摘要:

AbstractWe have used25Mg‐nmr to investigate the binding of magnesium ions to double‐stranded DNA. We have measured line shapes for25Mg in the presence of monodisperse calf thymus DNA (160 base pairs; b.p.) (magnesium : phosphate = 2.0) at two different field strengths, 11.75 T and 7.05 T, and used the isotropic model of two‐site exchange developed by Westlund and Wennerstrom to simultaneously fit the line shapes at both field strengths. This model does not reproduce the observed field dependence. This is in contrast to a previous study [E. Berggren, L. Nordenskiold, and W. H. Braunlin (1992),Biopolymers, Vol. 32, pp. 1339–1350] in which a similar model of isotropic two‐site exchange qualitatively reproduced the temperature dependence of the line widths.Relaxation rates were also measured as a function of magnesium : phosphate ratio and colon type. These measurements were used to assess the sensitivity of magnesium relaxation measurements to small changes in DNA structure induced by changes in the solvent environment. The temperature dependence of the line shape varies with the type of coion (chloride or sulfate) present. This coion dependence of the line shape is consistent with the coion dependence of the aggregation midpoint temperature reported by Bloomfield and co‐workers [O. A. Knoll, M. G. Fried, and V. A. Bloomfield (1988) inStructure and Expres‐sion, Vol. 2, R. H. Sarma and M. H. Sarma, Eds., Adenine Press, New York] and attributed to a lyotropic effect. These results suggest that even at low magnesium : phosphate ratios, relaxation parameters are specific to each magnesium–coion–DNA system. © 1994 Jo

ISSN:0006-3525    

DOI:10.1002/bip.360340602

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

年代:1994

数据来源: WILEY

摘要:

AbstractThe complex formation behaviors of an α‐helical polypeptide containing pyridyl ligands in the side chains, poly (Nω‐2‐pyridylmethylL‐glutamine) (P2PG), were investigated by absorption and CD spectroscopy using Cu2+ion as a guest molecule in 2,2,2‐trifiuoroethanol solution. In the low Cu2+ion concentration, P2PG exhibited the predominant formation of Cu2+and two pyridyl side‐chain complexes, involving a regular arrangement of the pyridyl side chains. On the other hand, the complexes were converted to Cu2+and one pyridyl side‐chain species with increasing Cu2+ion concentration. The conversion was accompanied by the disappearance of the side‐chain ordered structure without any changes in the backbone conformation. Moreover, the still remaining coordination sites of Cu2+were capable of complexing monomer pyridine (Py) added as a second guest to form ternary complexes, P2PG‐Cu2+‐Py, following the reconstitution of the ordered structure on the periphery of the α‐helix backbone. The unique characteristics of P2PG can be explained in terms of the restriction of the pyridyl side chains to form intramolecular chelate complexes, depending on the rigid α‐helix conformation.

ISSN:0006-3525    

DOI:10.1002/bip.360340603

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

年代:1994

数据来源: WILEY

摘要:

AbstractThe cyclo7,10[Cys7,Cys10,Nle12], cyclo7,10[Cys7,D‐Ala9,Cys10,Nle12], and cyclo7,10[Cys7,L‐Ala9,Cys10,Nle12] analogues of the α‐factor mating pheromone (WHWLQLKPGQPMY) of the yeastSaccharomyces cerevisiaewere studied in DMSO/water (80 : 20) and aqueous solution by nmr spectroscopy. In addition, the cyclo7,10[Cys7,D‐Val9,Cys10,Nle12] α‐factor was examined in DMSO/water. Nuclear Overhauser effect (NOE) and NHdδ/dTdata indicate that the cyclo7,10[Cys7,D‐Val9,Cys10,Nle12] α‐factor adopts a type II β‐turn in DMSO/water and that the cyclo7,10[Cys7,D‐Ala9,Cys10,Nle12] ‐ and cyclo7,10‐[Cys7,L‐Ala9,Cys10,Nle12] α‐factor analogues adopt type II and type I/III β‐turns, respectively, in both DMSO/water and aqueous solutions. In aqueous solution, residues 8 and 9 of the cyclo7,10[Cys7,Cys10,Nle12] α‐factor appear to adopt at least two distinct conformations, one of these being identified as a type I/III β‐turn. In contrast, the cyclo7,10[Cys7,Cys10,Nle12] α‐factor appears to adopt predominately a type II β‐turn in DMSO/water. Quantitative NOE measurements of the cyclo7,10[Cys7,Cys10,Nle12]‐, cyclo7,10[Cys7,D‐Val9,Cys10,Nle12]‐, and cyclo7,10[Cys7,L‐Ala9,Cys10,Nle12] α‐factors in DMSO/water were used to derive three‐dimensional structures of the cyclo7,10[Cys7,Pro8,X9Cys

