Conclusion
Theoretical analysis was performed to explore the influence of H-bond on
acidity of Deoxy-hexose sugars including CH3-pentose and
aldo-pentose sugars. The results of the calculation reveal that acidity
in the Deoxy-hexose sugars significantly depend on their OH groups
orientation (equatorial and axial position) and the nature and strength
of H-bonds in their conjugate bases. The axial OH groups in aldo-pentose
were found to be more acidic than the equatorial OH groups in
CH3-pentose sugars. Compared to aldo-pentose sugars,
methy-pentose sugars are less acidic due to existent of
CH3 group on C5. It was found that these
sugars are more acidic than normal alcohols such as ethanol and propanol
because their conjugate bases are stabilized through intramolecular
H-bonds. Thus, the nature of these H-bond interactions was analyzed by
means of the natural bonding orbital (NBO) and quantum theory atoms in
molecules (QTAIM) approaches. Analysis of the electron density in
methy-pentose reveals the existence O-H…O and C-H..O
intramolecular hydrogen bonds in methyl-pentose sugars. Also, the
results indicated the presence of two types of intramolecular (normal
and bifurcated) H-bonds in the conjugate bases of Deoxy-hexoses sugars.
The conjugate bases of D-ribose, L-lyxose and L-arabinose show
bifurcated intramolecular H-bonds; in contrast, those of other sugars
indicate only normal H-bonds. It was shown that intramolecular H-bonds
are electrostatic (closed-shell) interactions in the conjugate bases of
Deoxy-hexoes sugars due to at BCPs of H-bond. Although, depending on the
signs, some sugars are partially covalent-partially electrostatic and
some are van der Waals interaction in nature.