It is customary to define nanoparticles or nanostructures as entities in the range of sizes from 1 to 100 tun, so many biological materials are classified as nanoparticles. Bacteria, which range in size between 1 and 10 pm, are in the mesoscopic range, while viruses with dimensions from 10 to 200 nm are at the upper part of the nanoparticle range. Proteins, which ordinarily come in sizes between 4 and 50 nm, are in the low nanometer range. The building blocks of proteins are 20 amino acids, each about 0.6 nm in size, which is slightiy below the official lower limit of a nanoparticle. More than 100 amino acids occur naturally, but only 20 are involved in protein synthesis. To construct a protein, combinations of these latter amino acids are tied together one after the other by strong peptide chemical bonds and form long chains called polypeptides containing hundreds, and in some cases thousands, of amino acids; hence they correspond to nanowires. The polypeptide nanowires undergo twistings and turnings to compact themselves into a relatively small volume corresponding to a polypeptide nanoparticle with a diameter that is typically in the range of 4-50 nm. Thus a protein is a nanoparticle consisting of a compacted polypeptide nanowire. The genetic material desoxyribonucleic acid (DNA) also has the structure of a compacted nanowire. Its building blocks are four nucleotide molecules that bind together in a long double-helix nanowire to form chromosomes, which in humans contain about 140 x 106 nucleotides in sequence. Thus the DNA

Introduction to Nanotechnology, by Charles P. Poole Jr. and Frank J. Owens. ISBN 0-471-07935-9. Copyright © 2003 John Wiley & Sons, Inc.

molecule is a double nanowire, two nucleotide nanowires twisted around each with a repeat unit every 3.4 nm, and a diameter of 2nm. This long double-stranded nanowire also undergoes systematic twistings and turnings to become compacted into a chromosome about 6 pm long and 1.4 pm wide. The chromosome itself is not small enough to be a nanoparticle; rather, it is in the mesoscopic range of size.

To gain some additional perspective about die overall scope of nanometer-range sizes involved in the buildup of biological structures, let us consider the human tendon as a typical structure (Tirrell 1994). The function of a tendon is to attach a muscle to a bone. From the viewpoint of biology, the fundamental building block of a tendon is die assemblage of amino acids (0.6 nm) that form the gelatinlike protein called collagen (lnm), which coils into a triple helix (2nm). There follows a threefold sequence of fiberlike or fibrillar nanostructures: a microfibril (3.5 nm), a subfibril (10-20 nm), and a fibril itself (50-500nm). The final two steps in the buildup, specifically, die cluster of fibers called a fascicle (50-300 pm) and die tendon itself (10-50 cm), are far beyond the nanometer range of sizes. The lascicle is considered mesoscopic and die tendon, macroscopic in size. Since the smallest amino acid glycine, is ~0.42nm in size, and some viruses reach 200 nm, it seems appropriate to define a biological nanostructure as being in the nominal range from 0.5 to 200 nm With this in mind, the present chapter focuses on nanometer-size constituents of biological materials. In addition, we also comment on some special cases in which artificially constructed nanostructures are of importance in biology. See Gross (1999) for some additional discussions of biological nanostructures.

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