ISSN:0006-3525    

DOI:10.1002/bip.360340604

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

年代:1994

数据来源: WILEY

摘要:

AbstractThe membrane channel‐forming polypeptide, Leu1‐zervamicin, Ac‐Leu‐Ile‐Gln‐Iva‐Ile5‐Thr‐Aib‐Leu‐Aib‐Hyp10‐Gln‐Aib‐Hyp‐Aib‐Pro15‐Phol (Aib: α‐aminoisobutyric acid; Iva: isovaline; Hyp: 4‐hydroxyproline; Phol: phenylalininol) has been analyzed by x‐ray diffraction in a third crystal form. Although the bent helix is quite similar to the conformations found in crystals A and B, the amount of bending is more severe with a bending angle ≈ 47°. The water channel formed by the convex polar faces of neighboring helices is larger at the mouth than in crystals A and B, and the water sites have become disordered. The channel is interrupted in the middle by a hydrogen bond between the OH of Hyp (10) and the NH2of the Gln (11) of a neighboring molecule. The side chain of Gln (11) is wrapped around the helix backbone in an unusual fashion in order that it can augment the polar side of the helix. In the present crystal C there appears to be an additional conformation for the Gln (11) side chain (with ≈ 20% occupancy) that opens the channel for possible ion passage. Structure parameters for C85H140N18O22· xH2O · C2H5OH are space group P212121,a= 10.337(2) Å,b= 28.387(7) Å,c= 39.864(11) Å,Z= 4, agreement factorR= 12.99% for 3250 data observed>3

ISSN:0006-3525    

DOI:10.1002/bip.360340605

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

年代:1994

数据来源: WILEY

摘要:

AbstractSeveral rheological properties of aqueous alginate solution have been investigated in the absence and presence of four divalent cations: Ca, Cu, Mn, and Co. The concentration dependence of viscosity in alginate solution without addition of divalent cations shows a similar behavior to that found for other polysaccharides and synthetic polymers. We have found that for Ca(II) ‐added systems, sol‐gel transition curves for different alginate concentrations can be superimposed over the concentration range of this study by using a normalized parameter. Shear‐thinning behavior has been observed for all the alginate solutions by addition of four divalent cations over sol‐gel transition process. The values of exponentbin the power‐law relation\documentclass{article}\pagestyle{empty}\begin{document}$ {\eta \propto \dot \gamma }$\end{document}were found in the range of −0.3 ∼ −0.6. It is suggested that a considerable portion of divalent cations remains in a cross‐linking state even under shear flow. The results obtained from thixotropy measurements indicate the existence of a major framework formed in the gel‐like and well‐gelled samples, which can be maintained until shear rate (or shear stress) reaches a threshold value. © 199

ISSN:0006-3525    

DOI:10.1002/bip.360340606

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

年代:1994

数据来源: WILEY

摘要:

