摘要:

AbstractEquilibrium dialysis and electronic and nuclear resonance spectroscopy show that tRNA cooperatively binds divalent metal ions at very low concentrations (free metal concentration 3 × 10−6M). The first two methods show that different purified tRNAs have a very similar behavior, including initiator tRNAFmet. tRNAs with an extra arm in the clover‐leaf model, however, appear to have a slightly different behavior.The binding can be described in terms of two classes of sites. The cooperative association of divalent ions binding first does not parallel a cooperative change in the hyperchromism of the tRNA, while the non‐cooperative association of the second class of divalent ions corresponds to the concentrations needed to obtain a cooperative melting of the tRNA.The temperature dependence of the number of binding sites and of their binding constants is also presented.The nature of the divalent ion gives the following efficiency: for the cooperativity Co++>Mg++>Mn++for the weak binding sites Mn++>Co

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110702

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

年代:1972

数据来源: WILEY

摘要:

AbstractA structure is proposed for xylan hydrate as a result of investigations by x‐ray fiber diffraction and computer‐aided chain‐packing methods. The unit‐cell dimensions are a = b = 9.16 Å, c (fiber axis) = 14.85Å, γ = 120° and the proposed anti‐parallel chain arrangement corresponds to a space group symmetry of P3221. Left‐handed threefold screw helices are stabilized in this conformation by their interaction with chains of water molecules, so that a satisfactory hydrogen‐bonding scheme is achieved. The proposed structure provides an explanation of the changes in the x‐ray diagram with relative humidity and yields a very good structure factors agreement. An x‐ray fiber diagram corresponding to a higher hydrate (xylan dihydrate) is presented. Comments are made on the possible role of xylan in nature and in te

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110703

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

年代:1972

数据来源: WILEY

摘要:

AbstractA new type of polymer salt was prepared from an equivalent mixture of partly sulfated polyvinyl alcohol (polyanion, Q−) and partly aminoacetalyzed polyvinyl alcohol (polycation, P+). With respect to a three‐component system composed of this polymer salt (P+Q−), water, and a microsalt (K+A−), phase relationships, as represented by complex coacervation, were investigated. Experimental results were discussed according to a theoretical equation for the free energy of mixing derived by taking into account the entropy and enthalpy contributions as ascribed for non‐ionic polymer solution, and the electrostatic free energy expression as derived

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110704

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

年代:1972

数据来源: WILEY

摘要:

AbstractSpectrophotometric measurements were made on the extent of binding of formaldehyde to polyuridylic acid under conditions of varying temperature and formaldehyde concentration. The data is interpreted in terms of a temperature‐dependent stacking of the bases in poly U at 20°, but not at 40°C. A theory of cooperative stacking is developed which considers the base residues to be either non‐bonded, non‐bonded and methylolated, or stacked. The results indicate essentially non‐cooperative base stacking under these conditions with an equilibrium constant for base stacking of 0.9

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110705

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

年代:1972

数据来源: WILEY

摘要:

AbstractThe molecular parameters of pronase‐treated acid‐soluble bovine skin collagen (P‐ASC) were determined from low‐shear gradient viscosity, electric birefringence, and electron microscopic data in order to determine the shear gradient range in which viscosity studies yield data which can be correctly interpreted by use of the various hydrodynamic equations for prolate ellipsoids of revolution. The P‐ASC solutions could be characterized by a single relaxation process in electric briefringence with rotary diffusion coefficient θ of 810 sec−1and a corresponding molecular length of 2850 Å. Viscosity data were found to be shear gradient dependent and only the extrapolated zero‐shear value [η]D = 0 could be used with the viscosity hydrodynamic equations to provide a correct value of molecular length. Intrinsic viscosities obtained at shear gradients>250 sec−1are nearly 30% lower than the zero‐shear value. Untreated acid‐soluble collagen (ASC) solutions contain aggregates and these appear, from electric birefringence data, to be of endlinked character. ASC solutions show a much more marked shear gradient dependence than P‐ASC. For example, at D∼500sec−1,[η] = 22 dl/g, whereas the extrapolated zero‐shear value of[η]was found to be 44 dl/g. Thus, the shear gradient dependence of native collagen solutions is much more marked than previously assumed and, in contrast to the usual practice, only viscosities measured near zero shear can be interpreted in terms of molecular parameters for collagen

