摘要:

AbstractBiscyclo(Cys‐Sar) [I] and biscyclo(Cys‐Pro) [II]were prepared by the extension of amino acid moieties from cystine. Compound I equilibrates between two rotamers around the CSSC bond of the cystine residue in methanol, showing dual negative CD transitions (CS‐SC) at 270 and 255 nm, and dual S‐S vibrations at 507 and 539 cm−1in Raman spectra. In contrast, a conformation of P‐helix type is suggested in CH3CN, which shows a distinct negative CD at 270 nm and only one Raman band at 507 cm−1for the S‐S bond. Compound II rotates freely in dimethylsulfoxide (Me2SO) but takes a relatively stable conformer of P‐helix type in methanol, chloroform/Me2SO (9:1), and chloroform/trifluoroacetic acid (9:1). The conformation in chloroform is retained even on addition of trifluoroacetic acid. A more complete conformation of compound II in water was determined from the negative CD of S‐S transition and1H‐nmr spectra of

ISSN:0006-3525    

DOI:10.1002/bip.1981.360201202

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

年代:1981

数据来源: WILEY

摘要:

AbstractSmall‐angle x‐ray scattering of poly(γ‐methyl‐L‐glutamate), [Glu(OMe)]n, in m‐cresol and in pyridine was measured to determine the mass per unit length,Mq, and the radius of gyration of the cross section, 〈S q2〉1/2. It was confirmed from the values ofMqthat [Glu(OMe)]nexists in an α‐helical conformation in these solvents. It was elucidated from the calculations on 〈S q2〉1/2that the side chains come in moderately close contact with the main chain in these solvents. It was indicated from the analysis of the outer portion of the scattering curves that the side‐chain conformation va

ISSN:0006-3525    

DOI:10.1002/bip.1981.360201203

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

年代:1981

数据来源: WILEY

摘要:

AbstractThe thermodynamic parameters for the double‐strand formation of the molecules rCAmG + rCUnG,m,n= 5–7, and dCAmG + dCTnG,m,n= 5,6, were measured from optical melting curves. Normal helices are formed whenm=n. The deoxyoligomers are more stable than the ribo‐oligomers, due to a more favorable enthalpy. Double helices with mismatched bases can be formed by mixing oligomers withm≠n. Such helices may form several possible structures. A structure with a dangling base is favored over a structure with a bulged base. The destabilization of the double strands by the formation of a bulged base was determined to be greater than 1.6 kcal/mol at 10°C. The extent of aggregation in the oligomer double strand rCA7G·rCU7G was determined using ultracentrifugation equilibrium. The possible effects of aggregation on the determination of the thermodynamic parameters for double‐strand formation ar

ISSN:0006-3525    

DOI:10.1002/bip.1981.360201204

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

年代:1981

数据来源: WILEY

摘要:

AbstractThe conformational response of calf thymus DNA to solvent conditions altered by varying amounts of ethanol and NaCl has been monitored by circular dichroism (CD). These measurements, which extend over a much greater range of conditions than previously examined, reveal (above critical concentrations of ethanol and salt) a condensed form of the macro‐molecule with unusually largepositiveellipticity in the 250–300‐nm region [the Ψ(+) state]. Mere increase in NaCl concentration at constant 35% (v/v) concentration of ethanol suffices to convert such Ψ(+) samples—via a series of intermediate forms with CD spectra resembling those of A‐DNA, then B‐DNA—into Ψ(−) states having anomalously largenegativeellipticity similar to the well‐known Ψ(−) forms produced by above‐critical concentrations of poly‐(ethylene oxide) and salt. These ethanol/salt‐induced transitions are all completely reversible and can occur without formation of any visible precipitates. We suggest that they represent predominantly tertiary structural changes of B‐form DNA molecules analogous to the changes which occur in several other systems where Ψ(+) ⇌ Ψ(−) interconversion has been reported. A “skein‐of‐yarn” model for the condensed tertiary (and quaternary, i.e., aggregated) state of the DNA affords one possible explanation for the inversions of ellipticity in all these cases. Such a model accords well with the accept

ISSN:0006-3525    

DOI:10.1002/bip.1981.360201205

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

年代:1981

数据来源: WILEY

摘要:

