Abstract
Abstract The effects of varying polarity of solvent on the hydroxyl pK and on the rates of iodination of tyrosine and monoiodotyrosine have been studied by spectrophotometric techniques. When methanol replaces water to 50% (v/v), ΔpKa for the hydroxyl group is +0.74 for tyrosine and +1.29 for monoiodotyrosine. The pKa of the hydroxyl group of each compound correlates well with the dielectric constant of the solvent. When the observed rates of iodination of the two phenolic derivatives are corrected for varying iodide and hydrogen ion concentrations in the system according to appropriate pre-equilibria, increasing concentrations of methanol decrease the rate constant of iodination of tyrosine more than that of monoiodotyrosine. The data for tyrosine again correlate well with theoretical considerations of the dielectric constant of the solvent, but the data for monoiodotyrosine do not. It is concluded that the nonpolar environment favors diiodination relative to monoiodination, which may have biological significance in the case of the nonpolar internal environment of thyroglobulin.
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