STRUCTURE 



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molecule dissolved in salty water may be quite different — for example, one 

 can readily imagine the legs of this molecular octopus unfolding in the blood 

 stream. 



Structural knowledge of many other big molecules is rapidly becoming 

 available. This is a subject of intense interest. Straight chains and helices, 

 some coiled into balls, some folded back and forth to form rods, others with 

 randomly coiled shapes, are known or imagined. These forms are illustrated 

 in Figure 6-4. 



random coil 



helix 



globe 



rod 



Figure 6-4. Some Molecular Shapes in Solution (schematic). Transitions one to 

 another can be effected by change in pH, ionic composition, or temperature. 



Receiving much attention in the hands of F. O. Schmitt and the MIT 

 School has been collagen, the structural component of connective tissue, 

 tendon, skin, cartilage, etc. (Figure 6-5). Formed of three interwound 

 molecular helices of protein, with molecular dimensions approximately 

 3000 A long x 30 A in diameter, it cross-links end to end to form fibers, and 

 then side to side to form either sheets (two dimensions) or blocks (three di- 

 mensions) of connective tissue with very varied physical properties: for ex- 

 ample, tensile strength up to 100,000 lbs/in 2 , equivalent to that of a steel 

 wire of the same dimensions! 



Now thought to be the basic information-carrier of the gene, and an ex- 

 tremely important component of the nucleus of the cell, is desoxyribose- 

 nucleic acid (DNA). At about 70 per cent relative humidity, it is an ex- 

 tended, double-stranded helix, of molecular weight in the millions. Further 

 discussion of the structure of DNA, and its sister nucleic acid, ribosenucleic 

 acid (RNA), appears later in this chapter. 



Now, the backbone of the helices of DNA and RNA is ribose, a sugar, 

 polymerized through phosphate groups. Polymerized sugars are the second 

 major structural component of living tissue — cellulose and chitin are ex- 

 amples. Hyaluronic acid and glycogen are polysaccharides which take an 

 integral part in the biochemistry of life. Thus glycogen is the form in which 

 sugar is stored as an energy reserve in the liver. Polysaccharides, like 

 proteins, take many forms in tissue. One which seems to be unique is the 

 pleated sheet of cellulose. 