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110706

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

年代:1972

数据来源: WILEY

摘要:

AbstractA study of the effect of linkage on the possible conformations of di‐and polysaccharides of α‐D‐glucose and also the probable intramolecular hydrogen bonds has been made. The differences in the nature of linkage is shown to effect the energetically preferred conformations; (1 → 2) linkages lead only to righthanded helical conformations, (1 → 3) linkages lead to extended as well as both left and righthanded helical conformations; (1 → 4) linkages lead to both right‐and lefthanded wide helical conformations. The possible hydrogen bonds between adjacent residues are also dependent on the nature of the linkage. A comparison of the conformational data of α‐D‐glucans with those of β‐D‐glucans has indicated that the favored conformations and hydrogen bonds between contiguous residues in the chain are influenced by the configuration at the anomeric carbon atom in all the three types of polysaccharides. From the energy calculations a probable conformation (ϕM= −10°, ψM= −30°, ϕN= −23°, ψN= −19°) has also been proposed for crystalline mycodextran in conformity with x‐ray data. This conformation contains two types of hydrogen bonds between contiguous residues one between 0–2 and 0–3 atoms at (1 → 4) linkage and the other between 0−2 and 0–4 atoms at (1 → 3) linkage in the chain. The conformation of maltose unit (−10°,−30°) that is likely to occur in the crystalline mycodextr

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110707

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

年代:1972

数据来源: WILEY

摘要:

AbstractThe interaction between poly C and (Gp)nG(n = 1,2) in dilute solution was investigated spectrophotometrically in 0.1Mphosphate buffer pH 7.2 under conditions unfavorable for the formation of self‐associated complexes of oligoguanylic acids. Two isosbestic points were observed when poly C was titrated gradually with GpGpG, one at 232–233 mμ(in the range of 0–33% poly C) and one around 238 mμ (in the range of 50–100% poly C). The melting temperature (Tm) of the 1:1 poly C: (Gp)nG complexes (n = 1,2) of varying concentration were determined. The equilibrium properties of the 1:1 complexes can be described by two interaction parameters, namely, (i) cooperative stacking interaction between the first nearest neighbor of the adsorbed oligomer, and (ii) intrinsic association constant of the adsorbed oligomer with its polymeric site, since the cooperative helix–coil transition particularly in the smaller oligonucleotide can be described by an “all or none” model. Based on such a model the enthalpy of stacking inteaction‐dependent Tmvalues yielded directly the sum of the enthalpy of stacking interaction and of basepairing (which is dependent on the chain length of the oligomer) and the value of S, the stability constant of a G–C pair within a helix. The enthalpy of formation of G–C pair is then calculated as −6.3 kcal/base pair either from the chain length dependent enthalpy term or from the temperature coefficient of S values. From the S value and the association constant of 1:1 GpGpGpC:GpCpCpC complex, other thermodynamic parameters such as nucleation parameter (β) and free energy of stacking int

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110708

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

年代:1972

数据来源: WILEY

摘要:

AbstractThe polytripeptides poly(glu(OEt)‐gly‐gly) and poly(glu(OEt)‐glu(OEt)‐gly) have been synthesized and characterized using infrared linear dichroism, ultraviolet circular dichroism spectroscopy, and electron microscopy. Both polymers have been obtained in a cross β conformation with antiparallel chains, however, there appear to be significant differences in the stability of this conformation for the two polymers. Poly(glu(OEt)‐gly‐gly) has also been shown to exist in a 31polyglycine II type helix in aqueous solution whereas films cast from this solution appear to be random. Finally this same polymer can be obtained in either a random or an extended form trifluoroacetic acid depending on the nature of the substrate used for

