Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
charge-transfer interaction integrals associated with the i(Gly) → LUMO(Rec) and
HOMO(Rec) → x (Gly) transitions, respectively, where LUMO and HOMO are
related to the lowest unoccupied and the highest occupied molecular orbitals, respectively.
In turn, the C″ coefficients represent dispersion interaction integrals attributed
to the coupling of the sugar i → x MOs electron excitations with the receptor HOMO
→ LUMO excitation. Because the exact receptor geometry is unknown, these
coefficients are calculated by mathematical statistics according to Equation 5.4. It
is accepted that Equation 5.4 is a multiple regression equation. It applies to similar
sugars, for example, aldopyranoses, which react with the same receptor. In such
cases, the resulting glycophore–receptor complexes have a similar geometry for all
sugars in the set and the C 0, C a, C′ i, and C″ ix unknown coefficients in Equation 5.4
are common and constant for all the sugars in the set of j = 1,2,3, … n. These
coefficients can be found by solving Equation 5.4 as a system of n linear equations
with m unknown quantities, m < n, using the least-square procedure. If the R
coefficient of such multiple correlation exceeds 0.95 with m ≈ 1 /2 n, it points to a
strong dependence of sweetness on the electron structure of the saccharides.
The method presented permits finding a moiety of a sugar molecule responsible
for an interaction with the sweet-taste receptor and describes a general mechanism
of the saccharide–receptor interaction. It provides the recognition of the chemoreception
properties together with a semiempirical expression useful in determination
of RS j of an arbitrary, unknown saccharide, because its structure is similar to that
of sugars from the set under consideration.
5.7 DOMINATION OF THE B1,XH1 GLYCOPHORE IN
ALDOPYRANOSES 8
Atomic net charges on the O-4 oxygen and hydrogen atoms in the 4-OH group of
aldopyranoses (Figure 5.1) excellently correlate with logRS j expl (mole/mole) (Table
5.2). The corresponding correlation coefficients, Rs, are 0.981 and 0.964, respectively.
These atoms are identified as B1 and XH1 sweetener subsites in the
Nofre–Tinti heptagonal sweetener. The relations B1 = A and XH1 = H are valid for
the AH,B Shallenberger glycophore (Figure 5.2). However, generally, XH1 is identified
as AH. Because of the above-mentioned property, QSAR multiple correlation
Equation (5.4) reduces to the two-parameter Equation (5.5):
log ( RSj)=−6. 8368⋅ Qj( B1)+ 2. 5937 ⋅ Qj(
XH1)−3.
3754
(5.5)
with R = 0.9832 and S 2 = 0.0015.
Three constants, −6.8368, 2.5937, and −3.3754, satisfactorily estimate the RS j
values for all the eight aldopyranoses (Table 5.2). The QSAR equation leads to
the following conclusions: (1) The B1 and XH1 sweetener subsites situated on
the 4-OH group in aldopyranoses have pure electrostatic centers and they determine
the sweetness of these compounds. (2) The B2 subsite, equivalent to the B