摘要:

AbstractNormal modes of a linear array of iodine atoms with various repeat units, (I 3−), (I 3−I 3−), (I 3−I 2−), and (I 5−), have been calculated. The four resonance Raman bands (158, 107, 57, and 27 cm−1) observed with the starch–iodine complex can be assigned to the symmetric modes of either the (I 3−I 3−)nor (I 5−)nlinear chain. However, the nature of the valence force fields of these two structures is very different: the former consists of a weakly coupled linear chain of (I 3−) structure, while the latter, (I 5−)n, is a tightly bound chain approaching equidistant iodine spacings. The nearly equivalent I‐I stretching force constants obtained in the present normal mode analysis of the (I 5−)nstructure is supported by the results of previous x‐ray and I‐129 Mössbauer effect measurements. A numerical method of calculating the frequency dispers

ISSN:0006-3525    

DOI:10.1002/bip.360211102

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

年代:1982

数据来源: WILEY

摘要:

AbstractStarting with β‐methoxy methacryloylisocyanate, β‐methoxy methacrylisothiocyanate, and β‐isocyanatopropionyl chloride, on the one hand, andNα‐Z‐lysine orNα‐Z‐ornithine, on the other hand,Nα‐Z‐amino acids with pyrimidine bases in the side chain were synthesized. These Z‐protected nucleoamino acids were converted to the corresponding N‐carboxyanhydrides (NCAs) via the silylester method. In the case of 2‐thiothymine derivatives, the reaction intermediate of the NCA synthesis caused benzylation of the thioxo‐ group, so that a new class of 2‐mercaptopyrimidine derivatives was isolated unexpectedly. The poly(nucleoamino acids) obtained by polymerization of the nucleoamino acid NCAs were characterized by elemental analyses, optical rotations1H‐nmr and13C‐nmr spectra. Vapor pressure osmometry revealed that theDPs were in the range of 20–30. Their spectra suggest a helical secondary structure. While all homopolypeptides are insoluble in water, copolypeptides containingL‐lysineNε‐h

ISSN:0006-3525    

DOI:10.1002/bip.360211103

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

年代:1982

数据来源: WILEY

摘要:

AbstractIn an unfavorable solvent environment, DNA (and other polymers) undergo a conformational transition to a collapsed form, accompanied by a dramatic reduction in the effective volume of the molecule. Solvent conditions leading to the collapse are the same as those that cause aggregation. We give here a thermodynamic description of the collapse and its relations to aggregation (or precipitation). This is formulated in terms of the Flory‐Huggins theory of the thermodynamics of polymer solutions. The results show that it is possible for three different states of DNA to be stable under different conditions: (1) the extended random coil, (2) the collapsed coil, and (3) a concentrated phase of aggregated random coils. The collapsed coil is predicted to be stable against aggregation only at high dilutions, of the order of parts per million. For DNA the transition between the extended coil and the collapsed coil is predicted to be discontinuous, in the sense that intermediate states are not present, because of the relatively high stiffness of the chain. The transition should appear diffuse because of the small size of the single molecule in comparison to macroscopic system

ISSN:0006-3525    

DOI:10.1002/bip.360211104

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

年代:1982

数据来源: WILEY

摘要:

