Structures of helical β-tapes and twisted ribbons:: The role of side-chain interactions on twist and bend behavior

Molecular Dynamics (MD) simulations have been used to obtain a molecular insight into the origins of the twist, bend, and helix pitch of the tape-like and ribbon (double tape)-like P-sheet aggregates formed by the self-assembling peptides P-11-I (CH 3CO-OORQQQQQEQQ-NH2) and P-11-II (CH3CO-QQRFOWQFEQQ-NH2). P-11-II differs from P-11-I in that glutamines at positions 4, 6, and 8 have been substituted for F, W, and F, and this gives rise to left-handed helicoidal tapes having a significantly shorter helix pitch. The presence of these hydrophobic residues also enhances the cross-tape attractive forces in P-11-II ribbons, which foreshortens the helix pitch.