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Collagen - Imaging of flexible rod-like molecules with Atomic Force Microscopy

Collagen for medical use is commonly produced by digestion of bovine skin. This soluble collagen consists primarily of individual "monomers". A monomer is a triple helix of three protein molecules (polypeptide chains) with non-helical ends.  In medical applications, the monomers assemble to form collagen fibers (fibrils).  One use is to plump up the skin (remove wrinkles).

Whereas monomers are nominally 280 nm long, the soluble collagen may contain slightly shorter monomers or much shorter fragments. It may also contain longer chain aggregates such as dimers and higher oligomers. As with other biologic therapeutic materials, regulatory agencies such as the FDA (US Food and Drug Administration) or EMA (Europe) require that the consistency of this mix be documented and controlled. 

One way to characterize biopolymer materials such as collagen is to use Atomic Force Microscopy (AFM). Individual particles (molecules) can be seen and measured.  One can readily classify each molecule as a fragment, monomer, dimer, or higher oligomer. The principle indicators are length and geometry.  Fragments will be shorter than the monomers. Dimers and higher oligomers may be linear, branched or looped. By compiling measurements of many particles, one can get an indication of size distribution that is complementary to bulk analysis techniques such as chromatography. AFM analyses of different batches of soluble collagen provides an independent check on other quality assurance techniques, helping to ensure compliance with regulations and produce better quality materials.

AFM sample preparation is relatively simple, involving dilution of the soluble collagen, depositing a droplet of the dilute solution onto a cleaved mica surface, rinsing and drying.  Tapping mode in air is used to produce images of undisturbed molecules.

AFM image of collagen monomers deposited on Mica

Individual collagen molecules deposited on freshly cleaved mica from an aqueous suspension form wormlike patterns. 2 micron scan.

The collagen molecules, or "monomers", shown here are composed of three polypeptide chains which are arranged in a triple helix.  In the body, collagen monomers aggregate in a staggered manner with a longitudinal offset of ca. 67 nm to form fibrils and fibers.  These structures are the predominant matrix in tissues such as skin, tendon and bones.  To read more about Collagen, a good place to start is:
Karl A. Piez, "Collagen" in "Encyclopedia of Polymer Science and Engineering", John Wiley, New York, 1985




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