摘要:

AbstractThe probable number of hydrogen bonds has been calculated as a function of the imino acid content for water‐bridged collagen structures. With increasing imino acid content in collagen, the number of hydrogen bonds stabilizing triple‐helical structures become saturation. This might explain the asymptotic character of the empirical relation between thermostability and hydroxyproline content for colla

ISSN:0006-3525    

DOI:10.1002/bip.360210902

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

年代:1982

数据来源: WILEY

摘要:

AbstractThe interaction of silver ions with poly(A) was studied by potentiometric titration, uv spectrophotometry, and stopped‐flow spectroscopy. For 0

ISSN:0006-3525    

DOI:10.1002/bip.360210903

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

年代:1982

数据来源: WILEY

摘要:

AbstractBradley et al. [(1972)Biopolymers11, 645–652] used electro‐optical measurements to show that methylene blue (MB), like acridine orange, in its DNA complex is oriented more or less perpendicular to the helix axis as expected if intercalated. High‐precision flow linear dichroism (LD) here confirms that MB is coplanar with the DNA bases at low dye/DNA binding ratios and low ionic strengths. CD shows two origins of induced optical activity for the transition of lowest energy (polarized parallel to the long‐axis of the dye molecule): at low binding ratios (r0.4 mMMg2+). Nondegenerate coupled‐oscillator theory can explain the CD in terms of interactions of transition moments of the dye and the nearest nucleotide bases and indicates a change between two intercalation geometries: a Lerman type of mode, denoted γ−, and an orthogonal mode, denoted γ+. This rotation of MB in the base‐pair pocket is accomplished at Na+and Mg2+concentrations when the phosphates are effectively screened and the result suggests a more localized bonding of Mg2+than is expected from simple polyelectrolyte models. The exciton effect at high binding ratios, observed both in CD and in LD, can be interpreted in terms of an interaction between an intercalated and a nonintercalated MB. The geometry of this “accidental” dimer is consistent with a location of the nonintercalated MB in the minor groove, bridging the strands by the positively charged amino groups directed towards phosphate groups. The dihedral angle of the MB pairs, corresponding to a left‐handed helix, is opposite to that with acridine orange and proflavine on DNA, indicating that the latter ligands bind to

ISSN:0006-3525    

DOI:10.1002/bip.360210904

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

年代:1982

数据来源: WILEY

摘要:

AbstractA study of the oxygen replacement reaction of carbon monoxide‐saturated hemoglobin (HbA0) was carried out using spectroscopic, calorimetric, and pH titration methods. Under fully saturated conditions the replacement reaction can be defined by a single partition constant over all ratios of bound oxygen to carbon monoxide. This indicates that under saturating conditions Haldane's first law for the ligand binding of gas mixtures holds for any CO/O2ratio. It further shows that there is no appreciable difference in relative CO‐O2affinity between the α‐ and β‐chains. The same partition coefficient was found to hold for different pH, buffer, and allosteric effector conditions. The lack of any pH dependence of the partition coefficient was confirmed by the absence of proton changes for the replacement reaction. The temperature dependence of the partition coefficient and calorimetric results yield a value for the enthalpy of the reaction of −3.65 ± 0.29 k

ISSN:0006-3525    

DOI:10.1002/bip.360210905

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

年代:1982

数据来源: WILEY

摘要:

AbstractSome properties of α‐helices of polyclycine and polyalanine, up to the decapeptide, were investigated byab initiomolecular‐orbital calculations. These helices were found to be unstable relative to the corresponding “fully extended chain” conformation. The electric field of helices of 8–10 residues is about 20% stronger than that of models built from noninteracting monomers, for example. This is a result of cooperativity, which is essentially governed by the intramolecular hydrogen bonds. The cooperativity is manifest in all properties of the helices: relative stability, dipole moment, proton affinity, electrical potential. The electric potential of helices of three and four residues is such that their instability can be compensated for by a single charged group acting as an “initiator.” The computed proton affinity of the (Ala)8α‐helix is about 45 kcal/mol larger than that of formamide, which confirms that long helices may be protonated at the carboxy

ISSN:0006-3525    

DOI:10.1002/bip.360210906

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

年代:1982

数据来源: WILEY

摘要:

AbstractWe demonstrate that the isotropic absorption and linear dichroism in an unknown flow field can be used to determine base tilt in polynucleotides if three transitions are measured and the directions of the corresponding dipoles are known. The method is applied here to reach conclusions about the base tilt in poly(rA), poly(rA)+·poly(rA), and poly(rC). The respective values are: 28° tilt about the axis + 50° toward C8 from the C1′ → N9, and 25° tilt about the axis + 118° toward C5 from C1′ → N1. The results for poly(rA)+·poly(rA) are consistent with the accepted model. Spectra were measured for poly(rC)+·poly(rC), but definite conclusions must await reliable directions for transition dipoles. The dipole direction for the 218‐nm transition in rC is found to be +13° or +43° toward C5 from C1′ → N1. The CD spectra to about 168 nm are p

ISSN:0006-3525    

DOI:10.1002/bip.360210907

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

年代:1982

数据来源: WILEY

摘要:

AbstractThe effect of cytochromecon the redox behavior of 1,4‐benzoquinone and cystine has been investigated by differential pulse polarography. The adsorption of cytochromecon mercury surface produces a lowering of the reduction peak of 1,4‐benzoquinone and of the cystine prepeak. When the covering of the drop is maximal, no variation in peak height is observed. By using the Koryta equation, values between 1500 and 2300 Å2for the surface area of one adsorbed cytochromecmolecule were obtained. The capacitance dependences on concentration and time have been determined by alternating‐current polarography, and lead to the surface area values of 2800 and 2700 Å2, respectively. Results are interpreted on the basis of an unfolding of cytochromecmolecules during the adsorption process. It is likely that a rather compact covering is obtained, but about 10% of the mercury surface seems to be unoccupied and corresponds to the accessible interstices between adjacent mo

