摘要:

AbstractMass–length ratios of fibers in fibrin gels were obtained from measurements of the angular dependence of the intensity of light scattered by dilute gels and from the permeability of more concentrated gels. The permeability was determined by measuring the forced flow of buffer through a short column of gel held in a glass tube. The results obtained with the two methods are consistent. At high pH and high ionic strength the mass–length ratio is found to be that calculated for a protofibril, i.e., a double strand of fibrin molecules laid end to end at a separation of 450 Å. This same value is found under conditions where the polymerized fibrin is not gelled (pH 10.25) and where it is just gelled (pH 10.0). At pH 7.4, ionic strength 0.35, the fibers are found to have a higher mass–length ratio, the average fiber consisting of some three protofibrils associated laterally. At pH 7.4 and low ionic strength (0.10M) the fibers are up to a hundred times more m

ISSN:0006-3525    

DOI:10.1002/bip.1977.360160102

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

年代:1977

数据来源: WILEY

摘要:

AbstractCopolymers ofL‐lysine andL‐isoleucine [poly(L‐Lysf,L‐Val1 −f)] containing 4–15% isoleucine were investigated using potentiometric titration and circular dichroism (CD) spectroscopy. With increasing isoleucine content, β‐sheet formation is favored over α‐helix formation at high pH and room temperature. The fraction of β‐sheet present, as a function of pH, calculated from titrations of poly(L‐Lys85.2,L‐Ile14.8), agreed well with data obtained from CD studies for the same copolymer. Thermodynamic parameters were determined from titrations using the method of Zimm and Rice; the partial free energy (ΔG°C → β) at 25° for the coil‐to‐β‐sheet transition for isoleucine was estimated to be −515 cal/mol; from the temperature dependence of free energy, the partial entropy (ΔS°cβ), and the partial free enthalpy (ΔH°c → β) of the coil → β transition for isoleucine is estimated to be 2.6 e.u. and 260 cal/mol, respectively. The partial thermodynamic parameters obtained for lysine are in good agreement with literature values. It is concluded from these studies that isoleucine

ISSN:0006-3525    

DOI:10.1002/bip.1977.360160103

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

年代:1977

数据来源: WILEY

摘要:

AbstractIn the presence of sodium poly(L‐glutamate) at pH = 7.5 the dye pseudoisocyanine in dilute aqueous solution (Cd= 1.10 × 10−5M) and low P/D values exhibits an absorption spectrum with a very sharp red‐shifted J‐band. Under the same conditions circular dichroism (CD) in the visible region is observed with an extremely sharp peak at the position of the J‐band. At pH = 4.6, where the polypeptide is in the α‐helix conformation, no such J‐band is observed and no CD spectrum can be detected at the same P/D values. CD spectra in the uv range demonstrate that the occurrence of the dye–polypeptide complex which gives rise to the J‐band at slightly alkaline pH is not accompanied by a conformational transition of the polypeptide towards

ISSN:0006-3525    

DOI:10.1002/bip.1977.360160104

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

年代:1977

数据来源: WILEY

摘要:

AbstractOptical property calculations are presented for poly(A·U), poly[(A‐U)·(A‐U)], poly(G·C), and poly[(G‐C)·(G‐C)]in RNA, B‐DNA, and C‐DNA conformations. An all‐order classical coupled oscillator polarizability theory was used, and an effective dielectric constant of 2 was assumed. The calculated CD spectra were found to be sensitive to both geometry and sequence. Agreement with the measured CD spectra of poly(A·U), poly(G·C), and poly(dG·dC) is very good. Calculations for other sequences and geometries are less satisfactory and are particularly poor for poly[(G‐C)·(G‐C)] in RNA geometry and poly(A·T) in B‐DNA geometry. Attempts to improve agreement with measured spectra by varying monomer properties have been only partially successful for these calculations, but they illustrate the types of changes that may prove to be necessary. Calculations using other published X‐ray coordinates for certain deoxypolynucleotides of simple sequence, some of which are quite different from B‐DNA coordinates, did not result in better agreement with measured spectra. Finally, the dependence of the calculated CD on chain length is examined. Results show that non‐nearest neighbor interactions can be important when runs of 3 or more identical bas

ISSN:0006-3525    

DOI:10.1002/bip.1977.360160105

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

年代:1977

数据来源: WILEY

摘要:

AbstractThe influence on the excess scattering functionP(μ) of flutuations in the electron density ρ within a macromolecule is treated, to the approximation that the solvent is a structureless medium of constant electron density ρ0. The results forP(μ) and the apparent value of the mean square radiusRapp2, can be expressed as functions of the excess electron density Δρ:P(μ) =X(μ) + (Δρ)−1Y(μ) + (Δρ)−2Z(μ) andRapp2=Rx2+ (Δρ)−1Ry2+ (Δρ)−2Rz2, whereX(μ) andRx2depend only on the shape of the macromolecule, whileY(μ) andRy2as well asZ(μ) andRz2depend on the shape and the fluctuations in ρ. By varying the electron density of the solvent, the contributions of the shape and the internal structure of the macromolecule can be resolved. The quantitiesRx2,Ry2, andRz2are evaluated for seven models to illustrate the relative importance of these contributions

ISSN:0006-3525    

DOI:10.1002/bip.1977.360160106

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

年代:1977

数据来源: WILEY

摘要:

