FIGR ID=jo701926rn00001>figures/jo701926rn00001.gif" ALIGN="left" HSPACE=5> |
Can a benzene molecule di
fferentiate between two isomeric carbohydrates? It is generally accepted thattwo
factors govern molecular recognition: complementarity and preorganization. Preorganization requiresthe presence o
f cavities
for positioning the host's groups o
f complementary nat
ure to those o
f the guest.This study shows that, in
fact, groups should be complementary to recognize each other (
for the casepresented here, it is controlled by the CH/
fchars/pi.gi
f" BORDER=0 > interaction) but preorganization is not essential. Since weakinteractions have their origin in dispersion
forces, they also have impact on the enthalpic term o
f the
freeenergy, so it was considered that their participation can be demonstrated by measuring the energy involved.For recognition to happen, two conditions must be satis
fied: speci
ficity and associated stabilizing energy.In this study we evaluated the heat o
f dissolution o
f di
fferent carbohydrates such as methyl 2,3,4,6-tetra-
O-methyl-
fchars/alpha.gi
f" BORDER=0>-
D-mannopyranoside and methyl 2,3,4,6-tetra-
O-methyl-
fchars/beta2.gi
f" BORDER=0 ALIGN="middle">-
D-galactopyranoside usingdi
fferent aromatic solvents. The solvation enthalpies in benzene were -78.8 ± 3.9 and -88.7 ± 5.5 kJmol
-1 for each carbohydrate, respectively; and these values yielded a CH/
fchars/pi.gi
f" BORDER=0 > energy o
f interaction o
f 9.9kJ mol
-1. In addition, NMR studies o
f the e
ffect o
f the addition o
f benzene to chloro
form solutions o
fthe two carbohydrates showed that benzene speci
fically interacts with the hydrogen atoms o
f the pyranosering at positions 3, 4, and 5 located on the
fchars/alpha.gi
f" BORDER=0>
face o
f the methyl-
fchars/beta2.gi
f" BORDER=0 ALIGN="middle">-galactoside, so it is, in
fact, able torecognize it. Thus, the interactions between carbohydrates and the aromatic residues o
f proteins occur inthe absence o
f the con
finement generated by the protein struct
ure. By experimentally measuring theenergy associated with this interaction and comparing it to theoretical calculations, it was also possibleto unequivocally determine the existence o
f CH/
fchars/pi.gi
f" BORDER=0 > interactions between carbohydrates and proteins.