ISSN:0006-3525    

DOI:10.1002/bip.360210908

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

年代:1982

数据来源: WILEY

摘要:

AbstractExperimental evidence indicates that the cleavage sites for site‐specific DNA cleavage enzymes are randomly distributed throughout the DNA polymer [Botchan et al. (1973)Cold Spring Harbor Symp. Quantum Biol.38, 383–395; Hamer and Thomas (1973)Chromosoma49, 243–267]. Using this information we derive equations for the probability that a DNA base pair chosen at random can be found within the population of enzymatically generated partial cleavage products whose lengths fall betweenL+ 1 andL+rbase pairs. The results describe the fraction of a long‐chain DNA that can be incorporated in a recombinant DNA partial digest library. The weight‐fraction distribution of Kuhn (1930) is a special case of the distribution der

ISSN:0006-3525    

DOI:10.1002/bip.360210909

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

年代:1982

数据来源: WILEY

摘要:

AbstractMeasurements of stress relaxation in uniaxial extension and associated time‐dependent birefringence have been made on bovine fibrin film, prepared by gentle compaction of coarse fibrin clots, containing 13–22% fibrin plasticized with either aqueous buffer or glycerol. Both unligated and ligated (i.e., with α‐α and γ‐γ ligation by fibrinoligase, factor XIIIa) films were studied. Both types showed two stages of stress relaxation, with time scales of approximately 10 and 103–104s, respectively, with a plateau region between. In the plateau, the nominal (engineering) stress for ligated glycerol‐plasticized film is proportional to In λ, where λ is the stretch ratio, up to λ ≅ 2, and it decreases with increasing temperature. For unligated glycerol‐plasticized film, the stresses are smaller by a factor of one‐half to one‐third. For ligated film, the second stage of relaxation is relatively slight, and recovery after release of stress is often nearly complete. For unligated film, the second stage involves a substantial drop in stress, and after recovery there is a significant permanent set. A second relaxation for ligated film reproduces the first, but for unligated film it reproduces the first only if the initial relaxation is terminated before the second stage; otherwise, the second relaxation shows a weaker structure. The behavior of water‐plasticized film is similar to that of glycerol‐plasticized except that the second stage of relaxation occurs at shorter times. During the first stage of stress relaxation, up to about 100 s, the birefringence and the stress‐optical coefficient increase; during the plateau zone of stress relaxation, the birefringence of ligated films is approximately constant and is proportional to 2λ2/(λ2+ 1) − 1, where λ is the stretch ratio. This dependence is predicted by a two‐dimensional model in which rodlike elements in the plane of the film are oriented with independent alignment. During the final stage of stress relaxation, the birefringence of ligated films decreases slightly; that of unligated films decreases substantially, but less rapidly than the stress, corresponding to a further increase in the stress‐optical coefficient. With additional information from small‐angle x‐ray scattering reported in an accompanying paper, the first stage of relaxation is attributed to partial release of bending forces in the fibers by orientation, accompanied by increased birefringence. The second stage is attributed, for ligated films, to an internal transition in the fibrin units accompanied by elongation of some of the fibers; and in the unligated films, to a combination of the latter transition with slippage of protofibrils lengthwise within the fiber bundles that causes some loss of or

ISSN:0006-3525    

DOI:10.1002/bip.360210910

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

年代:1982

数据来源: WILEY

摘要:

AbstractMeasurements of small‐angle x‐ray scattering have been made on bovine fibrin film, prepared by gentle compaction of coarse fibrin clots, containing 12–15% fibrin plasticized with either aqueous buffer or glycerol. Most of the experiments were made with water‐plasticized films because of the better x‐ray contrast. Both unligated and ligated (i.e., with α‐α and λ‐λ ligation by fibrinoligase, factor XIIIa) films were studied. Theoretical angular scattering profiles were calculated for a simplified model of the fibrin protofibril as a function of stretch ratio in uniaxial elongation based on an orientation distribution derived in the preceding paper. Measurements were made on unstretched films both with slit geometry and with pinhole geometry and two‐dimensional position‐sensitive detection. From the former, which had better resolution, the fundamental peak corresponded to a repeat spacing of 230 ± 5 Å, in agreement with early results of Stryer et al. [(1963)Nature197, 793–794] and electron micrographs of stained fibrin, and with the half‐staggered overlapping model of the protofibril. Measurements were made with pinhole geometry on film stretched up to a stretch ratio λ = 1.59 at different times after imposition of strain and after release and recovery. The changes in meridional and equatorial scattering profiles, including the more prominent appearance of a harmonic in the former, show the orientation of the protofibrils as previously deduced from birefringence and related quantitatively to stretch ratio. In addition, the original repeat spacing is gradually replaced by one of about 290 Å. The “internal” stretch ratio λx, averaging about 1.28, is independent of the macroscopic stretch ratio λ from 1.14 to 1.59. The change, which is more nearly complete for unligated film and is reversible on release and retraction of the stretched strips, is attributed to an internal transition in the fibrin monomer units, probably involving extension of either the helical connectors or the terminal nodules. The results of stress and birefringence relaxation reported in the preceding paper of this s

ISSN:0006-3525    

DOI:10.1002/bip.360210911

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

年代:1982

数据来源: WILEY