THEORY OF INTERACTION BETWEEN HELICAL MOLECULES

This work builds a basis for understanding electrostatic and solvation forces between various types of helical molecules by explicitly incor porating the helical structure and symmetries into the theory. We deri ve exact expressions for interaction between molecules with cylindrica l inner cores and arbitrary distribution of discrete surface charges a nd analyze forces between single-stranded, double-stranded, and multis tranded helices. For example, we demonstrate that the traditional appr oximation by a homogeneously charged rod becomes inappropriate when ev en less than a third of the strand charge on a single-stranded helix i s neutralized by countercharges (adsorbed or intrinsic to the helix by their nature). The traditionally expected force is then complemented by helix-specific interactions. These helix-specific forces allow comm ensurate helices (with the ratio of pitches equal to a rational number ) to recognize each other at a distance and self-assemble into an aggr egate. Under certain conditions, these forces may induce a spontaneous symmetry loss, e.g., two DNA-type double helices rotate around their long axes to a separation-dependent angle when the molecules come clos er than a critical interaxial separation. In general, while a longer-r ange helix-specific attraction induces the self-assembly, a shorter-ra nge helix-specific repulsion prevents the tight molecular contact crea ting a force balance responsible for a nonzero surface separation in e quilibrium. The decay rates and the amplitudes of the attraction and o f the repulsion depend on the helical pitch and on the number and rela tive disposition of the helical strands. The theory of these forces al lows us to explain a number of puzzling features of interactions measu red between biological helices, including DNA, collagen, and four-stra nded guanosine macromolecules. (C) 1997 American Institute of Physics.