AbstractLight‐scattering studies were done to investigate the DNA collapse transition, a large and discontinuous reduction in the radius of gyration. Of particular concern was differentiating the compaction of a single DNA molecule from aggregation. Solutions of RK2 plasmid DNA (Mr= 37 × 106) or bacteriophage T7 DNA (Mr= 25 × 106) were titrated with the condensing reagents spermidine in aqueous solvent or magnesium ion in ethanol–water solvent. The transition was followed by the change in scattering at a single angle or by the change in the angular dependence of scattering. At concentrations below 1 μg/mL, only aggregation could be detected by observation at a single angle; therefore, to study the collapse transition, it was necessary to measure the angular dependence of scattering. The intensities measured between the angles 30° and 60° were fit to known scattering functions. At low concentrations of the condensing reagent, the data were consistent with the scattering function of a random coil. On the other hand, during the transition at higher reagent concentrations, the curve that fit the data required two components—the scattering function for a random coil with a large radius of gyration, plus that for a sphere with a radius about one‐fifth of that of the coil. The fractional concentration of the sphere increased with increasing condensing‐reagent concentration. This two‐component behavior is in apparent contrast to the situation with a more flexible polymer such as polystyrene, in accord with theoretical predictions. At still higher reagent concentrations, aggregation was apparent. Condensation to a collapsed state was reversible without hysteresis, while dissolution of the aggregated state nearly always occurred with hysteresis. Qualitative agreement between the observed DNA collapse transition and the theoretical phase diagram presented in the preceding paper was found, although the light‐scattering results did not show quantitative agreement with the simple

ISSN:0006-3525    

DOI:10.1002/bip.360211105

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

年代:1982

数据来源: WILEY

摘要:

AbstractVibrational circular dichroism (VCD) spectra for the principal amide stretching vibrations, amide A (NH stretch) and amide I (predominantly CO stretch), are presented and analyzed for a variety of polypeptides dissolved in chloroform, as well as for two examples in D2O. Our results for poly(γ‐benzyl‐L‐glutamate) confirm the first and only previous report of VCD in polypeptides carried out by Singh and Keiderling [(1981)Biopolymers20, 237–240]. Collectively, our spectra show that the sense of the bisignate VCD in these two regions depends on the sense of α‐helicity and not on the absolute configuration of the constituent amino acids. This conclusion is established by obtaining VCD for the two polypeptides, poly(β‐benzyl‐L‐asparate) and poly(im‐benzyl‐L‐histidine), that form left‐handed as opposed to right‐handed α‐helices. A new amide band having significant VCD intensity owing to its Fermi resonance interaction with the NH stretching mode has been identified as a weak shoulder on the low‐frequency side of the amide A band near 3200 cm−1and is assigned as a combination band of the amide I and amide II vibrations. VCD spectra of polypeptides in D2O solution, although weak, have been successfully measured in the amide I region, where spectra appear to be more complicated due to the presence of solvated and internally hydrogen‐bonded amide groups. Strong monosignate contributions to the VCD in the amide A and amide I regions for some of the polypeptides indicate coupling of an electronic nature between these two regions and is deduced by an application of the concept of local sum rules of rotational strength. It appears that a detailed understanding of the VCD obtained for polypeptides will not only be diagnostic of secondary structure, but also of more subtle structural and vibrational effects that give rise to local,

ISSN:0006-3525    

DOI:10.1002/bip.360211106

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

年代:1982

数据来源: WILEY

摘要:

AbstractDifferential scanning calorimetry was used to directly determine the transition enthalpies accompanying the duplex‐to‐single‐strand transition of poly[d(AT)], poly(dA)·poly(dT), poly[d(AC)]·poly[d(TG)], and d(GCGCGC). The calorimetric data allow us to define the followingaveragebase‐stacking enthalpies:InteractionΔH(kcal/stack)AC/TG, TG/AC5.6AT/TA, TA/AT7.1AA/TT8.6GC/CG, CG/GC11.9Comparison with published data on the corresponding RNA interactions reveals remarkably good agreement. By assuming transition enthalpies to result from the pairwise disruption of nearest‐neighbor stacking interactions, we used the enthalpy data listed above to predict the transition enthalpies for three oligomeric DNA duplexes. Excellent agreement was found between the predicted and the calorimetrically deter

ISSN:0006-3525    

DOI:10.1002/bip.360211107

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

年代:1982

数据来源: WILEY

摘要:

