C. Merino et al., Effect of the presence of beta-cyclodextrin on the solution behavior of procaine hydrochloride. Spectroscopic and thermodynamic studies, LANGMUIR, 16(4), 2000, pp. 1557-1565
The effect of the addition of beta-cyclodextrin (CD) to the aqueous solutio
ns of the local anesthetic drug, procaine hydrochloride, has been fully inv
estigated by means of spectroscopic (UV-vis, steady-sate fluorescence, and
NMR) and thermodynamic (density and speed of sound) studies. The global pic
ture of the results indicates that procaine hydrochloride penetrates the CD
cavity by the wider ring, -NH2 group end first, allowing up to the aromati
c ring of the drug inside the cavity, with a 1:1 stoichiometry. A new model
has been proposed to determine binding constants from UV-vis spectra, when
the addition of CD provokes a wavelength shift instead of an absorbance in
crease. The association constant, obtained from both emission fluorescence
and UV-vis data, ranges from 400 to 200 M-1, on going from 15 to 40 degrees
C. A linear decrease of the affinity of the cyclodextrin by the drug with
temperature drives the enthalpy (Delta H degrees = -19 +/- 5 kJ mol(-1)) an
d the entropy (Delta S degrees = -15 +/- 7 J K-1 mol(-1)) changes of the bi
nding process to negative values. These values indicate that the encapsulat
ion of procaine hydrochloride by beta-cyclodextrin is an exothermic and ent
halpy governed process, with a balance between van der Waals contacts, hydr
ophobic effect, and solvent reorganization being mainly responsible for the
overall stability of the complex. The thermodynamic study has shown that i
n the reorganization of water molecules after the association of the CD and
the drug, four to five water molecules are expelled from the CD cavity and
four to five water molecules are removed from the hydration shell of proca
ine.