BARBARA W. LOW AND JOHN T. EDSALL 



GONH groups in the peptide chain, the reaction involved may 

 be represented as 



\ / / ■ 



CO • • • HN , CO + HN 



/ \ \ 



and Ai/, the heat absorbed, is of the order of 7 or 8 kcal. per mole 

 of such bonds broken (82). In water, on the other hand, the 

 reaction may be written, following Kauzmann (53) 



\ / H\ 



G=0.-H.N + O.-H— O , 

 / \ H/ \ 



H 



C=0 • . • • H— O H\ / 



\ + 0..--HN 

 H H/ \ 



The net value of A// for this reaction, which may be considered 

 as hydrogen bond interchange rather than hydrogen bond 

 breakage, is naturally much less than for the other. Schellman 

 estimates a value of 1.5 kcal. per mole per bond, from data on 

 the association of urea molecules in aqueous solution and the 

 temperature coefficient of the association constant. 



A regular hydrogen-bonded chain configuration has low 

 configuration entropy. The gain in entropy on unfolding will 

 contribute a negative term to the free energy of unfolding. 

 Schellman has calculated that in general the free energy of un- 

 folding per residue AF^gs ^f ^ hydrogen-bonded peptide structure 

 at room temperature, neglecting side-chain interactions, is of 

 the order oi RT or less, that is about 500 ±100 cal. 



There are C=0 groups at one end of an a-helix and NH 

 groups at the other which project outwards and cannot form 

 hydrogen bonds. Terminal (a) — NH2 and (a) — COOH bonds 

 are not held in fixed positions. A helix may be either right- 

 handed or left-handed. These three factors must be allowed 

 for in calculating the over-all free energy of unfolding (AF^^f) 

 for a chain with n residues. In equation (1) 



AFunf = /zAi^res + C (1) 



416 



