The conformational properties of single polyelectrolyte chains of various lengths in the presence of counterions are investigated by molecular dynamics simulations. For Coulomb interaction strengths below the critical value for Manning condensation, the molecular chain exhibits an increase of the radius of gyration and of the end-to-end distance with increasing interaction strength. Above this critical value, counterions condense on the chain and ion pairs are formed. The ion pairs possess a net attraction such that beyond a certain interaction strength the chain with the condensed ions collapses into a dense coil. The scaling behavior of the radius of gyration and the end-to-end distance with changes in the number of bonds is discussed for various Coulomb interaction strengths. [S0031-9007(98)05910-9] PACS numbers: 36.20.Ey, 87.15.ByCharged macromolecules (polyelectrolytes and polyampholytes) constitute a large class of materials which are particularly important for biological systems. Among these proteins and nucleic acids are well known. A variety of theoretical studies have been undertaken to elucidate the structural properties of such molecules [1][2][3][4] and to gain insight into the coil to rod transition behavior. However, to predict the conformational properties of a charged macromolecule is a very complex problem due to the long range nature of the Coulomb interaction. This is particularly true for highly charged chains, where the interaction with the counterions has to be taken into account. Recent experimental and theoretical studies demonstrate that the condensation of counterions and the formation of ion pairs significantly influence the conformation of a chain [5][6][7]. The Coulomb interaction and the counterions add new length scales to those of the neutral polymer. This complicates scaling theories and other theoretical approaches considerably. Thus, a microscopic understanding of the underlying physical phenomena is currently only possible by computer simulations.To elucidate the conformational properties of highly charged polyelectrolytes in the presence of counterions, we performed a series of molecular dynamics simulations. We particularly investigated the distribution of the counterions for various interaction strengths between the chain molecule and the ions. For highly charged chains all the counterions are condensed on the chains, and compact globules are formed. As demonstrated below, the formation of ion pairs associated with dipole moments is essential for the collapse of the chain.In this Letter, we present simulation results for a polyelectrolyte chain without salt but taking into account the counterions explicitly. The chain comprises N (N 2 1 being the number of bonds) harmonically bound mass points. The excluded volume interaction between the masses of the chain and with an equal number of counterions is taken into account by a purely repulsive Lennard-Jones potential. In addition, each monomer of the chain carries a charge e and the counterions a charge 2e. Hence, the charges o...

The dynamic structure factor for molecular chains with variable stiffness in a dilute solution is investigated. In the limit of small scattering vectors q only the overall translational motion of the macromolecules contributes to the dynamic structure factor. The translational diffusion coefficient D exhibits a chain length dependence D∼1/√L for flexible chains and D∼ln L/L+const/L for rodlike chains. For flexible chains there is an intermediate scattering vector regime in which the decay rate or spectral linewidth of the dynamic structure factor is proportional to q3 indicating that stretching modes are dominant. Such an intermediate scattering vector regime cannot be observed for semiflexible or rodlike chains. At large scattering vectors q/2p≳1.5, where 1/2p is the persistence length of the macromolecules, the chain stiffness becomes important for any kind of molecules, i.e., even for very flexible ones. The dynamic structure factor and the decay rate are compared with experimental results of quasielastic neutron and light scattering experiments on different natural and synthetic macromolecules. These experimental results are in good agreement with the theoretical predictions. Furthermore, we determine the persistence length of F-actin from a dynamic light scattering experiment.

This wore is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, reuse of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under § 54 of the German Copyright Law where copies are made for other than private use, a fee is payable to ,,Verwertungsgesellschaft Wort", Munich.

The partition functions of discrete as well as continuous stiff molecular chains are calculated using the maximum entropy principle. The chain is described by mass points, and their connectivity is taken into account by harmonic constraints (flexible segments) in addition to the bending restrictions. For comparison and as a test of the formalism the freely rotating chain as well as the Kratky–Porod wormlike chain (rigid segments) are reexamined treating the bending restrictions as constraints. It is shown that the second moments for the chain of flexible segments agree exactly with those known from the freely rotating chain for the discrete as well as the continuous chain and for all stiffnesses. Moreover, the Green’s function for the continuous chain is determined, which allows to obtain any desired two point distribution function. The influence of various bending restrictions on equilibrium properties is discussed. Furthermore, a comparison to other existing models, especially the Harris and Hearst model, is given and the validity of the various models is examined.

No abstract