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SynopsisStudies on copolymerization kinetics of N-carboxyanhydride (NCA) of 0-acetyl-L-tyrosine (T) with NCAs of L-valine (V) and glycine (G) were carried out in dioxane a t 25°C with nbutylamine as initiator. The reactivity ratios for the VT system were found to be rV:rT: 0.57:0.22; and for the GT system, r~:r~:0.26:0.42. A comparison of this data with that obtained when L-alanine was used in place of L-valine or glycine showed that in the three systems, the reactivity ratio of the amino acid residue with a greater tendency t o form ordered secondary structure was greater than that of its comonomer. Among the variable comonomers (glycine, L-alanine, L-valine) of 0-acetyl-L-tyrosine, it is again L-alanine, whose tendency to form helical structure is very high, that has the greater reactivity ratio.

The binding of methylene blue (MB) to poly(rA) was studied using UV and CD spectroscopy at neutral and acidic pH, in which poly(rA) exists as a single strand and a duplex respectively. UV spectroscopic studies and analysis of equilibrium binding data show that salt increases the cooperativity parameter but reduces the binding between the cationic dye and the negatively charged phosphate groups. The binding constant for the single strand-MB complex decreased from 1.15 x 10(5) M-1 to 7.62 x 10(3) M-1 while the cooperativity parameter increased from 9 to 71, when salt concentration was increased from 1 mM to 200 mM. At neutral pH, changes in the induced circular dichroism of MB-poly(rA) complex with ionic strength show predominance of electrostatic binding to the phosphate group externally on the poly(rA) chain; a partial intercalation between the adenine bases is also indicated. A red shift in the visible absorption band and the ICD profile of the double stranded poly (rA)-MB complex suggest an electronically coupled intercalation mode for the dye binding at pH 4.2.

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