12.4. BIOLOGICAL NANOSTRUCTURES
325
(lo%), and it also has a high percentage of glycine (34%) and alanine (10%). The
proline and hydroxyproline residues have rigid five-membered rings containing
nitrogen, so collagen cannot form an a helix, but these residues do interject bends
into the polypeptide chains, facilitating the establishment of conformations that
strengthen the triplet helical structure. The smaller glycine residues make it easy to
establish close packing of the strands held together by hydrogen bonds. Artificial
genes in the size range from 40 to 70 kDa have been prepared that encode tripeptide
sequences such as -GlyProPre that resemble those in natural collagen.
The protein elastin possesses many properties similar to those of collagen. It
provides elasticity to skin, lungs, tendons, and arteries in mammals. Elastin has a
precursor protein called tropoelastin, which has a molecular weight of 72kDa. Its
structure involves sequences of oligopeptides, short polypeptides that are only four
to nine amino acid residues in length. An analog of such a chain has been
synthesized that contains the pentapeptide sequence -ValProGly ValGly-, which
exhibits a reverse turn *around a ProGly dipeptide. Additional amino acid residues
can occupy the space between turns, and make use of the high flexibility of glycine
as a spacer. Many turns joined together adapt a springlike structure that can be
stretched to over 3 times its resting length, and then return to rest without
experiencing any residual deformation.
The silk caterpillar Bombyx mori produces silk formed from hydrogen-bonded
p sheets of the protein fibroin. The fibers of the sheets are closely packed and highly
oriented, which gives them a large tensile strength. The amino acid residues are 46%
glycine, 26% alanine, and 12% serine (Ser), and the principal repeat sequence is the
hexapeptide -GlyAlaGlyAlaGlySer-. Artificial fibers with the properties of silk have
been prepared according to this hexapeptide sequence, and proteins of this type with
molecular weights between 40 and 100 kDa have been expressed @e., formed) in the
bacterium Escherichia coli. The polypeptide sequences -ArgGlyAspSer- from the
protein fibronectin and -ValProGlyValGly- fiom elastin and have been incorporated
into artificial silk polymers. The former serves as a substrate for cell culture, and the
latter renders the polymer more soluble and easier to process. The larvae of midge
spiders (Chironomus tentans) produce a silklike fiber with a molecular weight of
-1 MDa.
Deming et al. (1 999) have undertaken the de novo design and synthesis of welldefined polypeptides to assess the feasibility of creating novel useful proteins, and to
determine the extent to which chain folding and large-scale or supramolecular
organization can be controlled at the molecular level. They have incorporated
unnatural amino acids into artificial proteins by using intact cellular protein
synthesis techniques. “Unnatural” amino acids are those not included in the
set of 20 that have DNA codons. These artificial proteins formed folded-chain,
layered, or lamellar crystals of controlled surface configuration and thickness.
One such crystal was prepared via sequence-controlled crystallization by
employing a gene that incorporated the genetic codewords for 36 repeats of the
octapeptide -(GlyAla)3GlyClu- that could be expressed in the bacterium E. coli.
325
(lo%), and it also has a high percentage of glycine (34%) and alanine (10%). The
proline and hydroxyproline residues have rigid five-membered rings containing
nitrogen, so collagen cannot form an a helix, but these residues do interject bends
into the polypeptide chains, facilitating the establishment of conformations that
strengthen the triplet helical structure. The smaller glycine residues make it easy to
establish close packing of the strands held together by hydrogen bonds. Artificial
genes in the size range from 40 to 70 kDa have been prepared that encode tripeptide
sequences such as -GlyProPre that resemble those in natural collagen.
The protein elastin possesses many properties similar to those of collagen. It
provides elasticity to skin, lungs, tendons, and arteries in mammals. Elastin has a
precursor protein called tropoelastin, which has a molecular weight of 72kDa. Its
structure involves sequences of oligopeptides, short polypeptides that are only four
to nine amino acid residues in length. An analog of such a chain has been
synthesized that contains the pentapeptide sequence -ValProGly ValGly-, which
exhibits a reverse turn *around a ProGly dipeptide. Additional amino acid residues
can occupy the space between turns, and make use of the high flexibility of glycine
as a spacer. Many turns joined together adapt a springlike structure that can be
stretched to over 3 times its resting length, and then return to rest without
experiencing any residual deformation.
The silk caterpillar Bombyx mori produces silk formed from hydrogen-bonded
p sheets of the protein fibroin. The fibers of the sheets are closely packed and highly
oriented, which gives them a large tensile strength. The amino acid residues are 46%
glycine, 26% alanine, and 12% serine (Ser), and the principal repeat sequence is the
hexapeptide -GlyAlaGlyAlaGlySer-. Artificial fibers with the properties of silk have
been prepared according to this hexapeptide sequence, and proteins of this type with
molecular weights between 40 and 100 kDa have been expressed @e., formed) in the
bacterium Escherichia coli. The polypeptide sequences -ArgGlyAspSer- from the
protein fibronectin and -ValProGlyValGly- fiom elastin and have been incorporated
into artificial silk polymers. The former serves as a substrate for cell culture, and the
latter renders the polymer more soluble and easier to process. The larvae of midge
spiders (Chironomus tentans) produce a silklike fiber with a molecular weight of
-1 MDa.
Deming et al. (1 999) have undertaken the de novo design and synthesis of welldefined polypeptides to assess the feasibility of creating novel useful proteins, and to
determine the extent to which chain folding and large-scale or supramolecular
organization can be controlled at the molecular level. They have incorporated
unnatural amino acids into artificial proteins by using intact cellular protein
synthesis techniques. “Unnatural” amino acids are those not included in the
set of 20 that have DNA codons. These artificial proteins formed folded-chain,
layered, or lamellar crystals of controlled surface configuration and thickness.
One such crystal was prepared via sequence-controlled crystallization by
employing a gene that incorporated the genetic codewords for 36 repeats of the
octapeptide -(GlyAla)3GlyClu- that could be expressed in the bacterium E. coli.