摘要:

AbstractA general theory of polyelectrolyte solutions is here used to calculate the differences in Gibbs free energy, enthalpy, and entropy between the coil and helix forms of DNA at any temperature and salt concentration. The salt has univalent cations and is assumed present in excess over the base concentration. The results are restricted to sufficiently dilute solutions. It is shown that the salt concentrations effect is entirely entropic in origin. When applied to the melting temperature, the calculations yield a relation between the enthalpy difference at the melting temperature and the slope of the plot of melting temperature vs. the logarithm of the salt concentration. In accord with observation, both the Gibbs free energy difference at any fixed temperature and the melting temperature are predicted to be linear functions of the log of the salt concentration.However, the theory is not in quantitative agreement with enthalpy data. Data on various colligative and transport properties of both helix and coil forms are reviewed in the text and in Appendix B, and good agreement is found with theory for both forms. No attempt is made to explain why the theory is quantitative for these properties but not for heat measurements.Finally, in Appendix A, an approximate calculation is made of the free energy contributions due to ionic effects not associated with the salt concentration.

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110502

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

年代:1972

数据来源: WILEY

摘要:

AbstractThe techniques of the previous article are here applied to the case for which the solution contains, in addition to excess uni–univalent salt, one equivalent of divalent counterions per mole nucleotide. In agreement with the melting temperature measurements of Dove and Davidson for Mg++, it is predicted that a region of uni–univalent salt concentration then exists in which (dTm/dlogmA+) is negative. It is further predicted, in accord with experiment, that in the presence of divalent counterions, the helical form of DNA is much more stable than in their abse

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110503

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

年代:1972

数据来源: WILEY

摘要:

AbstractThe cooperative effect is developed for proteins containing four subunits. The extension of the primitive Adair model is considered and the present models based on this extension are compared.

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110504

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

年代:1972

数据来源: WILEY

摘要:

AbstractFollowing Simon and Zimm's recent work, both molecular dynamic and Monte Carlo methods are used to simulate on a computer the unwinding of a DNA‐like (N+ 1)‐unit helical strand. The strand unwinding is assumed to obey (N+ 1)‐coupled Langevin equations of motion (N= 10–50). The present computer “experiment” was done to elaborate dependence of unwinding behavior of the helix upon (i) configurations of its complementary strand [represented here by a square cylinder (Simon‐Zimm model), a circular cylinder, and a fixed helix (double‐helical model)]; (ii) interstrand potentials (a hard‐cylinder potential, an inverse twelfth‐power soft repulsion with or without an inverse third‐power attractive tail); (iii) amplitudes of the Brownian motions, and (iv) the molecular weights of the unwinding strand.We found that (i) the Brownian motions strongly couple with the interstrand potentials to produce a shorter “unwinding time” (τ0) (which characterizes an expotential decay of the unwinding) for a longer renged repulsive potential (without the Brownian motions, no such effect was present); (ii) addition of an attractive tail to the repulsive potentials further reinforces the unwinding, thereby giving a reduced value of τ0; (iii) τ0can be expressed as a product of two factors–unwinding time for a one‐dimensional spring‐bead model, which mostly accounts for theN2‐dependence in τ0, and a factor which depends on the Brownian motions and the interstrand potentials; (iv) among the three models described above, under similar situations, the three‐dimensional double‐helical model has the smallest τ0; and (v) unless the maximum Brownian displacement exceeds a certain value, the unwinding around a square cylinder takes place in a (unrealistic) stair‐step manner whose τ0decreaseswith increase in the Brownian displacements.TheN2‐dependence of τ0agrees with the Simon and Zimm's machine results as well as Crother's experimental data on T2 DNA; however, it contradicts experimental data of Massie and Zimm, and others. Further possible improvements in conne

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110505

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

年代:1972

数据来源: WILEY

摘要:

AbstractHigh‐molecular‐weight poly‐L‐alanine dissolved in hexafluoroisopropanol exhibits infrared, ultraviolet, circular dichroism, and optical rotatory dispersion spectra which are unique and unlike any other previously reported polypeptide spectra. Strong evidence that a helical conformation is present is shown by the high degree of hypochromism in the 187mμ absorption peak and by the positions of the amide infrared bands. The CD and ORD spectra are also similar to those of α‐helical polypeptides, though important qualitative and qualitative differences are observed. To explain the novel spectra, which are not mixtures of the spectra of previously reported polypeptide conformations, a new α‐helix‐like conformation is proposed. The postulated conformation (a doubly hydrogen‐bonded helix) is a distorted α‐helix in which the peptide carbonyl groups point slightly out from the helix axis and are hydrogen bonded simul taneously both to the NH of the fourth peptide residue to the carboxyl terminal side (as in the classical α‐helix), as well as to a solvent mole

