Toward Intelligent Materials - Molecular Self-Assembly: Advances and Applications - page 16

H-H dyad was replaced with two -(cytosine-1-yl)-alanine residues to

give ccQALVFFA [89]. At acidic pH near the cytosine pKa, the bases are
hemi-protonated, which stabilizes cytosine(+)-cytosine base pairing
(i-motif ) [90,91]. Under these conditions the peptide assembles as
nanotubes, while at higher pHs, fibers dominate [89]. The proposed
base pairing (Fig. 1.8C) places the cytosine ring in a very different
orientation from either the predicted packing of the phenylalanine
rings or the histidine chelation with Zn(II). While the MD models
are not yet sufficiently constrained to propose a single side-chain
orientation, extended laminations appear to be readily stabilized
through engineering side-chain complementarity. Diverse strategies
are now emerging for modulating global morphology through
simple structural interactions, and such control further extends the
conditionally responsive crystallinity of peptide assembly. However,
these crystalline phases represent distinct and well-ordered surfaces,
specific two-dimensional information that could be transmitted to
other molecular assemblies. A better understanding of the order and
dynamic nature of these surfaces and how they might contribute to
multi-component assembly will be important to any self-organizing
properties accessible with these materials.

b

1.5 Expanding Structural Diversity

Co-Factor Binding: The structure models that we have been discussing

β

for the cross- assemblies display distinct solvent-accessible faces

along their lengths. Ignoring for the moment the growing ends of the
assemblies, the anti-parallel

β

-sheet fibers (Fig. 1.9A) have two sets,
the pleat surface, containing alternate amino acid side chains of the
peptide backbone projecting from each pleat of the backbone (Fig.
1.9B), and the termini surfaces, composed of the accumulated ends
of the peptide strands (Fig. 1.9C). As shown more explicitly for the
A

β

(16-22) fibers in Fig. 1.10A-C, the two terminal faces are identical
with K-16 and E-22 residues defining the surfaces, but the two pleat
surfaces, assuming parallel sheet arrangements, project the side
chains of either KVFE or LFA on the surfaces, resulting in at least three
distinct surfaces [23]. In contrast, the parallel

β

-sheet assemblies, such

β β

as Cu(II)-A (16-22)E22Q [71] also present

distinct faces for the N- and C-termini, creating at least four different
solvent interfaces along the fibril long axis. To the extent that ligands
associate with these faces, the diversity becomes even greater and it

(13-21)H14A [81] and A