摘要:

AbstractThe structure of β‐chitin has been reexamined in anX‐ray diffraction study of the highly crystalline material available from pogonophore tubes. Oriented specimens were prepared by suspending the dispersed deproteinized tube in a fibrin clot which was then stretched into an oriented fiber. TheX‐ray diffraction patterns of pogonophore β‐chitin were explained by proposing that this material consists of a mixture of two structural phases which have been named β‐chitins A and B. This view was confirmed by the preparation of specimens which gave only the diffraction pattern of the A phase by treating the tubes with diaphanol. Both A and B phases gave rise to a series of hydrate structures. The lowest hydrate of β‐chitin A has been studied in detail and shown to consist of an anhydrous system of parallel chitin chains. For this form, the unit cell was monoclinic with dimensionsa= 4.85 Å,b= 10.38 Å (fiber axis),c= 9.26 Å, and β = 97.5°, and space groupP21. Reasonable agreement was obtained between the observedX‐ray intensities and those calculated for such a structure. Examination of the β‐chitin ofLoligopen and diatom spines showed that these materials also consis

ISSN:0006-3525    

DOI:10.1002/bip.1969.360070302

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

年代:1969

数据来源: WILEY

摘要:

AbstractA statistical analysis of the relation between the amino acid composition of proteins and the amount of helical secondary structure as characterized by the Moffitt b0parameter has shown a high degree of correlation of the b0parameter with those amino acids whose homopolymers can form helical structures. Using the data for 107 proteins, a linear relation was found between b0and the sum of the residue percentages of alanine, arginine, aspartic acid, cysteine, glutamic acid, leucine, andlysine. A statistical analysis of the Moffitt a0parameter, on the other hand, showed no statistically significant grouping of amino acids in relation to the amount of secondary β structure in a protein. A plot of b0versus a0–a 0R, where a 0Rrepresents the a0parameter for a fully denatured protein, for 55 proteins showed distinct nonlinearity. This nonlinearity was postulated to be due to presence of β structure, and a nomagram was constructed which allowed a semiquantitative estimate of the amount of helical and β‐type secondary structures from the b0versus a

ISSN:0006-3525    

DOI:10.1002/bip.1969.360070303

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

年代:1969

数据来源: WILEY

摘要:

AbstractIn this paper two points are considered: the methods of evaluating the helical content θ and the calculation of the parameters of the transition from experimental data and its interpretation. The parameter ΔHobtained is in good agreement with the calorimetric one andvis found to be independent of temperature and solvent and in agreement with the ordinarily accepted value for poly(γ‐benzyl‐L‐glutamate). The different methods of estimating θ are discussed for both pol

ISSN:0006-3525    

DOI:10.1002/bip.1969.360070304

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

年代:1969

数据来源: WILEY

摘要:

AbstractA method is described for determining the fractions of adenine‐uracil and guanine‐cytosine base pairs of partly double‐helical ribonucleic acids in aqueous solution. The method is based upon the ability to distinguish between paired and unpaired bases by means of infrared spectroscopy of their deuterium oxide solutions in the 1500–1800 cm−1frequency region of the infrared spectrum. An application of the method to yeast ribosomal RNA is described. At 30°C, ribosomal RNA is 64± 6% paired (35% GC, 29% AU). The method can be applied to determine the fractions of Watson‐Crick base pairs at a given temperature in any RNA containing the four common bases in

ISSN:0006-3525    

DOI:10.1002/bip.1969.360070305

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

年代:1969

数据来源: WILEY

摘要:

AbstractA new apparatus for the study of flow dichroism of macromolecules is described. The flow is down a long, narrow channel and an unpolarized light beam propagates along the flow direction. For a molecule such as DNA, in which the transition moments of the chromophores are perpendicular to the axis of orientation, an increase of absorbance is observed during flow. The apparatus is best suited for macromolecules which are readily orientable or at high shear gradients so that the extinction angle is close to 0°. The apparatus has the following advantages: dilute macromolecule solutions can be used; high shear gradients are easily obtained; only small volumes of solution are needed. The flow can be stopped rapidly so that relaxation times for disorientation can be studied. The flow dichorism of native, two‐stranded DNA has been measured for the molecular weight range of 0.6 × 106to 125 × 106, and for the shear gradient range (in aqueous solution at 25°C) from 200 sec−1to 21000 sec−1. At a fixed gradient the dichroism increases with molecular weight, but the curve is concave downwards. At a given molecular weight the dichroism increases with increasing shear gradient, but the curve is concave downwards. When the solvent viscosity and temperature are varied, the dichroism is a function of η〈G〉/Tshowing that the orientation is due to hydro‐dynamic shear stress and that the flexibility of DNA in a flow field is not due to local denaturation. The Zimm‐Rouse theory with no parameters taken from flow optical data predicts the correct order of magnitude of the dichroism but the experimentally observed shear gradient and molecular weight dependence do not fit the theory. This is an expected result, since the theory is believed to be applicable only at small distortions and extensions of

