Akiyoshi Wada

Helix-coil transformation and titration curve of poly-L-glutamic acid

The titration curve of poly-L-glutamic acid was studied in connection with the helix-coil transformation. In aqueous solution the transformation has its origin in the ionization of the polar group COOH in the side chain. Conversely the ionization and the titration curve of this molecule are affected by the change of the electrostatic interaction produced by its transformation. It is shown in this report that the experimental result, the titration curve and its modified plot, can be divided into three sections, that is, the ionization of the perfect helix, the region of the helix-coil transformation, and the ionization of the perfect coil.

Dielectric Properties of Polypeptide Solutions. IV. Several Problems on the Electric Dipole Moment of the α‐Helix

Relating to the dielectric properties of the α‐helical molecule described in a previous report, the following problems have been studied from the standpoint of the confirmed structure of this molecule.1. The electric dipole moment of the α‐helix has been estimated considering the dipole moment in the side chain besides that in the backbone chain.2. The dielectric constant of the solution of α‐helices has been obtained in terms of the dipole moment and polarizability of the solute molecules. The reaction field, which will be induced in such a cylindrical molecule immersed in the dielectric media, has also been examined.3. It was found that the deviation of the observed dielectric dispersion and absorption curve from Debyes theoretical one of single relaxation time could satisfactorily be explained when the polydispersity of the molecular weight was taken into account.

Regularity in the chain configuration of D,L-copoly-γ-benzyl glutamate†

The work described below leads to the following conclusions as to the structure of the D,L -copoly- γ -benzyl glutamate polymerized from a mixture of L - and D -N-carboxy anhydride (NCA) by initiation with strong bases. 1. Only one screw sense helix exists in the range f D = 0 to 0·25. 2. Regularity in the orientation of polar groups in the backbone chain is maintained up to f D = 0·35. 3. The side-chain of the α-helix appears to have a regular orientation, as does its backbone chain. ‡ 4. In the right-handed α-helix, the electric dipole moment of the side-chain is opposed to that in the backbone chain for L -residues, and in the same direction in D -residues. 5. The electric dipole moment of the backbone chain of the α -helix is about 6·5 Debye units per residue in formamide-saturated ethylene dichloride. 6. Some disorder appears when more than f D = 0·35 of antipode is incorporated in the α -helix. This seems to originate in the unfolded parts at the ends of the chain or in a few disordered points in the molecule. 7. Even in the copolymer polymerized from an equimolar mixture of L - and D -NCA there is good reason to believe that the major portion of the substance is composed of helices which may be a mixture of right- and left-handed helices of L -residue-rich and D -residue-rich polymers, respectively. Some of the above conclusions are derived by assuming rigidity in the side-chain. If the flexibility due to the internal rotational freedom allowed in carbon-carbon bonds in the side chain is taken into consideration, an alternative model is also possible: 1′. In an α -helix of pure L-polymer the electric dipole moment of the side-chain is forced in an opposite direction to that in the backbone by dipole-dipole interaction between the side-chain and backbone. 2′. When a D -residue is introduced, partial melting in the order of the side- chain occurs owing to steric hindrance, and the apparent residual moment will be increased by the decrease in the negative contribution of the side-chain. 3′. The contribution of the side-chain moment would not simply be offset in this case, and the moment of the backbone cannot be deduced from the above- mentioned extrapolation. But, since the melting in the order of the side-chain may be completed as the composition approaches that of the mesopolymer, the residual moment of the pure backbone of the α -helix is still believed to be 6 to 7 Debye units. Although either of these models is plausible at the present stage, it is already evident that the side-chain in the α -helix, whether flexible or rigid, takes an ordered form and it can be assumed that the whole side-chain plays a role in determining the stability of the α -helix and its screw sense. The latter is supported by the evidence that a left-handed helix is stable in the case of poly- β -benzyl aspartate as reported by Bradbury, Downie, Elliott & Hanby (1960) . Finally, it must be mentioned that the above conclusions indicate that even in a complicated group of side-chains in a protein, there will be a fixed pattern rather than random orientation and that this is important in giving specificity to the character of the protein.

Dynamical aspects of helix‐coil transitions in polypeptides. I

A dynamical theory for the helix‐coil transitions in polypeptides with finite length, which has the following specific features, is presented. (1) It includes size effects which result in the broadening of a critical slowing down (enlargement of the relaxation time of helix‐coil transition near its critical temperature). [M. Suzuki, Prog. Theor. Phys. 43, 882 (1970)] (2) It is formulated to give a clear physical picture for the relaxation modes from the final equations. (3) It is directly applied to experiments which measure physical quantities concerning a helical length and its orientation, such as dielectric dispersion, the Kerr effect, light scattering, sonic dispersion or T‐jump method. The relaxation spectra for dielectric dispersion are given.

Dust-Effect Reduction Facility of a Digital Correlator for Dynamic Light Scattering

A minicomputer-assisted digital correlator for dynamic light scattering (DLS) measurement has been made. The correlator has a unique circuit construction to minimize the effect of dust particles in sample solutions. Detailes of the operation of electronic circuits assuring the dust-effect reduction facility are described. An example of the measurement of DLS correlation functions for solutions of polystyrene latex spheres is presented demonstrating the correlators performance.

Dielectric Relaxation Measurements of Hydrated Powder at Subzero Temperatures by Time-Domain Reflectometry

Time-domain reflectometry (TDR) was applied to dielectric relaxation measurements of a powder sample at subzero temperatures. A sample cell for the powder was designed so as to tolerate the packing pressure. Temperature reduction of the entire system, constructed from a copper block embedding the sample cell and a heat insulating box, was performed using liquid nitrogen. The performance of the system was demonstrated by the measurement of partially hydrated silica gel powder from -105degC to room temperature, and the temperature-dependent dynamic hydration structure was elucidated.