AbstractExcess small angle X‐ray scattering in solvents of differing electron density has been calculated from the crystal structures obtained for rubredoxin, trypsin inhibitor, myogen, ferricytochrome c2, ribonuclease S, lysozyme, nuclease, myoglobin, α‐chymotrypsin, elastase, subtilisin, carboxypeptidase A, thermolysin, methemoglobin, deoxyhemoglobin, and a single polypeptide chain of M4lactate dehydrogenase. The scattering curves for each protein can be reproduced by the sum of three curves, with the weighting of the three curves depending on the electron density of the solvent. The radius of gyration obtained from the small angle X‐ray scattering by globular proteins in aqueous solution will usually exceed the values defined by the shape of the macromolecule. Deviations for certain of the proteins cited are calculated to be as large as 6%. These deviations arise from the tendency for the amino acid residues with low electron density to be situated closer to the center of the protein than the amino acid residues of high electron density. An upper limit of 19% is obtained for the discrepancy between the radius of gyration defined by the shape of a spherical globular protein of typical amino acid composition and the apparent radius of gyration measured for that protein in water by small angle X‐ray sc

ISSN:0006-3525    

DOI:10.1002/bip.1977.360160107

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

年代:1977

数据来源: WILEY

摘要:

AbstractThe helix–coil transition for poly(β‐benzyl‐L‐aspartate) [poly(Asp[OBzl])]in solvent mixtures of trifluoroacetic acid/deuterated chloroform (F3AcOH/CDCl3) was studied by means of proton and carbon‐13 nmr. Conformational fixation of the side chain occurs before the coil–helix transition of the backbone, when neighboring phenyl rings face each other. Another type of conformational fixation occurs in the side chain after the coil–helix transition of the backbone. These conformational changes of the side chain are due to the changes of the strength of the interaction between the side‐chain ester group and the F3AcOH molecule. In the absence of F3AcOH (coil‐forming solvent), the polymer has a rather rigid structure in which the side chain may wrap around the backbone. These conformational changes of the polymer are closely related to the changes of the interaction between the polymer an

ISSN:0006-3525    

DOI:10.1002/bip.1977.360160108

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

年代:1977

数据来源: WILEY

摘要:

AbstractThe charge‐transfer complexes of a poly(L‐tryptophan) sequence with imidazolium hydrochloride and poly(L‐histidine hydrochloride) have been investigated in 2,2,2‐trifluoroethanol by ultraviolet (uv), circular dichroism (CD), and fluorescence techniques. Both complexes exhibit absorption maxima centered at around 275 nm, whereas hypochromism with respect to the combined spectra of the constituents can be observed below 250 nm. All complexes show optical activity in the near uv and in the peptide absorption region, which is discussed in terms of the conformational properties of the donor. A marked decrease of the fluorescence intensity of theL‐tryptophan sequence is observed upon addition of imidazolium hydrochloride and poly(L‐histidine hydrochloride). From the fluorescence data the formation constant of the charge‐transfer complex between theL‐tryptophan sequence and imidazolium ions is

ISSN:0006-3525    

DOI:10.1002/bip.1977.360160109

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

年代:1977

数据来源: WILEY

摘要:

AbstractThe chiroptical properties of the cyclic dipeptidescyclo(L‐alanyl‐L‐tyrosine) andcyclo(L‐tyrosyl‐L‐tyrosine) have been investigated as a function of molecular conformation. Theoretical optical calculations and conformational energy calculations have been carried out as a function of the side‐chain dihedral angles χ1and χ2, and as a function of the angle of fold of the cyclic dipeptide backbone. The results of these theoretical calculations have been compared with experimental circular dichroism (CD) data. Theoretical predictions are in very good agreement with experiment forc(L‐Tyr‐L‐Tyr). Agreement was not quite as good forc(L‐Ala‐L‐Tyr), although the signs of all of the Cotton effects were apparently predicted correctly except for that associated with the lowest ener

ISSN:0006-3525    

DOI:10.1002/bip.1977.360160110

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

年代:1977

数据来源: WILEY

摘要:

AbstractThere are several studies of small molecule binding by biopolymers which use “Scatchard‐type” plots to determine the intrinsic binding constant (k) and pairwise interaction energy between bound sites (S). “Scatchard‐type” refers to the plot ln (r/m(n–r)) versusrin this text whereas “Scatchard plot” conventionally refers to the plotr/mversusr. The quantitiesr,n, andmare, respectively, the average bound ligand/site, ligand/site at saturation, and concentration of free ligand. Although these plots result from approximate solutions to the problem of binding small molecules on a general three‐dimensional lattice, they are often used to represent binding isotherms in linear systems for which the solutions are known exactly. A critical examination regarding the applicability of “Scatchard‐type” analysis to systems with a large number of binding sites is presented herein.Both experimental and simulated binding data are analyzed by “Scatchard‐type” and exact nearest neighbor plots (θ versus lnm, where θ =r/n). Numerical determination ofkandSfor the two theoretical approaches differ greatly in highly cooperative systems although their theoretical equivalence (interconversion) is demonstrated. A modified Scatchard theory, which correctly counts the degenerate energy states, is presented which reconciles this discrepancy. The “Scatchard‐type” plot for the modified theory in the highly cooperative limit is mathematically equivalent to the form ln (r/n–r) +p, wherepcontains all the molecular information and locates ther/n= 1/2 point. Contrary to the original theory, therdependence in the “Scatchard‐type” representation is devoid of molecular interpretation and is characterized by curvature in the low and high‐binding regions with a (maximum) slope of 4 atr/n= 1/2. The modified theory is shown to reproduce accurately the poly(cytidylic acid)‐guanosine data of Sarocchi et al. and to account for the m

ISSN:0006-3525    

DOI:10.1002/bip.1977.360160111

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

年代:1977

数据来源: WILEY