摘要:

AbstractThe coil to helix conformational transitions undergone by poly‐γ‐benzyl‐L‐glutamate in solutions of haloacetic acids and various cosolvents were studied by means of proton magnetic resonance. The results indicate a very small solvent dependence of the α‐CH Helix–coil chemical shift difference. The helical stabilities of PBLG in different solvent mixtures were interpreted in terms of modifications of the “sol

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120702

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

年代:1973

数据来源: WILEY

摘要:

AbstractThe concentration dependence of the ultraviolet absorption spectrum of aqueous solutions of caffeine has been studied. Individual species spectra have been derived for the monomer, dimer, and tetramer of caffeine. The emission spectrum of caffeine in aqueous solution and the dichroic spectra in oriented poly(vinyl alcohol) and polyethylene films have been measured. The long‐wavelength tail of the absorption spectrum of caffeine in non‐polar environment has been found to incorporate at least one carbonyl(π*,n) transition. Dichroic spectral data and molecular orbital calculations have been used to assign transition moment directions to the (π*,π) transitions. The lowest energy (π*,π) transition, responsible for the near‐ultraviolet absorption peak in aqueous solution of caffeine, has been used for the study of degenerate exciton interactions in the dimeric species of caffeine. Assuming that the caffeine molecules in the dimer are stacked in parallel planes, theoretical calculations of the ground‐state interactions and of the degenerate exciton interactions have been combined with experimental data and a unique model for the dimer of caffeine has been derived. The transfer rate of energy between the molecules in the dimer is of the orde

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120703

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

年代:1973

数据来源: WILEY

摘要:

AbstractA molecular field theory, taking into account long‐range electrostatic forces is used to study helix–coil transitions of polynucleotides. The theory predicts the existence of hysteresis when the electrostatic interaction parameter is large compared to the thermal energy. The theory is applied to the acid–base titration of poly(A)·2 p

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120704

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

年代:1973

数据来源: WILEY

摘要:

AbstractStudies of the d. c. conductive behavior of solid gels of DNA under high vacuum have shown that a reproducible energy gap of 2.4 eV and a photoelectric effect may be obtained following extensive evacuation procedures. Admission of air for short periods has a reversible effect on this photoelectirc effect and semiconductive behavior. After prolonged illumination in air the sample becomes permanently affected and will not reproduce the photoelectric effect or give rise to a semiconduction energy gap of 2.4 eV but give rise to a permanently lower energy gap of 1.6 eV. The effect of ultraviolet irradiation on the thermal denaturation of DNA in the presence of six aromatic hydrocarbons has been examined in order to determine the extent to which the presence of the hydrocarbon protects the DNA against ultraviolet damage.

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120705

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

年代:1973

数据来源: WILEY

摘要:

AbstractPolymerizations ofDL‐phenylalanine NCA by block copolymers of sarcosine andDL‐phenylalanine, designated by (Phe)m(Sar)nand capable of reaction at the phenylalanyl terminal, were investigated in nitrobenzene solution at 25°C. With increasingnfor constantm(m= 0, 1, 2, and 5), the polymerization rate greatly increased. Previously the acceleration of the initiation reaction in the polymerization ofDL‐phenylalanine NCA by polysarcosine (m= 0) was reported. The present results showing the acceleration by the copolymers of sarcosine andDL‐phenylalanine indicate the presence of the polymer effect in the propagation reaction as well. However, the polymer effect was most marked with polysarcosine (m= 0), and decreased with increasingm.The same polymerizations by sequential copolymers composed of ten sarcosine units and twoDL‐phenylalanine units were also investigated. Again with these copolymer catalysts the polymerization rate was larger than that by monomeric amines. But the polymer effect decreased sharply when the phenylalanine units take positions near the terminal amine group of the copolymer catalyst.These two deteriorating effects of the phenylalanine unit have been interpreted in terms of the decrease of the flexibility of polymer chain, caused possibly by an intramolecular hydrogen bond of the phenylal

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120706

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

年代:1973

数据来源: WILEY

摘要:

