Helix random-coil transition

B) Schematic presentation of the a-helix/random-coil transition of the photoisomerizable polymer 12. 

It remains for us to discuss the dimensions of polypeptides and other helix-forming chain molecules. Most of the theoretical works on models of such chains have been primarily concerned with the equilibrium of the helix-random coil transition and have not specifically treated the chain dimensions. An exception is found in the work of Nagai (195 ), who combined his theory of the transition with a very simple model of the chain dimensions. It amounts to the assumption that each helical sequence behaves like a rigid statistical chain element without correlation in direction with the randomly coiled sections which are adjacent to it. Then, if a fraction f of the monomer units are members of helical sequences, we may at once write... 

L. Peller, On a model for the helix-random coil transition in polypeptide. I. J. Phys. Chem. 63 1194-1199 (1959). 

Since the macromolecular form must be less orderly and different from the a form, the random-coil form seems plausible. We calculated the a-helix, random coil transition enthalpy from the difference of the spreading enthalpies of the forms on supports I and II (Table V) the average is 830 cal/mole of monomeric unit. Table V also shows the transition entropies calculated as the difference between the spreading entropies of the two forms the average is 3.0 eu. The AH and the single values agree well with those found by others using different methods (42, 43, 44,45, 46, 47, 48, 49,50,51,52, 53) in the bulk phase. 

Physical States of Biomolecules Calorimetric Study of Helix-Random Coil Transitions in Solution... 

Later work by Carlstrom, Miller and Bryant has thrown some doubt on to whether the reaction product obtained by Bryant actually was compound (I) above [424], Attempts to repeat the synthesis of (I) met with uniform failure and frequently yielded instead a silver-containing polyelectrolyte of unknown structure (silver ion was used as a catalyst). It seems quite possible that the line width changes observed by Bryant [422] reflect an entirely different process than a helix-random coil transition. 

Transition Temperatures for Helix-Random Coil Transition (n THE Absence op Side-Chain Hydrogen Bonds"... 

Although AFobs, AHob., and ASob are extensive quantities, the value of Tit will be unaffected if several, say 91, helices cooperate in an all-or-none helix-random coil transition. However, the sharpness will be increased since da/dT)Ti, of Eq. (IV-55) will then be proportional to 91 (Loeb and Scher-aga, 1961). 

Helix-random coil transitions can also be induced in polypeptides con-... 

Recent work in the short wavelength region indicates that there may be a potential application of ultraviolet difference spectrophotometry at short wavelengths to studies of helix-random coil transitions (Imahori and Tanaka, 1959 Gla-zer and Smith, 1960). 

Values of the transition pH, pHjj, which is defined as a midpoint of the pH-induced helix—random coil transition or complex dissociation, may also be given in the pK ono P oly columns. However, the pK values are shown as pHjj values (in the iMote column) only when pertinent information is given in the cited reference. 

R. B. Inman and R. L. Baldwin Helix-random coil transitions in synthetic... 

Helix-Random coil] transition vs. [Coil-Globule] transition... 

Helix-Random coil transition (induced by heat. pH. solvent compositions)... 

The thermodynamic parameters corresponding to the global reaction represent a balance for the rupture and formation of the different hydrogen bonds. These parameters have been determined by Schellman (1955a) on the basis of a helix random-coil transition. The treatment neglected the influence of the side chains, but accounted for the end effect. Therefore the energy for a helical polypeptide chain of n residues was given by the equation... 

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