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110709

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

年代:1972

数据来源: WILEY

摘要:

AbstractSpin‐labeled samples of poly rA, poly rU, and poly rG have been prepared, and physicochemical properties primarily of labeled poly rA are reported. The nitroxide radical, 4‐(2‐iodoacetamido)‐2,2,6,6‐tetramethylpiperidinooxyl, is incorporated to a greater extent in poly rA and poly rU, as compared to poly rG. No incorporation is observed in the case of poly rC. Special attention has been paid to the separation of the covalently attached labels from the free labels, and to the preservation of the integrity of the chain length of the labeled polymers. The determination of molar extinction coefficients of the three labeled polymers indicates virtually no difference from those known for the chemically unpertubed polyribonucleotides. The correlation times for the spin‐labeled single stranded poly rA and poly rU have been calculated. More mobile building blocks are found in poly rU as compared to poly rA. Conformational properties of labeled poly rA in aqueous solutions have been investigated using electron spin resonance, circular dichroism, and absorption spectroscopy. The objective of the study of labeled poly rA was to examine its conformational transitions upon the uptake of protons by the adenine bases. Based on electron spin resonance data there is strong evidence that the single strand‐double strand transition can take place in three steps. In addition to the already known two forms of double‐stranded poly rA in acidic solution, called A and B, it is suggested that a third phase, consisting possibly of large aggregates, is involved in the transition of the less protonated double strands to those of compl

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110710

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

年代:1972

数据来源: WILEY

摘要:

AbstractThe kinetics of the helix‐coil transition have been investigated for T2 and T7 phage DNA in a formamide‐water‐salt mixed solvent using a slow temperature perturbation technique (applicable to kinetic processes with rate constants ⩽ 3 min−1). In this solvent degradation of the DNA is effectively suppressed. Complex kinetic curves are observed by absorbance and viscosity measurements for the response to denaturing perturbations in the transition region. Analysis of the decay curves indicates that the denaturation reaction in this time range can be treated as a first‐order reaction with a variable first‐order rate parameter,k, the derivative of the logarithm of the absorbance or viscosity change with respect to time. In the approach to denaturation equilibrium in the transition region, the rate parameter is determined only by the instantaneous extent of denaturation of the molecules. Near equilibrium, the rate parameter assumes a constant value characteristic of the equilibrium state. In this region, where the denaturation reaction proceeds as a simple first‐order process, both the decay of absorbance (reflected local conformational change) and the decay of solution viscosity (reflecting macromolecular conformational change) are characterized by the same constant value ofk. In 83% formamide, 0.3MNa+, the rate parameterkfor T2 DNA decreases from an extrapolated value of 2.0 min−1at 0% denaturation to 0.11 min−1at 90% denaturation. Rate parameters determined for T7 DNA at the same counterion concentration and fraction of denaturation are approximately five times as large as those cited for T2 DNA, indicating an inverse proportionality of rate constant to molecular length. On the other hand, simple first‐order kinetic responses with constantkare obtained for renaturing perturbations within the transition, indicating that the mechanism of rewinding differs, in most cases, from that of unwinding. Only in the limit of very small perturbations about a given equilibrium position are the rate constantskobtained from denaturing and renaturing perturbations equal. For perturbations of finite size, it appears possible that an intramolecular initiation or nucleation event may precede rewinding and limit the rate of this reaction. The rate parameters again are approximately inversely proportional to molecular weight. The one exception to the first‐power dependence on molecular weight appears when temperature jumps are made upward into the post‐transition region. Here the molecular‐weight dependence is second power, but complications arising from the different strand‐separation properties of T2 and T7 DNA's make interpretation difficult. The previously used model of friction‐limited unwinding appears to fit all the observations except for

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110711

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

年代:1972

数据来源: WILEY