摘要:

AbstractWe describe conditions which lead to complete helix formation of poly(I) in the presence of NH 4+. Binding of NH 4+is shown to be specific in the presence of Li+, which does not by itself support helix formation under these conditions. The NH 4+–poly(I) complex is characterized by uv, CD, and ir spectroscopy. The CD spectrum is strikingly different from those of the Na+or K+complexes, the first extremum being changed from negative for the metal ions to positive for NH 4+. A stereospecific model is proposed for the NH 4+–poly(I) helix in which the N of NH 4+is located on the axis of the four‐stranded helix, midway between planar tetramers formed by the bases. The model is consistent with the tetrahedral symmetry of NH 4+, the requirement for four acceptable hydrogen bonds, the observed stability of the helix, and the accepted geome

ISSN:0006-3525    

DOI:10.1002/bip.360210112

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

年代:1982

数据来源: WILEY

摘要:

AbstractIt has recently been proven that the counterion condensate around an isolated line charge in an electrolyte, as characterized by nonlinear Poisson‐Boltzmann theory, is an encapsulating δ‐function. Here the identical result is shown to hold in the framework of the polyelectrolyte theory of Fuoss, Katchalsky, and Lifson. The proof fully exploits analytic solutions to the differential equation which are not available for the nonlinear, cylindrical Poisson‐Boltzmann eq

ISSN:0006-3525    

DOI:10.1002/bip.360210113

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

年代:1982

数据来源: WILEY

摘要:

AbstractSynthetic cyclic octapeptides of general structurecyclo[Glu(γOBzl)‐Sar‐Gly‐(N‐R)Gly]2(R=n‐hexyl and cyclohexyl) transport calcium ions selectively across organic phases and phospholipid membranes. We have now used proton nmr spectroscopy (360 MHz) to study the solution conformation(s) of their calcium complexes. When Ca(ClO4)2was added to solutions of these peptides in CDCl3, nmr spectra of the resulting calcium complexes were characteristic of a single C2‐symmetric conformer. From a Karplus‐Bystrov analysis of vicinal coupling constants in both the peptide backbone and Glu side chain (treated as anABCC′MXspin system), in conjuction with model‐building studies, a structure was proposed in which the calcium ion is bound in an octahedral‐type complex by the four (coplanar) carbonyl groups of the (all‐trans) Glu‐Sar and Gly‐(N‐R)Gly peptide bonds. Occurrence of preferred rotamers about Glu side chain Cα–Cβbonds indicated that restricted rotation in peptide side ch

ISSN:0006-3525    

DOI:10.1002/bip.360210114

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

年代:1982

数据来源: WILEY

摘要:

Abstract31P‐nmr has been used to investigate the specific interaction of three divalent metal ions, Mg2+, Mn2+, and Co+2, with the phosphate groups of DNA. Mg2+is found to have no significant effect on any of the31P‐nmr parameters (chemical shift, line‐width,T1,T2, and NOE) over a concentration range extending from 20 to 160 mM. The two paramagnetic ions, Mn2+and Co2+, on the other hand, significantly change the31P relaxation rates even at very low levels. From an analysis of the paramagnetic contributions to the spin–lattice and spin–spin relaxation rates, the effective internuclear metal–phosphorus distances are found to be 4.5 ± 0.5 and 4.1 ± 0.5 Å for Mn2+and Co2+, respectively, corresponding to only 15 ± 5% of the total bound Mn2+and Co2+being directly coordinated to the phosphate groups (inner‐sphere complexes). This result is independent of any assumptions regarding the location of the remaining metal ions which may be bound either as outer‐sphere complexes relative to the phosphate groups or elsewhere on the DNA, possibly to the bases. Studies of the temperature effects on the31P relaxation rates of DNA in the absence and presence of Mn2+and Co2+yielded kinetic and thermodynamic parameters which characterize the association and dissociation of the metal ions from the phosphate groups. A two‐step model was used in the analysis of the kinetic data. The lifetimes of the inner‐sphere complexes are 3 × 10−7and 1.4 × 10−5s for Mn2+and Co2+, respectively. The rates of formation of the inner‐sphere complexes with the phosphate are found to be about two orders of magnitude slower than the rate of the exchange of the water of hydration of the metal ions, suggesting that expulsion of water is not the rate‐determining step in the formation of the inner‐sphere complexes. Competition experiments demonstrate that the binding of Mg2+ions is 3–4 times weaker than the binding of either Mn2+or Co2+. Since the contribution from direct phosphate coordination to the total binding strength of these metal ion complexes is small (∼15%), the higher binding strength of Mn2+and Co2+may be attributed either to base binding or to formation of stronger outer‐sphere metal–phosphate complexes. At high levels of divalent metal ions, and when the metal ion concentration exceeds the DNA–phosphate concentration, the fraction of inner‐sphere phosphate binding increases. In the presence of very high levels of Mg2+(e.g., 3.1M), the inner‐sphere ⇄ outer‐sphere equilibrium is shifted toward ∼100% inner‐sphere binding. A comparison of our DNA results and previous results obtained with tRNA indicates that tRNA and DNA have very similar divalent metal ion binding properties. A comparison of the present results with

