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Although the visual information reaching the brain is very great, the 

 amount actually stored or analyzed is much smaller. This is similar to 

 the read-out limitation in a high speed computer. To completely 

 translate into the human memory, all the information received in a 

 0.1 sec flash takes more than 0.1 sec. By blinking the eyes open and 

 shut, it is possible to notice many details such as height, width, length, 

 brightness, hue, shade, tint, orientation, and shape. It takes many 

 seconds to store all these in the brain memory. 



4. Information Theory and Protein Structure 



Information theory is a mathematical technique used to give a quantita- 

 tive value to information. Its basic definitions, embodied in Equations 

 1 and 4, were used in the last section as a language to describe sensory 

 information. Information theory is not restricted to this field alone but 

 can also be used as a language to describe other types of information. 

 Only a few of these can be included in this text. In this section, informa- 

 tion theory is applied to protein structure. 



To assemble a protein, it is necessary to choose the proper amino 

 acids and arrange them in a given order with a suitable spatial con- 

 figuration. There are many ways in which this ordering can be done 

 with the same amino acids. It is too complex to illustrate this for a 

 complicated protein with a hundred amino acid residues, but some 

 intuitive feeling can be gained by considering a polypeptide with five 

 residues. Suppose these are all different; for example, one glycine (g), 

 one phenylalanine (p), one tryptophane (t), one valine (v), one methio- 

 nine (m) . These can be arranged in 5 ! fashions because the nature of 

 the peptide bond is asymmetrical; that is, gp # pg. These 5! forms 

 include 



There are four other equal sets making a total of 1 20, that is, 5 ! 



If two of the amino acids had been the same, the number of possi- 

 bilities would have been reduced by a factor of two. And if three were 

 the same the number of distinct possibilities would have been reduced 

 by a factor of 3 ! or 6. If only two different amino acids are present, 