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110506

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

年代:1972

数据来源: WILEY

摘要:

AbstractThe theoretical change of the mean‐square dipole moment of a polypeptide during the helix‐coil transition is compared with the change in helix content. It is shown that, according to the theory, the determination of the helix initiation parameter σ and the enthalpy of helix formation ΔHcan be determined. Experimental data on poly‐benzyl‐L‐gluatamate in two different solvent mixture

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110507

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

年代:1972

数据来源: WILEY

摘要:

AbstractThe ultraviolet absorption, optical rotatory dispersion, circular dichroism, and infrared absorption spectra of renatured 5S RNA have been measured at pH 7.0 in 0.1MNaCl at 25° and used to obtain four independent estimates of the number of base pairs. These four estimations are in reaonable agreement and average values of 28 ± 4 G.C and 13 ± 4 A.U. base pai

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110508

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

年代:1972

数据来源: WILEY

摘要:

AbstractUsing methods described in a previous publication the optical activity of a number of polypeptides and proteins has been calculated. The systems included the α‐helix, the two β‐structures, polyproline I, polyproline II, collagen and collagen models, and poly‐N‐methylalanine. In addition to these orderded structures, calculations were also performed on the α, β and nonperiodic regions of myoglobin, lysozyme, ribonuclease‐S and β‐chymotrypsin. The α and β structures in prteins differ from the polypeptide models by being very short and partially disordered. It is concluded that the 222‐nm band of the α‐helix is a good method for detecting helices in proteins but that the 207‐and 191‐nm bands of the helix will not fit a linear superposition model. The circular dichroism of the so‐called β regions of proteins differs markedly from that for ideal β structure because of breakdownin symmetry. As a result estimates of β‐structure in proteins based on polypeptide models are not likely to be quantitative.The theoretical methods give an adequate account of the optical activity of all the ordered polypeptides except polyproline II and collagen and (by inference) the nonperiodic chains in the various proteins. This difficulty is the remaining barrier to a complete theory of the optical activity of the polypep

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110509

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

年代:1972

数据来源: WILEY

摘要:

AbstractThe decay curves of electro‐optic transients are generally used to measure the rotary diffusion contants (D) of colloidal particles in suspension or biopolymers in solution. Analysis usually is made by plotting the logarithm of the decay as a function of time,t. Curvature in such plots the presence of several values ofDand hence a distribution of particle sizes. In this case, analysis of the complete curve to yield the distribution is severely restricted by lack of precision in the experimental data. The most reliable and reproducible part of the decay curve is the initial (t=0)slope.This paper develops theory for the initial slope for dilute suspensions of long thin cylinders with various electrical properties. It is shown that the inital slope depends not only on the particle‐size distribution and electric field strength, but also on the nature of the electrical properties of the molecules. Futhermore, for an identical polydisperse system. transient electric birefringence, electric optical rotation, and electric dichroism on the one hand, and transient electric light scattering on the other, yield different initial slopes and hence apperently different values forD. These important conclusions have not been appereciated in previous studies and indicate the need for caution when comparing data from different experimental methods. I11ustrative calculations are presented for a polydisperse system of long thin cylinders having a normal distribution in lengths, but of uniform diameter. Some preliminary measurement support the the

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110510

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

年代:1972

数据来源: WILEY

摘要:

AbstractCircular dichriosm (CD) spectra have been calculated for serveral dinucleoside phosphates using a variant of the Pariser‐Parr‐Pople π‐electron molecular orbital method. This method does not require the prior knowledge of the experimental absorption spectra of transition moments of the bases forming the dinucleoside phosphates. Calculated spectra were obtained in good agreement with experimental spectra for four dinucleoside phosphates, ApA, UpU, GpA, and UpA, and reasonable agreement was obtained for ApG and ApU. The effect of changing conformation on the CD spectrum was studied for ApA, UpU, UpA, and ApU; the spectra of UpU, UpA, and ApU were sensitive to small change in conformation, whereas ApA was insensitive over the range of conformation studied. Further studies await detailed knowledge of the structure of dinucleoside phosphates in so

ISSN:0006-3525    

DOI:10.1002/bip.1972.360110511

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

年代:1972

数据来源: WILEY