摘要:

AbstractAs an approach for elucidating the role of sequences of amino acids in protein structures, model polypeptides having the same composition but different sequences of amino acids, (L‐Ala‐L‐Val‐Gly)nand (L‐Val‐L‐Ala‐Gly)n, have been prepared by the method involving facile monomer synthesis usingN‐carboxy α‐amino acid anhydrides andN‐hydroxysuccinimide esters. The yields and the molecular weights of the polypeptides formed by polycondensation do not depend on the monomer concentrations, but on the sequences of the amino acids in the monomers. Infrared spectra in the solid state showed that (L‐Ala‐L‐Val‐Gly)ncan take the α‐helical conformation but (L‐Val‐L‐Ala‐Gly)ncannot. The results suggest that the conformations of polypeptides are influenced by the sequenc

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140702

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

年代:1975

数据来源: WILEY

摘要:

AbstractThe chain stiffness of linear native DNA is represented by a generalized bead and spring model recently proposed. It incorporates molecular rigidity by means of springs between beads, which are second neighbors along the contour of the chain. These springs are equivalent to elastic forces having longitudinal and transversal contributions. The model is compared with existing experimental data of sedimentation and low‐angle light scattering to obtain the statistical parameters of DNA. The value of the statistical length obtained with this model is 1300 Å. The same value is obtained with the wormlike chain. Throughout this analysis, excluded volume is left out as a simplifying assumpti

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140703

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

年代:1975

数据来源: WILEY

摘要:

AbstractThe effect of structural changes in the solvent (usually water) on the thermodynamics of the hydrophobic interaction process is examined within the framework of classical statistical mechanics.The concept of the “structure of water” is first defined in a precise way, yet reflecting the conventional definition that has been, implicitly and qualitatively, employed by many authors. Using this concept, we proceed to show that structural changes in the solvent, induced by the hydrophobic interaction process, cannot affect thestrengthof the hydrophobic interaction. On the other hand, the entropy and enthalpy changes, associated with the same process, may well be affected. Some qualitative arguments are presented showing that large structural changes are expected from a complex solvent such as wa

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140704

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

年代:1975

数据来源: WILEY

摘要:

AbstractFragmented and mitomycin C cross‐linkedE. coliDNA was fractionated according to base composition by means of hydroxylapatite chromatography and density‐gradient centrifugation in order to determine the effect of compositional heterogeneity on the breadth of the helix–coil transition. The transitions of some of the fractions are broader than that of the unfractionated DNA, due, presumably, to nonrandom sequences in molecules of 5 × 105daltons. Analysis of the transition breadths in terms of the known heterogeneity leads to reconsideration of current DNA helix–coil transition theory. We propose that partially denatured states include those for which the chains do not remain in strict register. Denaturation profiles are comprehensible only if this multitude of entropically favorable, degenerate states is included in th

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140705

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

年代:1975

数据来源: WILEY

摘要:

AbstractIn order to investigate the ability of neutral amino acids to support the α‐helix conformation, the coil–helix transition of poly(L‐lysine) and of lysine copolymers with these amino acids was studied in water/methanol using circular dichroism. The transtions were recorded at constant pH adding buffer to the methanol/water mixtures. With poly(L‐lysine), experiments were performed at several constant pH's; the transition midpoint on the water (methanol) concentration scale was found to depend strongly upon pH; the helix stability region is shifted towards higher water concentrations, when the pH is increased. Copolymers of lysine and several neutral amino acids revealed the same effect in that increasing amounts of, for example, norleucine also shifted the transition midpoint to higher water concentrations. A series of copolymers containingL‐lysine as the host and different hydrophobic amino acids were synthesized and the helix–coil transition in water/methanol was observed at constant pH. Different copolymers of equal composition showed significant differences with respect to the nature of the amino acid incorporated into polylysine. From these studies an α‐helix‐philic scale (in decreasing order): Leu, Nle, Ile, Ala, Phe, Val, Gly is deduced and discussed; the results obtained were compared with those of dif

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140706

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

年代:1975

数据来源: WILEY

摘要:

AbstractSome physical properties of DNA can be expressed reasonably as an expansion of the interaction among base pairs. For strictly random sequences, the terms of the expansion can be rearranged in such a way that an empirical parameterization of the nearest neighbor truncation is in fact correct to all orders. A parameterization obtained from data on real native DNA's is shown to be more accurate than the term “nearest neighbor approximation” implies. When synthetic polynucleotide duplexes are used in the parameterization, the degree of its accuracy depends on the selection of duplexes. It is shown here how to optimize such a parameterization for a given set of available duple

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140707

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

年代:1975

数据来源: WILEY

摘要:

