摘要:

AbstractThe lack of the positive band at around 226 nm in the CD spectra of poly(prolyl‐azetidine‐2‐carbonyl‐proline) in trifluoroethanol and of poly(azetidine‐2‐carbonyl‐prolyl‐azetidine‐2‐carboxylic) acid in F3EtOH and water, the hyperchromism of the absorption maximum at about 202 nm, and the extremely small intensity of the Cβ‐Pro, Cγ‐Pro, and Cβ‐Aze signals for thecispeptide bonds in the13C nmr spectrum of poly(Pro‐Aze‐Pro) in F3EtOH indicate that both polyproline analogs exist as disordered chains in this solvent, thetranspeptide group being maintained. The disordering of the chains is attributed to an increase in the accessible range of ψ due to the reduced dimensions of the square ring ofL‐azetidine‐2‐carboxylic acid residue relative to the pyrrolidine ring of proline and to strong interactions of the haloalcoho

ISSN:0006-3525    

DOI:10.1002/bip.1976.360150702

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

年代:1976

数据来源: WILEY

摘要:

AbstractIt has been observed that the amount of water associated with casein micelles is markedly higher when measured by intrinsic viscosity than by water content of the pelleted casein. A possible explanation of this discrepancy is that water is squeezed out of the pellet by the ultracentrifugal field. We have calculated the potential magnitude of this effect by considering a simple model system: an elastic gel swollen by solvent and compressed by the centrifugal field. Equilibrium is reached when the sum of the ultracentrifugal, elastic, and mixing free energies is a minimum. The equilibrium degree of swelling is calculated as a function of rotor speed, thickness of the unsolvated gel material, and enthalpy of mixing of solvent and solute. Sizable compressions can occur for highly swollen gels, if the enthalpy of mixing is moderately positive. Casein micelles from intrinsic viscosity measurements have a “hydration” of about 3.7 g water/g protein, corresponding to a swelling ratio of 6.3 relative to the dry protein. The observed pellet hydration is 1.9 g/g. Under the conditions of the experiment, pelleting at 25,000 rpm is predicted to decrease the swelling ratio to 4.9, and the hydration to 2.7 g/g, about half the observed decrease, if the enthalpy of mixing is 0.5RT/mol segment. These calculations may be relevent to the deterination by pelleting of the solvent content of other biopolymer g

ISSN:0006-3525    

DOI:10.1002/bip.1976.360150703

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

年代:1976

数据来源: WILEY

摘要:

AbstractAggregated complexes of acridine orange with dermatan and chondroitin sulfates have been studied in aqueous solution by absorption and circular dichroism spectroscopy. Aggregation was found to be favored at high‐dye and glycosaminoglycan concentrations, and in solutions where anionic sites of the glycosaminoglycan are effectively complexed with dye. The aggregates can be removed from solution by centrifugation at 27,000 ×gfor 1 hr or by filtration through a membrane containing pores of 0.1 μm diameter.The aggregated complexes exhibit large‐magnitude‐ellipticity circular dichroism bands. In addition, the circular dichroism spectrum observed for a solution containing aggregated acridine orange/chondroitin 4‐sulfate complexes is nearly a mirror image of that obtained for aggregated acridine orange/dermatan sulfate complexes.Cooperative alterations (sharp transitions) in the circular dichroism ellipticities of the aggregates occur at elevated temperatures, and result in spectroscopically distinct aggregates upon cooling. The circular dichroism properties and temperature effects are attributed to a supramolecular ordering of acridine orange/glycosaminoglycan complexes within the aggregates, which can be reorganized to a more stable form at high temperatures.Mixed aggregates, containing two different glycosaminoglycans, can be formed. The circular dichroism properties of the mixed aggregates also indicate the existence of long‐range order in the arrangement of the complexes. Mixed aggregates containing dermatan sulfate and either chondroitin 4‐sulfate or chondroitin 6‐sulfate resemble pure dermatan sulfate aggregates in circular dichroism

ISSN:0006-3525    

DOI:10.1002/bip.1976.360150704

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

年代:1976

数据来源: WILEY

摘要:

