Random Coil State Research Articles

Solution Properties of Synthetic Polypeptides. III. Viscosity Behavior of Poly-γ-Benzyl-L-Glutamate in the Helix—Coil Transition Region

Limiting viscosity numbers [η] and specific rotations [α]578 of five samples of poly-γ-benzyl-L-glutamate (PBLG) (ranging in weight-average molecular weight M̄w from 1.9×104 to 4.0×105) in mixtures of dichloroacetic acid (DCA) and ethylene dichloride (EDC) of various compositions were measured as functions of temperature in the range where the PBLG molecule underwent a thermal transition from a disordered, random-coil state to an ordered helical state. For one sample the measurements of optical rotatory dispersion were made at various solvent compositions and temperatures, and the data were analyzed in terms of the Moffitt plot. When the M̄w of a given sample was above a certain critical value, [η] first decreased (or was kept constant), passed through a shallow minimum, and then increased sharply to a limiting value as the temperature was raised. Below the critical value, the change in [η] with temperature was a monotonic decrease. Irrespective of the molecular weight of the sample, [α]578 increased sharply in a narrow range near 30°C and then gradually leveled off as the temperature was raised. This narrow region coincided with the region where [η] underwent a marked increase. It was found that the helical content fH of a given sample can be evaluated from the reduced mean residue rotation measured at 578 mμ. When the values of [η] obtained under different solvent compositions and temperatures are plotted against the fH values so calculated, the plotted points formed a single composite curve over the entire range of fH. This curve exhibits a quite similar character to the theoretical curves for 〈S2〉 (mean-square radius of gyration) vs fH calculated by Nagai on the statistical-mechanical treatment of helix-forming polypeptides.

The Dipole Moments of the Oligomer of Ethylenediamine

Abstract The dipole moments of ethylenediamine (N=1), diethylenetriamine (N=2), triethylenetetramine (N=3), and tetraethylenepentamine (N=4) were measured in a benzene solution at 25°C and 45°C. The observed dipole moments of N=1, N=2, N=3, and N=4 are 1.89 D, 2.22 D, 2.37 D, and 2.56 D respectively at 25°C, and 1.84 D, 2.22 D, 2.36 D, and 2.58 D respectively at 45°C. The dipole moments were theoretically calculated on the assumptions that; (i) the dipole moments of these chain molecules are given by the vector sum of the dipole moments of the polar groups of amine, and (ii) all internal rotations about single bonds are free. The calculated values of N=1, N=2, N=3, and N=4 are 1.84 D, 2.15 D, 2.43 D, and 2.68 D respectively. The agreement between the observed values and the calculated values is satisfactory. It may, therefore, be expected that the molecular chains will be in the random coil state. The intramolecular hydrogenbonding is very weak, as there is no temperature dependence of the observed dipole moments.

Transitions of DNA homopolymers

The DNA homopolymers‡ dC, d B C ¯ and dI have been examined by various physico-chemical methods. The polymers were prepared from the homopolymer pairs dG : dC and dI : d B C ¯ by density-gradient separation at high pH. Both dC and d B C ¯ form ordered structures with themselves and differ from all other DNA structures so far examined in requiring protons for the formation of the ordered state, dI was also found to interact with itself to form an ordered complex, which appears to be the least stable of all the DNA structures so far studied. The spectra, T m and hypochromicity of the various states of dC, d B C ¯ and dI have been investigated at various salt concentrations and pH values. Two complex transitions were studied by melting experiments. The first was the transition dI : dC → dI : dI + dC : dC, which proceeds at a fast rate at the T m of dI : dC. The other transition, dI : dC + d B C ¯ → dI : d B C ¯ + dC (where d B C ¯ and dC are in the random-coil states) proceeds at a measurable rate well below the T m of dI : dC.