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 β-sheet aggregates formed by the self-assembling peptides P 11 -I (CH 3 CO-QQRQQQQQEQQ-NH 2 ) and P 11 -II (CH 3 CO-QQRFQWQFEQQ-NH 2 ). 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.