AbstractA very low‐angle light‐scattering photometer is described with respect to optical features, scattering cell, correction factors, and absolute calibration in the angular range 2°–35°. An improved microfiltration apparatus was used to obtain essentially dust‐free aqueous solutions for very low‐angle light scattering. The instrument was calibrated with silicotungstic acid, an absolute molecular‐weight standard, and the calibration was confirmed with the use of several secondary standards.Very low‐angle light‐scattering measurements were made to determine the weight‐average molecular weightM̄randz‐average radius of gyrationRg,zof a commerical preparation of calf‐thymus DNA. Microfiltration of the solutions allowed measurements down to 6°. The valueM̄r= 20.0 × 106obtained by extrapolating 6°–9° data to 0° is more than three times that from 30°–75° data (6.38 × 106) but ∼20% smaller than that from 10–35° data (23.7 × 106). The experimental errors inM̄randRg,zare estimated to be ±8% and ±14%, respectively. Combined 6°–75° data from two photometers fit well a theoretical scattering curve for a m

ISSN:0006-3525    

DOI:10.1002/bip.1976.360150705

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

年代:1976

数据来源: WILEY

摘要:

AbstractTwo methods for the characterization of protein molecular weights from their diffusion coefficients are discussed. These measurements can be made quickly and reliably at low concentrations using quasielastic light‐scattering techniques. First, an empirical calibration of the diffusion coefficient at infinite dilution of denatured random coils against molecular weight is reported. The second method combines the measurement ofD0with the intrinsic viscosity [η]. ThisD0–[η] relationship proves to be very insensitive to polymers structure or solvent type. The data indicate that the ratio of the hydrodynamic radius measured by viscosity to the hydrodynamic radius measured by diffusion is about 15% smaller than that predicted by theoretical models. The nature of the molecular‐weight average obtained for polydisperse systems is defined for a Schulz distribution. These hydrodynamic methods have also been used to demonstrate the presence of chain branching in the glycoprotein ovomucoid. In addition, a method is proposed by which the effective segment length and an excluded volume parameter for random coils may be evaluated for diffusion measu

ISSN:0006-3525    

DOI:10.1002/bip.1976.360150706

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

年代:1976

数据来源: WILEY

摘要:

AbstractIt is shown that theoretical conformational analysis, based on the evaluation of semiempirical potential functions, can be used to compute the quantities relevant to the interpretation of energy‐transfer experiments. The relevant properties are computed for a segment of a polypeptide chain with the sequence Tyr‐Tyr. In particular, the average value of the orientation factor 〈κ2〉 and its distribution ƒ(κ2) are examined. It appears that the degrees of freedom for rotation of the side chains are not sufficient to randomize completely the orientation factor of the transition dipoles. Two additional degrees of freedom, namely the torsion angles around the valence bonds of the backbone, ψ1and ϕ2, bring 〈κ2〉 close to the value that corresponds to randomly oriented t

ISSN:0006-3525    

DOI:10.1002/bip.1976.360150707

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

年代:1976

数据来源: WILEY

摘要:

AbstractProton and phosphorus‐31 nuclear spin–lattice relaxation timesT1and spin–spin relaxation timesT2have been measured on the single‐stranded polyriboadenylic acid [poly(A)]–Mn2+system in a neutral D2O solution in the temperature range 10°–90°C at 100 and 40.5 MHz, respectively, with the Fourier transform nmr method. Minimum values ofT1have been found for all these nuclei, which have enabled the exact estimation of apparent distances from Mn2+to H2, H8, H1′, and the phosphorus nucleus to be 4.7, 4.1, 5.2, and 3.0 Å, respectively. The electron spin of Mn2+penetrates into the phosphorus nucleus, giving31P hyperfine coupling of more than 106Hz. Evidence of penetration of the electron spin into H8and H2is also obtained, suggesting direct coordination of nitrogen atoms of the adenine ring to the Mn2+Ion. Combined with the result from proton relaxation enhancement of water, it is concluded that every Mn2+ion added is bound directly to two phosphate groups with a Mn2+–phosphorus distance of 3.3 Å, while a part of the Mn2+ions are simultaneouly bound to the adenine ring. It is estimated that 39 ± 13% and 13 ± 5% of Mn2+are coordinated by N7and N3(or N1), respectively. The motional freedom of poly(A) in the environment of the Mn2+binding site has been found to be quenched to the extent that the rotational motion becomes several times slower than that of the corresponding Mn2+–free poly(A). The activation energies for the molecular motion are, however, practically unchanged from those for Mn2+–free poly(A), and are found to be 8.3, 8.5, 6.1, and 8.7 kcal/mol for H8, H2, H1′, and phosphorus, respectively.T2of phosphorus is determined by the dissociation rate (k−1) of Mn2+from the phosphate group for the whole temperature range studied with activation enthalpy of 6.5 kcal/mol. The dissociation rates of Mn2+from the adenine ring are also estimated f