AbstractHelix–coil transitions of poly(ε‐carbobenzyloxyL‐lysine) (PCBL) inm‐cresol and in mixtures of dichloroacetic acid (DCA) and ethylene dichloride (EDC) were investigated by optical rotation measurements, with special interest in the molecular‐weight dependence of helical content. The data obtained were analyzed according to an approximate expression for the helical content of Okita et al. The values obtained for the cooperativity parameter σ and transition enthalpy ΔHε (at the transition temperature) were: √σ = (0.25 ± 0.06) × 10−2and ΔHc= 930 ± 130 cal/mole inm‐cresol, and √σ = (0.49 ± 0.10) × 10−2, and ΔHc= 350 ± 150 kcal/mole in a DCA–EDC mixture (33 vol % DCA). The ΔHcvalue in the DCA–EDC mixture is much smaller than that derived by Giacometti et al. from heat of solution measurements. The transition inm‐cresol was extremely sharp. This was attributed to the unusually small value of √σ an

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120707

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

年代:1973

数据来源: WILEY

摘要:

AbstractHelix–coil transition of polypeptides in solution involves two elementary processes: unfolding of an α‐helical polypeptide chain and interaction of the unfolded chain with solvent molecules which are capable of forming hydrogen bonds. Optical rotation data for poly(β‐benzylL‐aspartate) in mixtures of dichloroacetic acid and carbon tetrachloride are analyzed according to the procedure developed by Sayama et al., who explicitly took these processes into theoretical account; and the enthalpy changes, ΔH0= −650 ± 200 cal/mole and ΔHa= −5.2 ± 1.6 kcal/mole. The latter value is favorably compared with heats of association (per hydrogen bond) of various amides and lactams in CCl4. There is excellent agreement with −5.15 kcal/mole for δ‐valerolactam in CCl4. In analogy with amides in non‐polar solvents, the difference between ΔH0and ΔHamay be attributed to the fact that the former is associated with intramolecular hydrogen bonding, whereas the latter is associated with inter

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120708

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

年代:1973

数据来源: WILEY

摘要:

AbstractCorrelation functions have been determined for the fluctuating intensity of the depolarized component of forward‐scattered laser light from solutions of DNA. The molecular correlation function of calf thymus DNA (mol wt ∼15 × 106) appears to exhibit a longest relaxation time (τ25,w, ∼ 18 msec) close to what one would predict from the flowdichroism measurements of Callis and Davidson and, in addition, manifests a spectrum of faster times down to tenths of milliseconds. Furthermore, a major fraction of the amplitude of fluctuations in the angular distribution of segment axes is relaxed on a very much shorter time scale (of the order of 20 microseconds) that appears to be relatively insensitive to molecular weight of the DNA, or to near‐melting temperatures.The temperature profile of the longest relaxation time has been obtained and found to exhibit a peculiar spike nearTm, which, together with the absence of a corresponding spike in the (high shear) viscosity, has been interpreted as indicative of an increase in the molecular weight of the DNA in a narrow temperature region nearTm. Correlation functions for polarized light scattered at finite angles were obtained in an attempt to determine the temperature dependence of the translational diffusion coefficient. Although the data contain an extremely slow component that does not admit a simple interpretation, there is some indication of a decrease in the translational diffusion coefficient nearTm, thus supporting the notion of an aggregation occurring nearTm. Finally, a “counterion escape” mechanisn is proposed for the apparen

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120709

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

年代:1973

数据来源: WILEY

摘要:

AbstractA new method of computing the geometry of a cyclic polymer from internal coordinates is discussed, which is particularly useful if the molecule has a center of inversion symmetry. When applied to the structure of valinomycin, the calculation is reduced to solving one transcendental equation in one variable.

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120710

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

年代:1973

数据来源: WILEY

摘要:

AbstractMalformin A is a cyclic pentapeptide with an intramolecular disulfide bridge. The conformation in solution of this molecule has been studied by NMR and CD. The 270 MHz Proton spectrum in dimethyl sulfoxide is well resolved and the peaks corresponding to the five residues have been assigned.From the temperature dependence of chemical shifts of the peptide protons and from the exchange rate of these protons, it is concluded that the NH proton of one Cys is shielded from the solvent. This observation and HNαCH angles, estimated from the corresponding coupling constants, a proposed conformation of the peptide backbone. From the HβCαCH coupling constants, aPchirality for the disulfide bridge is proposed. Such a conformation is confirmed by the circular dichroism spectrum which shows a negative band at λ>250 nm.It is concluded that the conformation of malformin A is rigid and that the disulfide bridge is exposed to interact with biolo

ISSN:0006-3525    

DOI:10.1002/bip.1973.360120711

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

年代:1973

数据来源: WILEY