ISSN:0006-3525    

DOI:10.1002/bip.1969.360070306

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

年代:1969

数据来源: WILEY

摘要:

AbstractAn expansion valid for short times is presented for the rectangular approximation to the Lamm equation when the sedimentation coefficient can be expressed ass=S0(1−kc). The expansion allows the study of meniscus perturbations on the Faxén approximation, as well as a determination of when reflections from the base become significa

ISSN:0006-3525    

DOI:10.1002/bip.1969.360070307

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

年代:1969

数据来源: WILEY

摘要:

AbstractPrevious work had indicated that the molecular weight calculated from the equilibrium distribution of DNA in a CsCl density gradient appears to be one half the estimated value of the true molecular weight. In the present study, the sedimentation equilibrium of a polyclectrolyte in a density gradient generated by a low molecular weight electrolyte is treated in terms of the representation according to which a constant fraction of the counterions is considered permanently immobilized and the remaining fraction completely free. Equations for the preferential hydration and the molecular weight are obtained. The validity of the treatment is tested by applying it to experimental data on the equilibrium sedimentation of ϕX 174 DNA in CsCl, and it is found that the molecular weight calculated in this way is in agreement with the accepted value for this molecule. Also, recalculation of published data on density gradient centrifugation of T2 DNA in CsCl according to the present treatment brings up the molecular weight to within the range of values given by other methods

ISSN:0006-3525    

DOI:10.1002/bip.1969.360070308

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

年代:1969

数据来源: WILEY

摘要:

AbstractThe Hydrogen–tritium exchange character of poly‐D,L‐alanine was studied in detail as a model for the hydrogen exchange behavior of the unhindered, polymeric peptide group. The random chain nature of poly‐D,L‐alanine was evident in the uniformity of exchange rate of all its hydrogens and in the similarity between this rate and that of random chain poly‐D,L‐lysine and other known, unhindered secondary amide groups. An equilibrium isotope effect favoring the binding of tritium over protium to the extent of 21% was measured. Specific acid and base catalysis of the exchange and the absence of detectable general catalysis were demonstrated. Apparent energy of activation is 17 kcal/mole for deprotonation, largely due to dependence of Kwon temperature, and 15 kcal/mole for protonation, which correlates with the extreme apparent pK. The hydrogen –tritium exchange half‐time rate; of poly‐D,L‐alamine at any pH and temperature (T: °C) is given by the equation:\documentclass{article}\pagestyle{empty}\begin{document}$$ \[t_{\frac{1}{2}} \,(\min )\, = \,200\,\, \times \,10^{0.05{\rm T}} /\,[10^{{\rm pH} - 3} \, + \,10^{3 - {\rm pH

ISSN:0006-3525    

DOI:10.1002/bip.1969.360070309

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

年代:1969

数据来源: WILEY

摘要:

AbstractCircular dichroism (CD) curves are reported for poly dA, (pdA)6, (pdA)2, poly A, ApAp, ApA, AMP, dApA, pdApA, A‐2′‐O‐methyl pA, and A‐2′‐O‐methyl pAp. Analysis of these curves indicated the presence of single CD bands at 228–230 mμ and at 278–280 mμ in oligomers longer than dinucleotides. In the case of dinucleotides and mononucleotides (from the literature, in addition to those studied here), the 230 mμ CD of band appears but the 280 mμ CD band does not. We assign the 230 mμ band to a very weak π–π* transition at this wavelength. From theoretical considerations, we show that the 280 mμ band is not an exciton component of the strong π–π* transition at 260 mμ in adenine. We conclude that the 280 mμ CD band must be assigned to a distinct absorption, not previously reported, which we suggest arises from ann–π* transition. The fact that then–π* CD band at 280 mμ is not seen in mononucleotides or dinucleotides is ascribed to solvation of the adenine ring by water, which shifts the band to shorter wavelengths. Therefore, only interior residues of oligomers have the 280 mμ band, and the optical activity of a polymer cannot be computed from that of a dinucleotide, by using a nearest‐neighbor approximation. The existence of this end effect hag been tested, by taking it into account in computing the rotational strengths of the 278 mμn–π* transition for several oligomers; it is pointed out that a more sensitive test of this end effect would require CD data for the oligo dA series of 3 to 5 residues. We speculate about the structural and optical differences between poly dA and poly A, and point out the need for a theoretical t

ISSN:0006-3525    

DOI:10.1002/bip.1969.360070310

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

年代:1969

数据来源: WILEY

摘要:

AbstractThe isomerization of poly‐L‐proline in different solvents has been studied by NMR spectroscopy. Different resonance signals for the CHαprotons have been obtained for the two different helical conformations of thus compound, namely form I and for

ISSN:0006-3525    

DOI:10.1002/bip.1969.360070311

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

年代:1969

数据来源: WILEY