The electron density and H-bond energies are two criteria to explain and evaluate the strength of intramolecular H-bonds. The values vary from 5.73 to12.15 kcal.mol-1 for the conjugate bases CH3-pentose as well as 4.74 to 24.94 kcal.mol-1 for conjugate bases of aldo-pentose sugars. The calculated values of and for neutral D-ribose at BCPs of O4….HO2-O2 H-bond are positive (+0.0596 and +0.0010) a.u, respectively, implying that O4….HO2-O2 H-bond should be considered as a weak H-bond. The L-arabinose deprotonated at C3-HO3 indicates the O3…HO1-O1H-bond, which is the strongest bifurcated H-bond in aldo-pentose sugars with the largest value of = 24.94 kcal.mol-1, =0.0752 a.u.; =0.1380 (e/au5).
On the other hand, the strongest H-bond in the conjugate bases of CH3-pentose sugars is O4…HO3-O3 H-bond formed from deprotonation of L-fucose at C4-HO4 with the largest value of = 12.15 kcal.mol-1; =0.0439 a.u.; =0.1257 (e/au5). L-fucose deprotonated at C4-HO4 indicates the O3…H2-C6 H-bond (Figure 2d) with = 1.43 kcal.mol-1; and L-rhamnose deprotonated at C2-HO2 indicates the O3…H3-C6 bond (Figure 2f) with = 2.08 kcal.mol-1; which are categorized as weak H-bonds (van der Waals) with > 0 and > 0.
The L-arabinose deprotonated at C3-HO3indicates the O3…HO1-O1 and O3…HO4-O4 H-bonds with = 24.94 and 8.38 kcal.mol-1, respectively; and D-ribose deprotonated at C4-HO4indicates the O4…HO2-O2 and O4…HO4-O4 H-bonds with = 17.44 and 6.14 kcal.mol-1, respectively. These are considered as bifurcated intramolecular H-bonds. Thevalues for the conjugate bases of methyl-pentose vary between 1.43 and 2.08 kcal.mol-1 for C-H…O H-bonds and from 5.73 to 12.15 kcal.mol-1 for O-H…O H-bonds. Whereas, in the conjugate bases of aldo-pentose sugars, values for O-H…O H-bonds vary from 4.81 to 24.94 kcal.mol-1. Consequently, these results show that H-bonds in the conjugate bases of methyl-pentose sugars are weaker than those in the conjugate bases of aldo-pentose sugars. The CH3 group on C5of methyl-pentose sugars causes the H-bonds strength to decrease in their conjugate bases, as compared to the H-bonds strength in the conjugate bases of aldo-pentose sugars. This rationalizes why methyl-pentose sugars are less acidic than the aldo-pentose sugars; the role of the CH2OH group is crucial in stabilizing hydrogen bonding in the conjugate bases of aldo-pentose sugars. Recently, Bayach et al. [49] showed that the hydrogen bond strength in the Octyl-xylopyranoside in which CH2OH group is absent on C5 is weaker than that in Octyl-glucopyranoside containing CH2OH group.