ISSN:0006-3525    

DOI:10.1002/bip.360210115

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

年代:1982

数据来源: WILEY

摘要:

AbstractThe extent and modes of binding of the divalent metal ions Mn2+and Co2+to DNA and the effects of salt on the binding have been studied by measurements of the effects of these paramagnetic metal ions on the longitudinal and transverse relaxation rates of the protons of the solvent water molecules, a technique that is sensitive to overall binding. The number of water molecules coordinated to the DNA–bound Mn2+and Co2+is found to be between five and six, and the electron spin relaxation times and the electron‐nuclear hyperfine constants associated with Mn2+and Co2+are little or not affected by the binding. These observations indicate little disturbance of the hydration sphere of Mn2+and Co2+upon binding to DNA. An average 2–3‐fold reduction in the exchange rate of the water of hydration of the bound metal ions and an order‐of‐magnitude increase in their rotational correlation time are attributed to hydrogen‐bond formation with the DNA. The binding constants of Mn2+to DNA, at metal concentrations approaching zero, are found to be inversely proportional to the second power of the salt concentration, in agreement with the predictions of Manning's polyelectrolyte theory. A remarkable quantitative agreement with the polyelectrolyte theory is also obtained for the anticooperativity in the binding of Mn2+to DNA, although the experimental results can be well accounted for by another simple electrostatic model. The various modes of binding of divalent metal ions to DNA

ISSN:0006-3525    

DOI:10.1002/bip.360210116

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

年代:1982

数据来源: WILEY

摘要:

AbstractDNA with Mn2+as the only counterion has been prepared, and the extent of the Mn2+binding was determined under a variety of conditions through measurements of the proton relaxation enhancement of water. The total extent of Mn2+binding per DNA phosphate is found to be 0.43 ± 0.04, independent of the metal ion concentration in the experimental range of 2.8 × 10−5to 2.1 × 10−3M. The predictions of Manning's condensation theory and those obtained from solution of the generalized Poisson‐Boltzmann equation regarding the extent of divalent ion binding to polyelectrolytes, in the presence and absence of monovalent counterions, are compared with one another and with the experimental results. Good agreement between the two theoretical approaches is found, with less than 14% variance in the predicted extent of binding over a large range of mono‐ and divalent ion concentrations. While the predictions of both theoretical approaches generally agree with the experimental results, some discrepancies are noted and their possible origins

ISSN:0006-3525    

DOI:10.1002/bip.360210117

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

年代:1982

数据来源: WILEY

摘要:

AbstractUsing quasielastic light scattering, we have determined the diffusion coefficients of low‐density lipoprotein (LDL) and polystyrene latex spheres over the temperature range of 293–318 K. The results show that after correction for thermal changes in the solvent, there remains a residual negative temperature coefficient in the diffusion amounting to about −0.6%/K that is independent of the chemical type of complex. Confirmation of these results was obtained for LDL through sedimentation studies over a similar range of temperatures. The residual temperature coefficient was similar to that reported for oxyhemoglobin [W. B. Veldkamp and R. Votano (1980)Biopolymers19, 111–124] and greater than that found for bovine serum albumin by these and earlier workers. These observations show that the residual dependence on temperature is not an isolated phenomenon and could in part be explained by increased aggregation of particles, although this is not the primary cause of the

ISSN:0006-3525    

DOI:10.1002/bip.360210118

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

年代:1982

数据来源: WILEY

ISSN:0006-3525    

DOI:10.1002/bip.360210101

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

年代:1982

数据来源: WILEY