ISSN:0006-3525    

DOI:10.1002/bip.1976.360150708

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

年代:1976

数据来源: WILEY

摘要:

AbstractThe bimolecular rate constantk2for the association of complementary polynucleotide strands has been observed to increase strongly with increasing ionic strength—in fact, proportional to its third or fourth power. This effect is here interpreted quantitatively by means of polyelectrolyte theory starting with the Wetmur–Davidson postulate of a pre‐equilibrium between separated strands and aligned segments close to one another but unbonded. The correct form, a power dependence ofk2on ionic strength, is predicted. Comparison of the theoretical exponent with data allows the conclusion that each of the two single‐stranded segments in the aligned but unbonded configuration consists of about 13–16 nucletides (not to be confused with the much smaller number of bonded base pairs in the nucleus), and that this number, denoted byQ, is possibly correlated either with a minimum length for duplex stability or with the persistence length of a single polynucleotide strand. It is suggested that experimental determination of the dependence ofQon (G+C)‐content may distinguish between these possibilities. It is also suggested that addition of sufficient amounts of divalent metal ions such as Mg2+, Ca2+, or Co2+may reverse the dependence ofk2on ionic strength; under these conditions,k2is predicted todecreasewith about the first power of ionic strength. At fixed ionic strength,k2should increase with increasing concentration of divalent metal ion, and, in fact, the published observation that the formation of poly(A)·2 poly(U) from poly(A)·poly(U) and poly(U) is second order in Mg2+concentration is here correctly predicted froma priorimolecular considerations. Finally, published association rate data for oligonucleotides are discussed in the present theore

ISSN:0006-3525    

DOI:10.1002/bip.1976.360150709

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

年代:1976

数据来源: WILEY

摘要:

AbstractTheoretical calculations are conducted on the helix–coil transition of DNA, in the presence of large, cooperatively binding ligands modeled after the DNA‐binding proteins of current biological interest. The ligands are allowed to bind both to helx and to coil, to cover up any number of bases or base pairs in the complex, and to interact cooperatively with their nearest neighbors. The DNA is treated in the infinite homogeneous Ising model approximation, and all calculations are done by Lifson's method of sequence‐generating functions. DNA melting curves are calculated by computer in order to expolore the effects on the transition of ligand size, binding constant, free activity, and ligand–ligand cooperativity. The calculations indicate that (1) at the same intrinsic free energy change per base pair of the complexes, small ligands, for purely entropic reasons, are more effective than are large ligands in shifting the DNA melting temperature; (2) the response of the DNA melting temperature to increased ligand binding constantKand/or free ligand activityLis adequately represented at high values ofKL(but not at lowKL) by a simple independent site model; (3) if curves are calculated with the total amount of added ligand remaining constant and the free ligand activity allowed to vary throughout the transition, biphasic melting curves can be obtained in the complete absence of ligand–ligand cooperativity. In an Appendix, the denaturation of poly[d(A‐T)] in the presence of the drug, netropsin, is used to verify some features of the theory and to illustrate how the theory can be used to obtain numerical estimates of the ligand binding parameters from the experimental mel

ISSN:0006-3525    

DOI:10.1002/bip.1976.360150710

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

年代:1976

数据来源: WILEY

摘要:

Abstract(DLLys)10–(LVal)20–(DLLys)10, a copolymer of a valine block in the β conformation with randomly coiled flanking blocks of lysine [Kubota, S.&Fasman, G. D. (1975)Biopolymers14, 605–631], was studied by nuclear magnetic resonance. The peak areas of the NH and CH resonances of valine and lysine are concentration‐dependent with little chemical shift change. A model is presented describing the aggregation of the copolymer. Additional evidence, obtained by monitoring the spin‐lattice relaxation times of amino, amide, γ‐methyl‐valine, and ϵ‐methylene‐lysine, is consistent with the aggregation model. Hydrodynamic studies are presented which show monomer → oligomer aggregatio

ISSN:0006-3525    

DOI:10.1002/bip.1976.360150711

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

年代:1976

数据来源: WILEY