AbstractThe angular dependence of light scattering and the concentration dependence of the relative viscosity have been measured in solutions ofo‐nitrophenylthio‐hexa‐(γ‐benzyl‐L‐glutamate) ethylamide in ethylene dichloride. Both the reduced intensity of scattered light and the reduced viscosity of the solution suddenly increase above a certain critical concentration, below which both of them remain low and constant. The Debye plot of light scattering indicates that primary micelles having an aggregation number 48 are formed at the critical micelle concentration and that secondary micelles, each consisting of 294 molecules, then appear in increasing amounts with increasing concentration beyond the critical micelle concentration. The secondary micelle is rodlike and has a length of 1170 Å, if it is rigid. An analysis of the reduced viscosity leads to the intrinsic viscosity for the primary micelle, 0.360 dL g−1, and to that of the secondary micelle, 1.28 dL g−1. If the secondary micelle is represented by a prolate ellipsoid, it should have an axial ratio of 47. If the polypeptide chains are extended in the micelle, the observed aggregation number and axial ratio of the secondary micelle can well accommodate the intermolecularly hydrogen‐bonded in‐register β‐structure of anti‐parallel chains. In the primary micelle, some folded polypeptide chains are involved, and an intermolecularly hydrogen‐bonded out‐of‐register structure

ISSN:0006-3525    

DOI:10.1002/bip.1981.360201206

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

年代:1981

数据来源: WILEY

摘要:

AbstractThe dielectric features of poly(L‐glutamic acid) are studied by the Fourier synthesized pseudorandom noise method in a time domain combined with a four‐electrode cell. Polymer concentration dependence, the effect of the solvent viscosity, salt effects, and pH dependence are studied concomitantly with measurements of CD. A helix‐to‐coil transition occurs near pH 5.6 for a salt‐free solution; at higher pH values, the polymer has an ionized random‐coil conformation, and at lower pH, it has a deionized α‐helical conformation. When it is in the ionized random‐coil conformation, with the usual features of an electrolytic polymer, the solution shows a relaxation spectrum with a large dielectric increment at low frequencies. In the deionized α‐helical state, no distinct relaxation curves are obtained, which does not deny the existence of a permanent peptide dipole. The pH dependence of the dielectric increment does not mainly correspond to the conformational change from helix to coil, but rather corresponds to the change of chain expansion on account of a charge–charge interaction under low ionic strength, which is conceived of by a

ISSN:0006-3525    

DOI:10.1002/bip.1981.360201207

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

年代:1981

数据来源: WILEY

摘要:

AbstractWe present the results of molecular‐mechanics studies on base‐paired dinucleoside phosphates and hexanucleoside pentaphosphates. Starting from B‐DNA‐like conformations, we have refined the nucleic acid conformations, allowing all degrees of freedom to relax. The calculated energies of different base sequences are used to investigate the basis for the different stabilities of DNA polymers of different sequences, as found by Wells et al. [J. Mol. Biol.54, 465–497, 1970]. Our calculations appear to reproduce the relative melting temperatures of sequence isomers as well or better than any of the previous calculations which addresssed the question of nucleotide stability. We offer a detailed physical explanation of three observations, that poly(dA)·poly(dT) melts higher than poly[d(A‐T)]·poly[d(A‐T)] by 6°C, that poly(dG)·poly(dC) melts lower than poly[d(G‐C)]·poly[d(G‐C)] by 12°C, and that poly(dA‐dG)·poly(dT‐dC) melts lower than poly(dA‐dC)·poly(dT‐dG) by 6°C. The dihedral angles found after refinement are similar to those of Levitt [Proc. Natl. Acad. Sci. USA75, 640–644, 1978] and differ by ∼20° from the B‐DNA values. We also see evidence of base tilting and twisting similar to that found by Levitt. The two main differences in the results of our calculations and Levitt's lie in the sugar puckers [we find mainly C(2′)endo] and in the tendency to stay in local “torsional” minima (we find a number of examples of C(3′)endosugar puckering and ω′ =transrather thangauche). Both of these results are dependent on the nature of the potential function used in our study. However, our finding of movement from local torsional minima is suggestive of the signifi

ISSN:0006-3525    

DOI:10.1002/bip.1981.360201208

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

年代:1981

数据来源: WILEY

摘要:

