摘要:

AbstractThe theoretical kinetics are analyzed for reactions between torsionally stressed DNA and another compound which reacts at different rates with portions of the substrate molecule which are in different conformational states. These reactions are assumed to obey Michaelis‐Menten kinetics, so no cooperative effects occur. The DNA is regarded as being susceptible to a stress‐induced local conformational transition as described recently by Benham. Briefly, alterations in torsional stress consequent on superhelicity effectively change the relative concentrations of the two conformational states of the substrate, thereby influencing the course of the reaction. This theory is developed for transition between the B‐form helix and the single‐stranded, random‐coil states. To illustrate the influence of posited stress‐induced melting on kinetics, calculations are made on simple models of two biochemical phenomena. First, the variations in initial nicking rates of single‐strand‐specific endonucleases with substrate superhelicity are interpreted as arising from changes in the concentration of (stress‐induced) single‐stranded binding sites. Second, the observed dependence of the transcription rate of RNA polymerase core enzyme on substrate superhelicity is interpreted in terms of a model in which the rate‐limiting step in the initiation event is the formation of a complex between the enzyme and a single‐stranded region. Related experimental results are shown to be qualitatively consistent with the suggestion that sufficiently supercoiled DNA contains

ISSN:0006-3525    

DOI:10.1002/bip.1980.360191202

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

年代:1980

数据来源: WILEY

摘要:

AbstractA detailed assignment of the13C chemical shifts of κ‐ and ι‐carrageenans in their Na+and K+forms in D2O and dimethylsulfoxide is given. Evidence of the conformational transition induced by temperature variation in the absence of any gel formation on κ‐carrageenans is also presented. This evidence is based on13C‐nmr and optical rotation e

ISSN:0006-3525    

DOI:10.1002/bip.1980.360191203

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

年代:1980

数据来源: WILEY

摘要:

AbstractThe preferential interaction of sodium poly(α‐L‐glutamate) and poly(α‐L‐glutamic acid) with the solvent components in water/2‐chloroethanol mixtures has been determined using density‐increment measurements. The degree of preferential interaction was deduced from the density increments at constant molality of 2‐chloroethanol and at constant chemical potential of 2‐chloroethanol. Sodium poly(α‐L‐glutamate) and poly(α‐L‐glutamic acid) are both preferentially hydrated in the whole range of solvent composition. A dehydration process occurs during the 2‐chloroethanol‐induced coil‐to‐helix transition of sodium poly(α‐L‐glutamate). This dehydration process was attributed to the release of some moles of water from the neighborhood of the peptide bond during the nucleation of the helix. After the conformational transition, sodium poly(α‐L‐glutamate) is solvated by one 2‐chloroethanol molecule. The location of water and 2‐chloroethanol molecules in the different parts of the residue (more

ISSN:0006-3525    

DOI:10.1002/bip.1980.360191204

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

年代:1980

数据来源: WILEY

摘要:

AbstractA study of the near‐uv CD spectrum of lysozyme was carried out in the presence and absence of the inhibitor tri‐N‐acetylglucosamine, and theoretical chiroptical calculations based on the tetragonal crystal structure of the enzyme and the enzyme‐inhibitor complex were performed. The results of these calculations indicate that the near‐uv CD spectrum of lysozyme can be adequately explained in terms of negative rotatory strengths arising from the tryptophan1La(293–300 nm) and the disulfiden‐σ* bands (250 rm), and positive rotatory strength contributions from the tryptophan1Lbbands (291 nm) and the tyrosine1Lbbands (275 nm). Contributions to the rotatory strength of each band were approximated in terms of specific interactions between chromophores. It was found that the rotatory strength of most of the near‐uv transitions arises primarily from coupling interactions involving other side‐chain chromophores and amide groups which are in close proximity. Changes which are observed in the lysozyme CD spectrum on binding of tri‐N‐acetylglucosamine may be explained in terms of changes in the rotatory strength which result from interactions of the1Latransitions of the active‐site tryptophans with the acetamide groups of the inhibitor. The reasonable agreement which is found between the experimental and calculated rotatory strengths implies that the crystal conformation of lysozyme must resemble t

