110 L. G. AUGENSTINE 



3) A reasonable value for the number associated with specifying the interhelical 

 bonds would seem to be 7?/2 bits. This arises by assuming RjA interhelical 

 bonds, i.e. one bond per turn of the helix, and the previous discussion of intra- 

 molecular bonding indicates that the identity of each interhelical bond requires 

 about 2 bits of information. Another reasonable value for this factor is i?/4; this 

 would occur for 1 one-bit interhelical bond or 1 two-bit bond every other turn, 

 which attempts to take into account that disulfide and "strong" H-bonds 

 are probably the most important interhelical bonds. Actually this factor could 

 be zero since it may not be possible to specify interhelical bonds independent 

 of the sequence. 4) The information necessary to specify the orientation of 

 each helix with respect to some reference point in the protein is the most 

 difficult factor to estimate. It may be almost zero, since the interhelical bonds 

 may unequivocally determine the orientation of the helix. On the other hand, 

 it should not be larger than (log2 R + 30) bits, where logg R bits is sufficient 

 to determine a specific residue and 30 bits to specify its orientation. The 30 

 bits would be assigned to the six parameters associated with the two vectors 

 necessary to specify orientation. An average 'grain' of 1 :32 is undoubtedly 

 too coarse for specifying the orientation of a. single isolated helix, but is probably 

 adequate for specifying a helix which is oriented in relation to others in the same 

 molecule. 



The 7?/2 and 30 and the zero terms have been combined to give 'high' 

 and 'low' values for the estimation of the minimum of /,.. These are calculated 

 as /^residue (in bits) by 



/./residue - "!' LO W (1) 



and 



30+3 logo R 

 = 0.50H n ff^GH (2) 



The results as a function of R are shown in Fig. 3. Pauling, Corey and 

 Branson (8) cite examples of heUcal polypeptides for which Ris 11, 18 and 36. 

 The corresponding region of Fig. 3 has been shaded. From these considerations 

 it would appear that the minimum value of /<. should be about 1 to 4 bits/residue 

 depending upon R. 



Although many proteins appear to be helical in nature, there are others, 

 such as ribonuclease (RNase), which from the available evidence would seem 

 not to be. In RNase the structural specificity appears to be determined pre- 

 dominantly by the S — S bonds with the other intramolecular bonds adding 

 stabihty to the structure. A further discussion of the relative importance of 

 the specific and non-specific intramolecular bonds in maintaining structure 

 will be presented later. 



It is obvious that an upper limit cannot be assigned to /. as readily as to 

 7^.. However, since the structures proposed by Pauling, Corey and Branson 

 probably represent polypeptide configurations for which /. is near minimum, 

 it would appear that one bit/residue is a reasonable lower limit for 7^. From 

 the estimates of 7, and 7^ presented here, it appears that for the proteins of 

 general interest 7^ should have a value in excess of 4.5 bits/residue although 



