Voltammetric evaluation on poly α-aspartic acid-zinc ion complex in the helix-coil transition pH region.

Helix-coil transitions of poly α-aspartic acid (PASP) were studied by dc polarography in the presence of Zn(2+) as a marker attached to the polymer. The diffusion current (i(d)) of Zn(2+) declined markedly in the pH range of 3.5-7.4 due to a formation of metal ion-PASP macromolecular complexes. The complex formation also reflects on an increase of the magnitude at ca. 222 nm of CD spectrum, suggesting that PASP forms the helix structure by coordination of Zn(2+) in the corresponding pH region. Helix content, determined by the decrease in i(d) of Zn(2+), corresponds favorably to that by CD measurements. In the lower acidic pH region, the coordination mode of Zn(2+) to PASP is different from that at neutral pH region. The decrease in i(d) of Zn(2+) is independent of the further formation of helix structure. Zn(2+) coordinates with sparsely dissociated carboxylate groups of the helical part of PASP, which bring about an aggregation of polypeptide strands. The diffusion current of the ion attached to the polymer, therefore, is a parameter sensitive to conformational changes of PASP from acidic through neutral pH region.

Pulsed NMR study of the curing process of epoxy resin.

To analyze a curing process of epoxy resin in terms of molecular motion, we adapted a pulsed NMR method. Three kinds of (1)H spin-spin relaxation times (T(2L) (long), T(2S) (short) and T(2M) (intermediate)) were estimated from observed solid echo train signals as the curing process proceeded. A short T(2S) value below 20 micros suggests the existence of a motion-restricted chain, that is, cured elements of resin, and its fraction, P(S), sigmoidally increased with the curing time. On the other hand, the fraction of T(2L), P(L), decreased with the reaction time reciprocally against P(S), suggesting the disappearance of highly mobile molecules raised from pre-cured resin. The spin-lattice relaxation time, T(1), was also measured to check another aspect of molecular motion in the process. T(1) of the mixed epoxy resin and curing agent gradually increased just after mixing both of them. This corresponds to an increment of a less-mobile fraction, of which the correction time is more than 10(-6) s, and also means that the occurrence of a network structure whose mobility is strongly restricted by chemically bonded bridges between the epoxy resin and curing agent. The time courses of these parameters coincided with those of IR peaks pertinent to the curing reaction. Therefore, pulsed NMR is a useful tool to monitor the hardening process of epoxy resin in real time non-distractively in terms of the molecular motion of protons.

Pulsed NMR study of network formation in the course of bulk polymerization of methyl acrylate.

The proton spin-spin relaxation time (T2) during the bulk polymerization of methyl acrylate was measured as a function of the reaction time at various temperatures. Three kinds of T2 (T2L (long), T2S (short) and T2M (intermediate)) were obtained as the polymerization proceeded. The fraction of T2S (FS) increased sigmoidally at a certain reaction time, while that of T2L (FL) decreased reciprocally. The former corresponded to the amount of a polymer whose molecular weight was sufficiently high enough to cause a tight entanglement that produced a transient network structure; the latter reflected a decrease in the mixture of the monomer and the low molecular weight of the polymer. T2M is considered to arise from a relatively mobile region of the entanglement. The relationship between the fractions of T2S + T2M and the polymer yield was found to be linear, which led us to monitor the polymer yield in real time during the polymerization in a non-distractive manner. 13C DD (dipolar decoupling)/MAS (magic angle spinning) NMR spectra were also measured to monitor the polymerization process in terms of the molecular motions between the main chain and the side chain in the formation of a network structure. The 13C DD/MAS NMR spectra show that the side chain motion became restricted as well as the main chain when the "Trommosdorff effect" (gel effect) was observed, and a part of the monomers were trapped in the network structure.

Polarographic study on poly alpha-L-glutamic acid-Cd2+,Co2+ complexes in the helix-coil pH region.

Interactions between poly alpha- L-glutamic acid (PGA) and metal ions Cd(2+), Co(2+) were studied by direct current polarography. The diffusion currents of these ions decreased sharply in the presence of PGA in the pH region from 5.0 through neutral. A corresponding increase in the helix content of the PGA-metal ion complex was revealed by CD measurements on the same solutions. Helix contents determined by polarography were in good agreement with those by CD in the neutral pH region. On the contrary, the decrease of current in lower acidic pH regions was independent of helix formation and suggested that metal ions coordinate to sporadically-dissociated carboxylate groups to cause aggregation of the intra and/or inter polymer chains. The diffusion current of the ions, therefore, is a parameter sensitive to the conformational changes of PGA from acidic through neutral pH region.