ISSN:0006-3525    

DOI:10.1002/bip.1980.360191205

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

年代:1980

数据来源: WILEY

摘要:

AbstractThe potentiometric data concerning the dependence of the apparent pKwith the degree of neutralization and the nature of counterions (Na or Ca) of galacturonic oligomers and polymers are discussed. They are characterized by an apparent charge density λappwhich predicts their electrostatic behavior. The calculated osmotic coefficients from Katchalsky and Manning's theories are in good agreement with the experimental data obtained by potentiometry. The dependence of the osmotic coefficient (ϕCa) as a function of the polymer concentration is established by potentiometry and interpreted in terms of a multichain aggregation. In addition, it is proven that no Ca2+is ever fixed in excess of stoichiometry. The dependence of CD spectra obtained under the same conditions as the pKmeasurements shows analogies between the two sets of results. The CD data, in agreement with13C‐nmr measurements, suggest that in dilute solutions the polymer adopts two conformations: one (acidic form) which may be represented by threefold screw symmetry and a second (Na or Ca form) which can be related to a twofold screw symmetry. The twofold screw symmetry is known to allow the formation of cooperative “egg‐box” fixation of Ca without conformati

ISSN:0006-3525    

DOI:10.1002/bip.1980.360191206

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

年代:1980

数据来源: WILEY

摘要:

AbstractComplex formation between tryptamine and mononucleotides and dinucleoside phosphates containing adenine and/or cytosine has been studied at five pD's ranging from 1.1 to 7.4 by proton magnetic resonance spectroscopy. Chemical shifts of base ring protons and the ribose anomeric proton in the nucleotides and indole ring protons in tryptamine were monitored and their changes with pD and intermolecular interactions interpreted qualitatively. Stacked complexes were found to exist at all pD's in the range studied. Complex geometries differ depending on pD. An electrostatic interaction between the tryptamine amino group and the nucleotide phosphate group contributes to complex formation above pD 4 but is not strong enough to shift the dinucleoside phosphate equilibrium towards the unstacked conformer.

ISSN:0006-3525    

DOI:10.1002/bip.1980.360191207

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

年代:1980

数据来源: WILEY

摘要:

AbstractCyclo(L‐Pro‐Sar)n(n= 2–4) with moderate flexibility and hydrophobicity of molecular structure was synthesized, and the characteristics of these cyclic peptides and their metal complexes in acetonitrile were investigated in connection with the residual properties using13C‐nmr measurements. The cyclic tetrapeptide cyclo(L‐Pro‐Sar)2showed a sterically hindered phenomenon in acetonitrile in which the amide backbone adopted acis‐trans‐cis‐transsequence. The cyclic hexapeptide cyclo(L‐Pro‐Sar)3existed as a mixture of several conformers whose interconversion is slow on the nmr time scale, includingcis‐cis‐transand/orcis‐trans‐transarrangement of the Sar‐Pro bond. Finally, it was demonstrated that the cyclic octapeptide cyclo(L‐Pro‐Sar)4behaved as a mixture of multiple conformers which allowed forcis‐transisomerism about the Pro‐Sar peptide bond, of which 20–30% had the all‐cisSar‐Pro bond isomer and the remaining 70–80% had one (or more)cisSar‐Pro bond isomer.13C‐nmr spectra also demonstrated that cyclo(L‐Pro‐Sar)n(n= 3,4) formed a 1:1 ion complex whose conformation was characterized by an all‐transpeptide bond in the presence of excess metal salt. Cation binding studies, using CD measurements, established that the ion selectivity of cyclo(L‐Pro‐Sar)4i

ISSN:0006-3525    

DOI:10.1002/bip.1980.360191208

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

年代:1980

数据来源: WILEY

ISSN:0006-3525    

DOI:10.1002/bip.1980.360191209

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

年代:1980

数据来源: WILEY

ISSN:0006-3525    

DOI:10.1002/bip.1980.360191201

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

年代:1980

数据来源: WILEY