ISSN:0006-3525    

DOI:10.1002/bip.360010102

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

年代:1963

数据来源: WILEY

摘要:

AbstractPoly‐L‐proline in the molecular weight range 50,000–90,000 has been synthesized fromL‐proline‐N‐carboxyanhydride. The molecular weight obtained is dependent on the initiator concentration. The rate constants for the polymerization have been determined by measuring the CO2evolution. By observation of rotation during the polymerization, it is possible to deduce the conformation of the polymer in the polymerization solution. It is concluded that the immediate product of the polymerization is neither pure form I nor pure form II. The differences between form I and form II of poly‐L‐proline have been further characterized by measurements of their ultraviolet rotatory dispersion and their ultr

ISSN:0006-3525    

DOI:10.1002/bip.360010103

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

年代:1963

数据来源: WILEY

摘要:

AbstractThe statistical mechanics of the helix–coil transition in DNA is treated by means of a grand partition function. This method is simpler than methods used previously, although it is suitable only in the limit of great chain lengths. Expressions for the number of base pairs bonded are obtained and shown to be identical to those obtained by other methods in the limit of great chain lengths. The physical meaning of the grand partition function is also discusse

ISSN:0006-3525    

DOI:10.1002/bip.360010104

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

年代:1963

数据来源: WILEY

摘要:

AbstractThe grand‐partition‐function theory of the preceding paper is used to derive expressions for the number of base pairs bonded in DNA with explicit consideration of the copolymeric nature of the DNA. The following features of the DNA molecule are taken into account: (1) the DNA's from different sources have different ratios of a–t and g–c pairs; (2) the a–t pair is weaker than the g–c pair; (3) the nearest‐neighbor frequencies of the two kinds of base pairs are nonrandom; (4) the stacking energies (nearest‐neighbor energies) of the various combinations of pairs may be different. An ensemble is constructed in which the features (1) through (4) are introduced by means of statistical weights on the various pairs and combinations of pairs, and an expression for the corresponding partition function is written. Expressions are derived for the number of bonded base pairs and the number of he

ISSN:0006-3525    

DOI:10.1002/bip.360010105

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

年代:1963

数据来源: WILEY

摘要:

AbstractWe determined the onset of helical structures in solution for oligopeptides by means of the decrease in intensity of the 190 mμ band (hypochromism), and related these findings to optical rotatory dispersion data. Measurements of ultraviolet absorption spectra in trifluoroethanol show that helical conformations commence at about the tetradecamer for peptides derived from β‐methyl‐L‐aspartate. In the γ‐methyl‐L‐glutamate series the hypochromic effect starts at the nonamer, increasing in magnitude with peptide chain length. Essentially the same results were obtained by using optical rotatory dispersion. By dividing theb0(Moffitt‐Yang constant) for the oligopeptides into helical and nonhelical components, we have better indications that in previous work for the onset of helicity. We have also studied the far ultraviolet spectra and optical rotatory dispersion for high polymers derived from β‐methyl‐L‐aspartate and γ‐methyl‐L‐glutamate. Assuming complete helicity for these high polymers, we assigned helical contents to the oligopeptides. In addition, ultraviolet spectra were measured for the polymer from γ‐methyl‐DL‐glutamate in o

ISSN:0006-3525    

DOI:10.1002/bip.360010106

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

年代:1963

数据来源: WILEY

摘要:

