Polypeptides helix-random coil transitions

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). 

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

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... 

The studies of the molecular structure of representative classes of synthetic polypeptides in solution focus on four areas (1) the structure of the random coil, (2) the helix-coil transition, (3) the a-helix to conformation transition in polylysine, and (4) the stability of the internal structure of intramolecularly cross-linked synthetic polypeptides. The random coil is not truly random in solution, and its structure depends upon its... 

The research on polypeptides and their assembly behaviors is important and beneficial for several areas. First, polypeptides can be used as a model polymer with various chain rigidities. Polypeptides can adopt conformations of a-helix, p-sheet and random coU, which can transform into each other under controlled conditions. The a-helix to random coil transition in solutions is especially interesting. Thus, polypeptides can serve as an ideal model for investigating the influence of polymer rigidity on the assembly behavior of polymers. Second, the synthetic polypeptides... 

Current investigations on dilute polymer solutions are still largely limited to the class of macromolecular solutes that assume randomly coiled conformation. It is therefore natural that there should be a growing interest in expanding the scope of polymer solution study to macromolecular solutes whose conformations cannot be described by the conventional random-coil model. The present paper aims at describing one of the recent studies made under such impetus. It deals with a nonrandom-coil conformation usually referred to as interrupted helix or partial helix. This conformation is a hybrid of random-coil and helix precisely, a linear alternation of randomly coiled and helical sequences of repeat units. It has become available for experimental studies through the discovery of helix-coil transition phenomena in synthetic polypeptides. 

Chapter E is devoted to the mean-square dipole moment and mean rotational relaxation time derived from dielectric dispersion measurements. Typical data, both in helieogenic solvents and in the helix-coil transition region, are presented and interpreted in terms of existing theories. At thermodynamic equilibrium, helical and randomly coiled sequences in a polypeptide chain are fluctuating from moment to moment about certain averages. These fluctuations involve local interconversions of helix and random-coil residues. Recently, it has been shown that certain mean relaxation times of such local processes can be estimated by dielectric dispersion experiment. Chapter E also discusses the underlying theory of this possibility. 

The question arises as to whether or how closely Eq. (D-8) is obeyed by non-randomly coiled macromolecules, especially, by polypeptides in the helix-coil transition region. An answer has been given by a recent work by Norisuye (S3), who measured [fj] and for two high-molecular-weight samples of PBLG... 

It is seen that

characteristic behavior suggests that the molecular shape of PBLG in the mixed solvent studied does not differ very much from swollen spheres of randomly coiled polymers at stages where the helical fraction is less than about 0.6. In this connection, it is worth recalling from Chapter C, Section 2.b that the dimensional features of a polypeptide remain close to Gaussian at such stages of helix-coil transition, provided the chain is sufficiently long. 

Wada et al. (120) studied dielectrically the normal transition of two PBLA samples in m-cresol and observed that the higher-molecular-weight sample exhibited a secondary dispersion at frequencies above the region which could be associated with orientation polarization of the polypeptide. Their contribution will be discussed in the next section, in which such a dispersion is attributed to a relaxational alternation of helix and random-coil units. 

N 002 "Theory of the Phase Transition between Helix and Random Coil in Polypeptide Chains"... 

Polypeptides and poly(a-amino acid)s have a quite unique position amongst synthetic polymers. The reason for this is that most common synthetic polymers have very little long range order in solution and their properties are the products of statistical random coil conformations. Polypeptides, in contrast, can adopt well defined, ordered structures typical of those existing in proteins, such as a-helix and P-struc-tures. Moreover, the ordered structures can undergo conformational changes to the random coil state as cooperative transitions, analogous to the denaturation of proteins. 

The structures of spiropyran-modified poly(L-glutamate)s are strongly affected by light or dark conditions, as demonstrated by the CD spectra in Figure 10. Before irradiation, the colored solutions show the CD spectrum of a random coil conformation. After exposure to sunlight, the colorless solutions display the typical CD pattern of the a-helix, thus indicating that the isomerization of the side chains causes a transition from coil to helix in the polypeptide chains. The photoinduced conforma-... 

From this point of view, polypeptides containing photochromic units in the side chains are quite special polymers. They can exist in ordered or disordered conformations, and photoisomerization of their photochromic side chains can produce order = disorder conformational changes. These photostimulated structural variations, such as random coil a-helix, take place as highly cooperative transitions therefore photochromic polypeptides actually work as amplifiers and transducers of the primary photochemical events occurring in the photosensitive side chains. 

---