AbstractUsing the free energy difference between double‐helix and random‐coil forms of DNA as a measure of the stability of the double helix, we calculate the dependence of the stability on excess univalent cation concentration and on polynucleotide phosphate concentration, both as functions of the equivalent ratiorof divalent cation‐to‐phosphate concentrations. The theoretical tool is merely to compare the free energy of one polyelectrolyte solution, characterized by the polyelectrolyte linear charge density, with the free energy of another, characterized by a different value of the charge density. It is assumed only that the charge density of the double helix is greater than that of the coil form. The calculation represents the only molecular theory given to date (forr≠ O) for these aspects of helix stability.We find that, as excess univalent cation concentration increases, the helix stability increases ifris small but decreases ifris large (i.e., of the order of unity). Moreover, as the concentration of nucleotide phosphate increases, the helix stability does not change for small values ofrbut increases for large values. For both effects, a continuous transition as a function ofrbridges the low‐rand high

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140708

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

年代:1975

数据来源: WILEY

摘要:

AbstractThe association of chloroform‐soluble derivatives of uracil and adenine has been examined in chloroform solution in the presence of dissolved water. Analysis by infrared spectroscopy shows that complex formation still occurs in these conditions, and that the extent of association is substantially unchanged by the presence of water. Evidence is presented for the coexistence of two kinds of base pair (involving, respectively, the C2and C4carbonyl groups of the pyrimidine) in the solutions, and for some displacement in their relative balance by the added water. The binding of water to the C2and C4carbonyl groups can be separately observed in both the free uracil derivative and its 1:1 complex with 9‐ethyladenine. Little or no competition has been found to occur between the formation of base pairs and binding of water to the bases, as judged by measurements of water solubility in chloroform solutions of the bases individually and in 1:1 mixtures. The evidence suggests that this phenomenon can be largely explained by the formation of double hydrogen bonds by the uracil carbonyl groups. Taken together with recent published observations, the results indicate that hydrogen bonding may make a much greater energetic contribution to conformational stability of biopolymers in aqueous solution that has been suppo

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140709

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

年代:1975

数据来源: WILEY

摘要:

AbstractBy use of a polycondensation procedure free of racemization, stereoregular polymethionines have been synthesized from C‐activatedD‐methionyl‐L‐methionine andL‐methionyl‐D‐methionyl‐L‐methionine.The poly(D‐methionyl‐L‐methionine) and poly(L‐methionyl‐D‐methionyl‐L‐methionine) so prepared are soluble in chloroform and can be purified through dissolution in this solvent and precipitation by ligroin.Poly(D‐Met‐L‐Met)which is obtained in a 25% yield, is about 5000 in average molecular weight. It has no discernible optical activity when examined between 400 and 600 nm in a trifluoroacetic acid solution.Poly(L‐Met‐D‐Met‐L‐Met) (40% yield,M. W.= 10,000) is an optically active polymer. [α]43624

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140710

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

年代:1975

数据来源: WILEY

摘要:

AbstractInfrared spectroscopy, X‐ray diffraction, and nuclear magnetic resonance spectroscopy have been used in investigating the conformation of two stereoregular polymethionines, poly(D‐methionyl‐L‐methionine) and poly(L‐methionyl‐D‐methionyl‐L‐methionine). When dissolved in a helicogenic solvent, such as chloroform and hexafluoroisopropanol, the polytripeptide is in an α‐helical conformation. A helix‐to‐coil transition can then be induced by addition of trifluoroacetic acid.On the other hand, it appears that the most stable conformation of poly(D‐Met‐L‐Met) is a β antiparallel folded structure in which the linear polypeptide segments are near to the planar extension. This structure has been evidenced through X‐ray examination of oriented films, casted from solutions in chloroform. It has also been identified in solution in the same solvent, by use of infrared spectroscopy and by measuring the δHα chemical shift which characterizes the Hαproton in the peptide units. This δHα value is found equal to 5.4 ppm and differs significantly from those which are usually attributed to the α‐helical conformation (δHα = 4.2 ppm) and to the random coil (δHα = 4.6 ppm). The β folded conformation of the poly(D‐Met‐L‐Met) appears to be comparatively less stable than the α‐helical one for the poly(L‐Met) macromolecular stereoisomer since hexafluoroisopropanol is a helicogenic solvent for this last solute and a destabilizing one for the poly(D‐Met‐L‐Met) β folded conformer. X‐ray examinations carried out with stretched films, casted from a solution of poly(D‐Met‐L‐Met) in chloroform, result in several data concering th

ISSN:0006-3525    

DOI:10.1002/bip.1975.360140711

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

年代:1975

数据来源: WILEY