AbstractThe thermodynamic properties derived earlier for the formation of side‐chain hydrogen bonds were re‐evaluated, taking into account medium effects and interactions of the side chains with nearby nonpolar groups. In determining the enthalpy of formation of the hydrogen bond, competitive hydrogen bonding with water has to be considered, except for “buried” groups, i.e., groups surrounded entirely by nonpolar groups. For such buried groups, the earlier estimate of ΔH H°= −6 kcal./mole is still valid, but for groups exposed to water, a lower value has to be used, estimated to be −1.5 kcal./mole. The internal bond rotations of side chains involved in hydrogen bonding are not restricted completely; therefore, ΔS H°is only −2 to −5 e.u. per side chain, instead of the much higher values estimated earlier. As a result, the new estimated value of ΔF H°is 0 to −0.5 kcal./mole, not much different from the earlier estimate. The ease of hydrogen bond formation may be changed by cooperative or competitive hydrophobic bonding of the nonpolar portions of the polar side chains with the backbone or nearby nonpolar side chains. Various cases of such interactions are discussed, including the burying of polar groups and the formation of hydrogen bonds between buried groups. It is shown that, in aqueous environment, the polar groups form more than one hydrogen bond with water, an effect to be taken into account when considering buried groups. Medium effects due to the presence of nonpolar groups and the effects of changes in the water structure on the ease of formation of the α‐helix are mentioned. Estimates for the thermodynamic parameters of the various hydrogen‐bonding and hydrophobic interactions are given. It is shown that hydrophobic interactions will, in general, enhance the strength of hydrogen bonds. The net free energy of formation of a simple structure involving both interactions may range from about −0.5 to −2.4 kcal./mole. It may reach much more highly negative values for large structures. The corresponding ranges of the net enthalpy and entropy of formation are −1.5 to −0.5

ISSN:0006-3525    

DOI:10.1002/bip.360010107

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

年代:1963

数据来源: WILEY

摘要:

AbstractThe base‐catalyzed hydrolysis rate of the cationic esters obtained by quaternization ofN,N,N′,N′‐tetramethylethylenediamine with phenylbromoacetate was studied in buffers and in solutions of partially neutralized polymeric acids. The polyanions were found to be powerful inhibitors of the reaction. This effect was interpreted as due to the association of the ester with the polyanion, from which the catalytically active hydroxyl groups are repelled. It is shown that kinetic data may be interpreted by a procedure similar to the Lineweaver‐Burk plot to yield the reactivity of the bound ester and the dissociation constant of the polymer–es

ISSN:0006-3525    

DOI:10.1002/bip.360010108

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

年代:1963

数据来源: WILEY

摘要:

AbstractA method is described for formation of hydroxamic acids by direct coupling, in aqueous medium, of hydroxylamine hydrochloride and the sodium salt of a carboxylic acid. The reaction is mediated by a water‐soluble carbodiimide, 1‐cyclohexyl‐3‐[2‐morpholinyl‐(4)‐ethyl]‐carbodiimide metho‐p‐toluene sulfonate. Using model compounds, the production of α‐, β‐ and γ‐acyl hydroxamic acids was studied. Hydroxamic acid derivatives of α‐polyglutamic and γ‐polyglutamic acids were also prepared. Dinitro‐phenylhydroxamate derivatives were prepared and subjected to Lossen rearrangement, and the reaction mixtures hydrolyzed. Analysis showed that α‐polyglutamic acid yielded as many molecules of α,γ‐diaminobutyric acid as there had been glutamic acid residues in the form of the dinitrophenylhydroxamate, and gave rise to no ammonia and succinic semialdehyde. In contrast, γ‐polyglutamic acid yielded exactly twice the molar quantity of ammonia as there had been glutamic acid residues in the form of dinitrophenylhydroxamate, and also gave rise to significant quantity of succinic semi‐aldehyde but no α,γ‐diaminobutyric acid. Since production of the latter is characteristic of α‐glutamyl hydroxamates and production of the former is associated with γ‐glutamyl hydroxamates, the results indicated that the side‐chain carboxyl groups of either polymer retained their identities during formation of the hydroxamic acid derivative, and no interchange had occurred between α‐ and γ‐carboxyl groups. These methods were then used to establish that gelatin of ichthyocol contains (per 1000 total residues) at least 20 residues of glutamic acid in γ‐peptide linkage. Due to the incomplete dinitrophenylation of the hydroxamic acid derivative of the protein, gelatin of calf skin collagen gave a lower figure of 10 such linkages per 1000 total amino acid residues–a value which must be considered a minimum va

ISSN:0006-3525    

DOI:10.1002/bip.360010109

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

年代:1963

数据来源: WILEY

ISSN:0006-3525    

DOI:10.1002/bip.360010101

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

年代:1963

数据来源: WILEY