AbstractBovine carbonmonoxy hemoglobin investigated with light scattering studies is found to dissociate from its native tetramer structure into dimers and monomers. The values of the hydrated tetramer radius,RT= 32.1 Å, and the fractional dissociation vs pH, have been obtained at different ionic strengths from the autocorrelation function of the scattered light.The results suggest that a relevant contribution to Hb dissociation is due to electrostatic effects and, by means of a model derived by Tanford, it has been possible to predict the behavior of dissociation. Among the findings of this approach, we recall the estimates of the electrostatic energy contributions to Hb dissociation, up to ≃ GRT, and the predicted charge of tetrameric Hb vs pH, which agrees very well with the experimental data. © 1994 John Wiley&Sons,

ISSN:0006-3525    

DOI:10.1002/bip.360340607

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

年代:1994

数据来源: WILEY

摘要:

AbstractA new method for calculating a set of low‐frequency normal modes in macromolecules is proposed and applied to the case of proteins. In a first step, the protein chain is partitioned into blocks of one or more residues and the low‐frequency modes are evaluated at a low‐resolution level by combining the local translations and rotations of each block. In a second step, these low‐resolution modes are perturbed by high‐frequency modes explicitly calculated in each block, thus leading to the exact low‐frequency modes. The procedure is tested for three cases–decaalanine, icosaleucin, and crambin–using a perturbation‐iteration scheme in the second step. Convergence properties and numerical accuracy are assessed and tested for various partitions. The low‐resolution modes obtained in the first step are always found to be good starting approximations. Potential advantages of the method include a central processing unit time roughlyN2dependent on the size of the problem (Nbeing the number of degrees of freedom), the possibility of using parallel processing, the nonrequirement for loading the complete mass‐weighted second‐derivative input matrix into central memory, and the possibility of introducing in the procedure further structural hierarchy, such as secondary structures or motifs. In addition, any improvement or refinement of the algorithm benefits from the efficient formalism of the effective Hamiltonian theory. © 1

ISSN:0006-3525    

DOI:10.1002/bip.360340608

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

年代:1994

数据来源: WILEY

摘要:

AbstractThe conformational properties of fragment 18–47 of rabbit uteroglobin in aqueous solution containing SDS micelles were investigated by two‐dimensional nmr spectroscopy and molecular dynamics calculations. The fragment comprises helices II and III and the β‐turn connecting the two helices.The nmr results and nmr‐restrained molecular dynamics calculations showed that in the isolated fragment the elements of secondary structure present in the intact protein are preserved only in part. Specifically, a well‐defined α‐helix was found in the sequence 33–44, corresponding to helix III of uteroglobin, while the regions of helix II and β‐turn are characterized by high flexibility in the fragment. © 1994 J

ISSN:0006-3525    

DOI:10.1002/bip.360340609

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

年代:1994

数据来源: WILEY

摘要:

AbstractThe secondary structure of xanthan in solutions of relatively low salt concentration and at room temperature has been investigated using static light scattering experiments. Additional evidence has been found for a dimeric structure at 25°C in 0.01MNaCl. From the experimentalz‐average mean square (ms) radius of gyration, a value for the persistence lengthphas been estimated, taking explicitly into account the polydispersity of the three samples used, which has been established by gel permeation chromatography (GPC) measurements. The experimental particle scattering functions of the three samples are consistent with theoretical estimates for polydisperse systems with the same value ofp= 65 ± 10 nm and the molar mass per unit length for a dimeric structure.This secondary structure remains unaffected by the ionic strength in the 0.005–0.0lM range. Partial aggregation seems to occur at higher NaCl concentrations.Light scattering and GPC data show that heating the xanthan 0.01MNaCl solutions to about 70°C considerably reduces the Mwof the low molar mass sample (2.3 × 105‐g·mol−1), contrary to what is observed for the high molar mass sample (1.8 × 106‐g·mol−1). These experimental findings can be accounted for by a partial temperature‐induced dissociation of the xanthan dimers according to an all‐or‐none mechanism. ©

ISSN:0006-3525    

DOI:10.1002/bip.360340610

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

年代:1994

数据来源: WILEY

ISSN:0006-3525    

DOI:10.1002/bip.360340611

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

年代:1994

数据来源: WILEY