Structure and dynamics of self-assembling β-sheet peptide tapes by dynamic light scattering

Oligomeric peptides can be designed which undergo one-dimensional self-assembly in solution to form beta-sheet tapes a single molecule in thickness and micrometers in length.(1) In this paper, we present the first systematic investigation of the size, shape, dynamics, and interactions of beta-sheet tapes formed by a self-assembling 24-residue peptide, K24, in 2-chloroethanol, over a wide range of peptide concentrations c (c: 10(-7)-1 mM), using photon correlation spectroscopy. The tapes behave like semiflexible chains with persistence lengths of several hundred nanometers and much longer contour lengths, even at c approximate to 0.1 nM. The polarized q-dependent light-scattering intensity I fits a model of a prolate object with major and minor axes alpha approximate to 630 nm and beta approximate to 40 nm. This is an unexpected result in view of the previous theoretical predictions that tapelike polymers could form oblate coinlike structures in solution.(2) This experimentally observed behavior is attributed to the pronounced twist and bend of the P-tapes, which do not allow them to form the coinlike structures, but instead they favor the formation of elongated polymers. At c* approximate to 10(-2) mM, the tapes are seen to start overlapping and forming networks with unusually large mesh sizes (e.g., ca. 400 nm at 15 muM), much larger than those of conventional polymers. With increasing peptide concentration the mesh size decreases and the network becomes a physical gel at c approximate to 0.4 mM. These semidilute solutions are characterized by one main relaxation mode associated with the cooperative diffusion of the entangled tape network, and a weaker slower mode, associated with gel cluster formation. The concentration dependence of xi (xi(c) similar to c(-0.34)) for is much weaker compared to the expected scaling for Gaussian or swollen chains (xi(c) similar to c(-1) for Gaussian chains, or xi(c) similar to c(-3/4) for swollen ones), but is not inconsistent with the expected scaling for rigid rods. On the basis of the concentration dependencies of the light-scattering intensity I and of the cooperative diffusion coefficient D, the cooperative friction coefficient f(c) is found to display a stronger concentration dependence (f(c) similar to c(1.34)) than in the case of semidilute flexible and semiflexible polymer solutions (f(c) similar to c(0.5)).(3) Thus, we may conclude that the network of entangled tapes approximates in its behavior that of semirigid polymers.