Abstract1H‐Nmr was used to measure the rate ofcis–transinterconversion ofX‐Pro bonds in linear and cyclic oligopeptides.k(cis → trans) = 2.5 × 10−3s−1at 25°C was found for the zwitterionic form of H‐Ala‐Pro‐OH, in good agreement with earlier measurements. Replacement of Ala by Phe, Tyr, or Trp resulted in a 10‐fold slower interconversion rate, whereas after substitution of Ala by His or Glu, the rate decreased only slightly. Independent of the residuesX, the interconversion rate was increased by a factor of ca. 20 when the peptide chain was elongated by addition of Ala to the C‐terminal Pro. An additional increase by a factor of 6 was observed when going from the protected linear peptide CF3CO‐Gly‐Gly‐Pro‐Ala‐OCH3to the closely related cyclic compoundc[‐Gly‐Gly‐Pro‐Gly‐Ala‐]. These data are evaluated with regard to their possible use in future studies on the role ofX‐Procis–tran

ISSN:0006-3525    

DOI:10.1002/bip.1981.360201209

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

年代:1981

数据来源: WILEY

摘要:

AbstractAn azo pyrimidine nucleotide has been prepared and enzymatically attached to oligo(A) primers. The nucleotide's azo pyrimidine group has previously been shown to initiate polymerization of methacrylate esters designed to bind marker groups for visualization by microscopy. When attached to RNA molecules complementary to a chromosomal DNA segment, these nucleotides may allow localization of the DNA segment followingin situhybridization of the probe, methacrylate polymerization, and marker attachment. Since mRNA molecules of potential interest as probes bear a 3′‐poly(A) tail, the modified nucleotides were added to oligo(A) primers as models. First,N4‐ureidocytosine nucleotides were enzymatically added to ApApA, (Ap)9A, or [5′‐32P]‐(pA)10, using the modified cytidine 5′‐diphosphate and “primer‐dependent” polynucleotide phosphorylase (M. luteus). In the case of the ApApA‐primed reaction, theN4‐ureidocytosine nucleotides in the product polynucleotide were converted into azo nucleotides by oxidation withN‐bromosuccinimide. The other two primers were employed to study the time course of polynucleotide formation and to verify that primer was indeed being utilized by the enzyme. The suitability of the modified nucleotide forin situhybridization studies was examined. Poly(N4‐ureidocytidylic acid) was prepared from poly(C) and semicarbazide by the bisulfite‐catalyzed transamination reaction. It was found that 95% of theN4‐ureidocytosine nucleotides in this polynucleotide survive the elevated temperatures typically required for DNA:DNA denaturation and RNA:DNA annealing. When poly(N4‐ureidocytidylic acid) was mixed with poly(I) in buffered aqueous salt solutions, no evidence for hybridization was found, so binding of the probe RNA to the denatured chromosomal DNA molecule via the modified nucleotides is not expected. Upon oxidation of poly(N4‐ureidocytidylic acid) withN‐bromosuccinimide, the azo nucleotides were formed, as judged by the appearance of a characteristic peak at approximately 350 nm in the uv‐absorption spectrum of the yellow‐orange product, azoRNA. The azo nucleotides in azoRNA exhibited the expected acid lability, which is known to be accompanied by 1‐glyceryl methacrylate polymerization in the case of the simple azo pyrimidine. Because 1‐glyceryl metharcylate bears substituent glycol groups for attaching heavy atoms or fluorescent markers, it is possible that probe RNA molecules bearing azo nucleotides may be useful for localizing low

ISSN:0006-3525    

DOI:10.1002/bip.1981.360201210

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

年代:1981

数据来源: WILEY

摘要:

AbstractIn the keratin structure, the presence of water having the usual properties of “bulk water” has been discussed for many years. Results of experiments designed to study this question are conflicting and thus do not allow a precise idea of the state of water molecules in keratin at high hydration. A recent depolarization thermal‐current (DTC) study gave good evidence for the presence of a well‐defined DTC peak in the depolarization spectrum of ice. Initially, we verified the exact characteristics of this particular peak under our own experimental conditions. Subsequently, we looked for and found this ice peak in keratin samples. It appears when the water content is 19% or greater. The meaning of this result is discussed in relation to the other experimental results reported in the literature dealing with this q

ISSN:0006-3525    

DOI:10.1002/bip.1981.360201211

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

年代:1981

数据来源: WILEY