1.2 Nanoscale Actors and Their Properties

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which carries a specific amino acid that is bonded to form a protein with a specific se-

quence with the help of ribosomes. Since there are 20 different amino acids with differ-

ent side chains (Figure 1.10), and the length of a protein can be anywhere between tens

to thousands of amino acids, the possible range of three-dimensional structures that can

be built is large.

The linear chain of amino acids is called the primary structure (Figure 1.11 a). This

chain forms either an α-helix, a β-sheet, or a random β-turn (Figure 1.11 b, secondary

structure). The α-helix is mechanically elastic and acts like a spring that can stretch

as well as bend. The β-sheet can easily stack and form a strong crystal, creating a

stiff section in the protein. The β-turn is a flexible linker between the different sec-

ondary structures. Additional strength can be gained by disulfide linkages that act as

crosslinks. All these secondary structures combined make up the tertiary structure,

or three-dimensional structure, of the protein (Figure 1.11 a). If several proteins ag-

Figure 1.11: (a) Protein structure; (b) Different secondary structures.