Biomedical Engineering Reference
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which can yield entirely different molecules. These well-defined sequences al-
low the peptides to undergo ordered self-assembly, in a process resembling
some situations found in well-studied polymer assemblies. A broad range
of peptides and proteins have been shown to produce very stable nanofiber
structures, also called amyloid fibers [24-34].
2.1
Peptides as Construction Motifs
Similar to the construction of a house, many other parts of the house, such
as doors and windows can be prefabricated and program-assembled accord-
ing to architectural plans. If we shrink the construction units many orders of
magnitude to the nanoscale, we can apply similar principles for construct-
ing molecular materials and devices, through molecular self-assembly and
programmed molecular assembly.
2.2
Modulus I: “Peptide Lego”
Type I peptides, also called “molecular Lego” are the first member of the
“peptide Lego”, which was serendipitously discovered from a segment in
a left-handed Z-DNA binding protein in yeast and named Zuotin [14]. Lego
bricks have pegs and holes, which can be assembled into particular struc-
tures. In a similar way, these peptides can be assembled at the molecular
level. The nanometer scale “peptide Lego” resembles Lego bricks that have
both pegs and holes in a precisely determined organization and can be pro-
grammed to assemble into well-formed structures. This class of “peptide
Lego” can spontaneously assemble into well-formed nanostructures at the
molecular level [15].
The molecular structure and proposed complementary ionic pairings of
the modulus I peptides between positively charged lysines and negatively
charged glutamates in an overlapping arrangement are modeled in Fig. 1.
This structure represents an example of this class of self-assembling
-sheet
peptides that spontaneously undergo association under physiological con-
ditions. If the charged residues are substituted, i.e. the positively charged
lysines (Lys) are replaced by the positively charged arginines (Arg) and the
negatively charged glutamates (Glu) were replaced by negatively charged as-
partates (Asp), the peptide would still be able to undergo self-assembly into
macroscopic materials. However, if the positively charged residues, Lys and
Arg, were replaced by negatively charged residues, Asp and Glu, the peptide
would not be able to undergo self-assembly and form macroscopic materials
although
β
-sheet structures have been observed in the presence of salt. If the
alanines (Ala) were changed to more hydrophobic residues, such as Leu, Ile,
Phe or Tyr, the molecules had a greater tendency to self-assemble and formed
β
 
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