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The influence of guest size on the guest exchange behavior of the δ form of syndiotactic polystyrene was investigated using a series of n-alkanes from n-hexane to n-decane as a guest. The bands due to sorbate and desorbate molecules and the conformational sensitive bands of sPS were followed by ATR-FTIR spectroscopy. The guest exchange of the δ form containing chloroform proceeded smoothly for all n-alkanes, but it became slow with the length of n-alkane. Although the amount of TTGG helices characteristic of the clathrate structure was kept during the exchange with n-hexane and n-heptane, a decrease in conformational regularity was observed in the beginning of the guest exchange of longer n-alkanes. The absorbed amount of n-alkanes decreased with the amount of the TTGG helices. The frequency shift of the methyl asymmetric stretching band of n-alkanes suggested that the packing of n-alkane molecules in the cavity became tight as the chain length increased.

The guest exchange mechanism in the clathrate phase of syndiotactic polystyrene (sPS) was investigated by ATR−FTIR spectroscopy utilizing the selectivity of the sPS crystalline region about the conformation of 1,2-dichloroethane (DCE). 1,2-Dichloroethane (h-DCE) and its deuterated compound (d-DCE) were used as guests to analyze the sorption behavior of the sorbate molecules and the desorption behavior of the guest molecules stored in the crystalline region beforehand, simultaneously. The spectral changes on the exposure of an sPS film containing d-DCE to h-DCE vapor were followed, and the information about the guest exchange processes in both the crystalline and amorphous regions were obtained. In the earliest stage, the exchange occurred mainly in the amorphous region, while the exchange in the crystalline region proceeded gradually for a long time. On the basis of this finding, the diffusion coefficients of DCE molecules in the crystalline and amorphous regions were estimated.

In higher plants, O-phosphohomoserine (OPH) represents a branch point between the methionine (Met) and threonine (Thr) biosynthetic pathways. It is believed that the enzymes Thr synthase (TS) and cystathionine gamma-synthase (CGS) actively compete for the OPH substrate for Thr and Met biosynthesis, respectively. We have isolated a mutant of Arabidopsis, designated mto2-1, that over-accumulates soluble Met 22-fold and contains markedly reduced levels of soluble Thr in young rosettes. The mto2-1 mutant carries a single base pair mutation within the gene encoding TS, resulting in a leucine-204 to arginine change. Accumulation of TS mRNA and protein was normal in young rosettes of mto2-1, whereas functional complementation analysis of an Escherichia coli thrC mutation suggested that the ability of mto2-1 TS to synthesize Thr is impaired. We concluded that the mutation within the TS gene is responsible for the mto2-1 phenotype, resulting in decreased Thr biosynthesis and a channeling of OPH to Met biosynthesis in young rosettes. Analysis of the mto2-1 mutant suggested that, in vivo, the feedback regulation of CGS is not sufficient alone for the control of Met biosynthesis in young rosettes and is dependent on TS activity. In addition, developmental analysis of soluble Met and Thr concentrations indicated that the accumulation of these amino acids is regulated in a temporal and spatial manner.