pair is shown to be an energetically strained left‐handed form. The A‐form dinucleoside monophosphates also have a low value ofn. A model of UpAl polymerized as a fourfold left‐handed helix with the bases on the outside and phosphates on the inside is investigated for its sharp 90° turn angle characteristics. UpA2 cannot be polymerized due to a low values ofh(1.31 Å) andr(2.72 Å), which cause steric hindering. An eightfold model of poly(rA) is discussed as are the nonhelical residues of tRNA. Finally, the effects of small changes in dihedral angles and bond lengths and angles on the helical parameters are investigated and discussed b

ISSN:0006-3525    

DOI:10.1002/bip.1979.360180807

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

年代:1979

数据来源: WILEY

摘要:

AbstractIn this paper we analyze theoretically the observable details of the differential melting curves (DMC) and the denaturation maps (DM) of a DNA. With the help of a mathematical model, we explore their implications, their relation with each other and with the genetic map of the molecule, and discuss possible future applications. ϕX174 is used as the example, since its sequence and genetic map are available. We find that each gene section of ϕX174 has a characteristic DMC. A reconstruction scheme to get the DMC of a whole piece from those of its constituent genes is shown to be fairly successuful. The relations between the melting curve and the denaturation maps are clarified. We observe that nearly always, the beginning and end of a gene melt at lower temperatures. The sharp features in the DM indicate that despite the long‐range cooperative interactions, the DM do reflect the local sequence effect. Denaturation maps (theoretical) of ϕX174 and SV40 are presented. From available data of other authors, we estimate that the dependence of the melting temperaturetmonGC, the fraction of (G+C)‐content, and onx, the ionic concentration in fractions of the standard saline citrate solution, can be expressed astm(x,GC) = ‐5.2 (logx)GC+ 18.4 logx+ 41.0GC+ 69.4. The first two coefficients are less

ISSN:0006-3525    

DOI:10.1002/bip.1979.360180808

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

年代:1979

数据来源: WILEY

摘要:

AbstractSpectrophotometric techniques have been employed to study the binding of bromophenol red (BPR) to hen egg white lysozyme and the consequent inhibition of enzyme activity. Experimental evidence is given from the dye binding studies in the presence of hexasaccharide and from the studies on activity that BPR binds at a site outside the proposed cleft region (A–F) in such a way that it inhibits the lytic activity towards cell walls but does not inhibit the activity towards hexasaccharide. These observations are consistent with the kinetics of binding [studied using temperature‐jump (T‐jump)] in the presence of Co++or chitotriose in large concentrations and the experiments with acetylated lysozyme which suggest that the binding site of BPR is closer to a lysine residue near the cleft. It is suggested that the binding site of BPR could be important in positioning the peptide segment of the cell walls, which are cleaved in the cleft.Evidence for the statement that this binding takes place at least by a two‐step process, in which the bimolecular step is followed by a slower monomolecular step, is given from the observations of two types of 1:1 complexes at 24°C in equilibrium studies and from the concentration dependence of the relaxation observed at 605 nm in the T‐jump experiments. The binding process is examined by analyzing the T‐jump data obtained between 18 and 33°C in the pH range 5.2–9.2 and ionic strength 0.01–01. The ionic strength and pH dependences of the equilibrium constant associated with the bimolecular stepk2/k1and the forward rate constant associated with monomolecular stepk3have been given as evidence for the suggestion that a Coulombic interaction is involved in the first step of binding. However, the final state of binding is hydrophobic in nature. The enthalpy of activation ΔH f≠and the entropy of activation ΔS f≠associated withkf[=k3(k1/k2)] showed compensation behavior with pH variation, with maxima around pH ∼ 7.5 in H2O. This has been interpreted as a maximal disordering of water structure in a region of the enzyme at this pH during the monomolecular step. However, the binding of chitotriose or Co++in the cleft reduces the ΔH f≠and ΔS f≠associated with the monomolecular step of BPR binding, probably by disordering the structured water during their binding in the cleft. The differences in the kinetic parameters obtained in H2O and in D2O probably arise due to subtle differences in the conformation of the enzyme in the two solvents and apart from isotope effects.The correlation between the pH (or pD) dependence of the “intrinsic activity” towards cell walls and ΔH f≠or ΔS f≠indicates that ordered water structure could be playing a role in controlling the catalytic activity. It is also suggested that this factor is associated with the rate constantk3sof the monomolecular step leading to the formation of the final bound state of the substrate

ISSN:0006-3525    

DOI:10.1002/bip.1979.360180809

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

年代:1979

数据来源: WILEY

摘要:

AbstractWe report on experiments showing that the presence of some monohydric alcohols in the solution medium perturbs the kinetics of bovine liver β‐galactosidase. The effect increases with increasing alcohol concentration and alkyl group size. To explain this effect we suggest a model that considers the existence of functional and nonfunctional enzyme conformations in thermodynamical equilibrium. This equilibrium is dependent both on alcohol concentration and on alkyl group si

ISSN:0006-3525    

DOI:10.1002/bip.1979.360180810

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

年代:1979

数据来源: WILEY

摘要:

AbstractWe studied the effect of methanol, ethanol,iso‐propanol, andn‐propanol on the reaction of hemoglobin with oxygen. The oxygen affinity was found to decrease with increasing alcohol concentration and alkyl group size; no detectable effect on Hill's constant was found. Difference spectroscopy indicatedKRnot to be affected by the presence of alcohols; the lowered affinity was then attributed to an altered equilibrium between T and R conformations of hemoglobin. The results have been analyzed in such a way as to allow separation of electrostatic contributions to free energy difference between the T and R states from nonelectrostatic ones. The nonelectrostatic term has been attributed to protein–solvent hydrophobic interactions. Values of hydrophobic free energy are in good agreement with analogous data estimated by correlating different results previously reported in the liter

ISSN:0006-3525    

DOI:10.1002/bip.1979.360180811

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

年代:1979

数据来源: WILEY