Electrostatic Persistence Length

The persistence length is calculated for polyelectrolyte chains with fixed bond lengths and bond angles (pi - theta), and a potential energy consisting of the screened Coulomb interaction between beads, potential wells alpha phi(2)(i) for the dihedral angles phi(i), and coupling terms beta phi(i)phi(i +/- 1). This model defines a librating chain that reduces in appropriate limits to the freely rotating or wormlike chains, it can accommodate local crumpling or extreme stiffness, and it is easy to simulate. A planar-quadratic (pq), analytic approximation is based on an expansion of the electrostatic energy in eigenfunctions of the quadratic form that describes the backbone energy, and on the assumption that the quadratic form not only is positive but also adequately confines the chain in an infinite phase space of dihedral angles to the physically unique part with all vertical bar phi(i)vertical bar < pi. The pq approximation is available under these weak constraints, but the Simulations confirm its quantitative accuracy only under the expected condition that a is large, that is, for very stiff chains. Stiff chains can also be Simulated with small alpha and small theta and compared to in OSF approximation suitably generalized to chains with finite rather than vanishing theta, and increasing agreement with OSF is found the smaller is theta. The two approximations, one becoming exact as alpha -> infinity with fixed theta, the other as theta -> 0 with fixed alpha, are quantitatively similar in behavior, both giving a persistence length P = P-0 + aD(2) for stiff chains, where D is the Debye length. However, the coefficient a(pq) is about twice the value of a(OSF), Under other conditions the simulations show that P may or not be linear in D-2 at small or moderate D, depending on the magnitudes of alpha, beta, theta, and the charge density but always becomes linear at large D. Even at a moderately low charge density, corresponding to fewer than 20% of the beads being charged, and with strong crumpling induced by large P, increasing D dissolves blobs and recovers a linear dependence of P on D-2, although a lower power of D gives an adequate fit at moderate D. For the class of models considered, it is Concluded that the only universal feature is the asymptotic linearity of P in D-2, regardless of flexibility or stiffness.