AbstractThe conformation of several samples of poly(α,β‐L‐Asp) with a molar fraction of β‐bonds ranging from 0.1 to 0.55 was investigated by means of ir and CD spectroscopy and potentiometric titration and compared with the results obtained previously with poly(α‐L‐Asp). All samples investigated underwent a conformational change induced by changes in their degree of ionization: unpronounced ir absorption of amide V at 650 cm−1was shifted to 620 cm−1and substantially increased on deionization; CD spectra changed with the degree of ionization, passing through an isosbestic point; and the pattern of the titration curves was more complex than that of a simple polyelectrolyte. The conformation developing with the decreasing degree of ionization may be considered to be α‐helix, as deduced according to the analogous behavior of other polypeptides. The extent of the conformational change in the individual samples depends on the molar fraction of β‐bonds: the higher it is, the lower is the helix‐form

ISSN:0006-3525    

DOI:10.1002/bip.360211108

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

年代:1982

数据来源: WILEY

摘要:

AbstractThe polymerization of fibrin induced by the enzyme thrombin was investigated in the pregelation phase by combining measurements of the release of the fibrinopeptides A with static and dynamic light‐scattering at scattering angles ranging from 5°–150°. Without making any assumptions about the polymer distribution, one is led to the following conclusions: The aggregates are rodlike; the Flory‐Stockmayer model is to be preferred over the percolation model, i.e., cyclic bonds are infrequent; in the early stages the experiments indicate a functionalityf= 2 (number of reactive binding sites per monomer) that increases with increasing release of fibrinopeptides A, eventually approaching the valuef= 4; the number of binding sites involved in a bond is about twice the number of the released fibrinopeptides A; and in the polymers the monomer units aggregate end‐to‐end in the very early stage and then in a stagge

ISSN:0006-3525    

DOI:10.1002/bip.360211109

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

年代:1982

数据来源: WILEY

摘要:

AbstractThe use of1H‐nmr spectroscopy is demonstrated to be a useful analytical method to characterize the structure of synthetic peptides attached to soluble, macromolecular polyoxyethylene (POE) supports in the liquid‐phase method (LPM) of peptide synthesis. We report an extensive 360‐MHz1H‐nmr study of POE‐bound homo‐oligo‐L‐methionine peptides. A combination of high field and selective saturation or Redfield pulse methods allows resolution of individual backbone NH and α‐CH resonances of dilute peptides in the presence of strong resonances from macromolecular POE and/or protonated solvents. The nmr spectra for the POE‐bound peptides in CDCl3are qualitatively similar to those of the low‐molecular‐weight Boc‐L‐Metn‐OMe peptide esters. This corroborates other observations that POE has little effect on peptide stucture. The backbone α‐CH region of peptides is overlapped by signals from the terminal oxyethylene group of POE, but the peptide side‐chain and low‐field backbone NH resonances are well resolved. In trifluoroethanol the Boc‐(L‐Met)n‐NH‐POE heptamer and octamer adopt the right‐handed α‐helical structure, and the present nmr studies provide evidence fortwostrong intramolecular hydrogen bonds to stabilize the helices. In water, the N‐deblocked derivatives, (L‐Met)n‐NH‐POE oligomers adopt β‐sheet structure and manifest well‐resolved n

ISSN:0006-3525    

DOI:10.1002/bip.360211110

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

年代:1982

数据来源: WILEY

摘要:

AbstractThe concerted model of Monod, Wyman, and Changeux is generalized so that all effects of interactions for an enzyme operating at a nonequilibrium stationary state are considered. In contrast to the original model, which is based on an analogy to equilibrium ligand binding, the generalization may show both “positive” and “negative cooperativity” in both catalytic binding and conformational processes. Furthermore, in contrast to any equilibrium binding model, the Hill coefficients may be greater than the number of sitesn. For catalysis, the maximum value is 2n, and for conformational changes,n+ 1. These points are illustrated by two cases that yield simpler analytic expressions. The first obtains when catalysis occurs on a much faster time scale than the conformational changes, and the second, when this situation is r

ISSN:0006-3525    

DOI:10.1002/bip.360211111

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

年代:1982

数据来源: WILEY