Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
Inclusion complexes can be isolated in stable crystalline form. On their dissolution,
the equilibrium established between dissociated and associated species (Equation
17.1)
CD + D CD.D (17.1)
can be expressed with the stability constant K a defined in Equation 17.2:
K1: 1
[ ]
=
[ ][ ]
CD.D
CD D
(17.2)
There are several essential consequences of the interaction between a poorly soluble
guest and a CD in aqueous solution.
1. The concentration of the guest in solution increases significantly, whereas
concentration of dissolved CD decreases. The latter statement is not universally
true, because either ionizing guests or hydrogen bond containing
compounds (e.g., phenols) might facilitate the solubility of CD.
2. Spectral properties of the guest are modified. For example, NMR chemical
shifts of the anisotropically shielded atoms are modified. Achiral guests
introduced into the cavity of the chiral CD become optically active, showing
strong induced Cotton effects in the circular dichroism spectra. Sometimes,
the UV spectral bands are shifted by several nanometers.
Fluorescence is very strongly improved when fluorescing molecules are
transferred from the aqueous milieu into a nonpolar environment.
3. Reactivity of included molecules is modified. Frequently, the reactivity
decreases, i.e., the guest is stabilized, but in many cases CD behaves as
an artificial enzyme, accelerating various reactions and modifying the
reaction pathway.
4. Diffusion and volatility of the included guest strongly decrease.
5. On complexation, hydrophobic guests become hydrophilic, and therefore
their chromatographic mobility also changes.
6. In the solid state, complexed substances are molecularly dispersed in the
CD matrix, forming either microcrystalline or amorphous powder, even
when guests are gaseous. Guests are effectively protected from any type
of reaction, except reactions with the CD hydroxyl groups, and catalyzed
by these groups. The complexes are hydrophilic, readily wettable, and
rapidly soluble. If the formation of the CD inclusion complexes in aqueous
solution can be detected, for example, by NMR spectroscopy, circular
dichroism, or through a catalytic effect, this does not necessarily mean
that a well-defined crystalline inclusion complex can be isolated. Repulsion
between included water molecules and the nonpolar CD cavity on
the one hand and between the bulk water and the nonpolar guest on the
other are the main components of the driving force of the inclusion. The
second type of repulsion is absent in the crystalline (dry) state. For this