Metachromasy. V. The effect of the chain length of polyphosphate on the metachromatic behavior of crystal violet and trypaflavine with emphasis on the binding curve and the spectra of bound dyes.

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Absorption spectra of two metachromatic dyes, Crystal Violet, CV, and Trypaflavine, TF, were measured in the presence of three sodium polyphosphates, NaPP, of different chain lengths. The optical titrations of these systems were performed at the phosphate residue-to-dye ratios, P/D, between 0 : 1 and about 1 : 1, where isosbestic points are present in the absorption spectra. The apparent molar absorption coefficients of the two dyes in each system changed sigmoidally with the increase in P/D. The extended principal component analysis was applied to the observed spectra of each dye–NaPP system in order to determine both the equilibrium constant for the binding reaction between the dye and the polymer and the pure spectra of the bound dyes. An empirical parameter α was introduced into the expression for the equilibrium constant, K=[complex]/[free dye] [unoccupied binding site]α, to reproduce the sigmoidal titration curve (α>1). The amount of the bound CV and TF increased with the increase in the chain length of NaPP at a given P/D, although the pure spectra of those bound dyes were independent of the chain length for low P/D values. The binding behavior of those dyes for high P/D values was also discussed by comparing the previous data with the binding curves calculated in this work.
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