(+)-Tubocurarine chloride

British Journal of Pharmacology, 1982, 75(3):503-512.

The ionization of morphine, hydroxyamphetamine and (+)-tubocurarine chloride and a new method for calculating zwitterion constants.[Reference: WebLink]

METHODS AND RESULTS:
1 An improved method for estimating the zwitterion constants of phenolic amines is described which involves the exploratory least-squares fit of absorbance (at a suitable wavelength) to pH, starting with estimates of pK1 and pK2 obtained electrometrically. 2 With the method it is possible to see that hydroxyamphetamine (alpha-methyltyramine) has a higher zwitterion constant than tyramine and the zwitterion constants of both compounds are lower at 37 degree C than at 25 degree C. 3 The zwitterion constant of morphine is not reduced by raising the temperature from 25 degree to 37 degree C and the effect of temperature is much greater in compounds with a primary or secondary amino group than with those containing a tertiary amino group. Some zwitterions may be stabilized by hydration and their formation will be reduced by a rise in temperature which will break up water structure. 4 From electrometric titrations with (+)-Tubocurarine chloride in 0.1 M NaCl estimates of pK1, pK2 and pK3 were 7.6, 8.65 and 9.65 at 25 degree C and 7.4, 8.6 and 9.7 at 37 degree C, compared with 7.8, 8.85 and 9.75 given by Perrin (1980).
CONCLUSIONS:
However, the effects of pH on absorbance show that the phenolic groups lose a proton before the ammonium group so there is extensive zwitterion formation which is probably greater at 25 degree than at 37 degree C. the p-phenolic group (position 13) probably ionizes first with the phenate form stabilized by hydration involving water molecules and the protonated form of the (1-) ammonium group.