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Can a benzene
molecule differentiate between two iso
meric carbohydrates? It is generally accepted thattwo factors govern
molecular recognition: co
mple
mentarity and preorganization. Preorganization requiresthe presence of cavities for positioning the host's groups of co
mple
mentary nature to those of the guest.This study shows that, in fact, groups should be co
mple
mentary to recognize each other (for the casepresented here, it is controlled by the CH/
![](/i<font color=)
mages/gifchars/pi.gif" BORDER=0 > interaction) but preorganization is not essential. Since weakinteractions have their origin in dispersion forces, they also have i
mpact on the enthalpic ter
m of the freeenergy, so it was considered that their participation can be de
monstrated by
measuring the energy involved.For recognition to happen, two conditions
must be satisfied: specificity and associated stabilizing energy.In this study we evaluated the heat of dissolution of different carbohydrates such as
methyl 2,3,4,6-tetra-
O-
methyl-
![](/i<font color=)
mages/gifchars/alpha.gif" BORDER=0>-
D-
mannopyranoside and
methyl 2,3,4,6-tetra-
O-
methyl-
![](/i<font color=)
mages/gifchars/beta2.gif" BORDER=0 ALIGN="
middle">-
D-galactopyranoside usingdifferent aro
matic solvents. The solvation enthalpies in benzene were -78.8 ± 3.9 and -88.7 ± 5.5 kJ
mol
-1 for each carbohydrate, respectively; and these values yielded a CH/
![](/i<font color=)
mages/gifchars/pi.gif" BORDER=0 > energy of interaction of 9.9kJ
mol
-1. In addition, NMR studies of the effect of the addition of benzene to chlorofor
m solutions ofthe two carbohydrates showed that benzene specifically interacts with the hydrogen ato
ms of the pyranosering at positions 3, 4, and 5 located on the
![](/i<font color=)
mages/gifchars/alpha.gif" BORDER=0> face of the
methyl-
![](/i<font color=)
mages/gifchars/beta2.gif" BORDER=0 ALIGN="
middle">-galactoside, so it is, in fact, able torecognize it. Thus, the interactions between carbohydrates and the aro
matic residues of proteins occur inthe absence of the confine
ment generated by the protein structure. By experi
mentally
measuring theenergy associated with this interaction and co
mparing it to theoretical calculations, it was also possibleto unequivocally deter
mine the existence of CH/
![](/i<font color=)
mages/gifchars/pi.gif" BORDER=0 > interactions between carbohydrates and proteins.