58
P. Panwaria and A. Das
π
* interaction [66–79] etc. Among all these non-covalent interactions, the conventional hydrogen bond is the most abundant and well-studied interaction. Conventional
hydrogen bonding, which is denoted by X-H…Y with both X and Y as strong electronegative atoms, is generally dominated by electrostatic interaction [14–16, 80–85].
On the other hand, either X or Y or both X and Y are very weak in electronegativity
in the unconventional hydrogen bonding interaction.
Although conventional hydrogen bonding is very much popular due to its presence in nucleic acids and proteins, it has been found recently that unconventional
hydrogen bonding is not very far away from its conventional counterpart in terms
of its strength and application. In general, unconventional hydrogen bonds were
not explored earlier in detail, probably due to the conventional wisdom that these
hydrogen bonds are very weak in nature. However, the study of these unconventional hydrogen bonds, considering various elements (C, P, S, Se, etc.) apart from
the conventional electronegative elements in the periodic table as hydrogen bond
donors and acceptors, has been increased to a great extent after the re-definition of
the hydrogen bond by the IUPAC committee in 2011 [16, 50, 80, 86–93].
In general, various experimental techniques such as solution-phase FT-IR spectroscopy, Nuclear magnetic resonance (NMR) spectroscopy, and X-ray crystallography are used to characterize different types of hydrogen bonds as well as other
non-covalent interactions present in small molecules to large supramolecular assemblies and biomolecules [89, 92, 94–98]. Solution-phase FT-IR spectroscopy is a
prevalent technique, which exploits, in general, the red-shift in the frequency as well
as the intensity of the vibrational transition of the hydrogen bond donor as a characteristic signature of the strength of the hydrogen bond present in a molecular system
or complex [99]. The presence of the hydrogen bond in a system is also characterized
by the chemical-shift of the NMR of the proton of the hydrogen bond donor as well
as the hydrogen bond acceptor atom [100]. Temperature-dependent NMR, as well
as 2D-NMR spectroscopy, render additional confirmation in support of the presence
of the hydrogen bond in a system [101]. X-ray crystallography is another powerful
method to determine the presence of the hydrogen bonding and other non-covalent
interactions by measuring the distances and angles of the atoms in consideration for
the attractive interaction [96–98].
Despite the versatility of the X-ray crystallography technique on determining
the positions of the atoms in the crystal structure with atomic resolution in small
molecules as well as large macromolecules such as proteins, nucleic acids, etc.,
this technique suffers some limitation in terms of accurate identification of the noncovalent interactions, especially, the hydrogen bonding. In the crystal structure, two
atoms can come close to the distance within the sum of their van der Waals (vdW)
radii due to the crystal packing forces, as well as the optimization of some other interactions between neighboring units. Consequently, parameters of these non-covalent
interactions in terms of distances and angles vary widely from one protein to another.
Further, it is quite uncertain to confirm the presence of the hydrogen bond in the X-ray
crystal structure as the position of the hydrogen atom is not known there. Similarly,
in solution phase FT-IR and NMR spectroscopy, the non-covalent interactions under
investigation are perturbed by the intermolecular interaction with the solute as well
P. Panwaria and A. Das
π
* interaction [66–79] etc. Among all these non-covalent interactions, the conventional hydrogen bond is the most abundant and well-studied interaction. Conventional
hydrogen bonding, which is denoted by X-H…Y with both X and Y as strong electronegative atoms, is generally dominated by electrostatic interaction [14–16, 80–85].
On the other hand, either X or Y or both X and Y are very weak in electronegativity
in the unconventional hydrogen bonding interaction.
Although conventional hydrogen bonding is very much popular due to its presence in nucleic acids and proteins, it has been found recently that unconventional
hydrogen bonding is not very far away from its conventional counterpart in terms
of its strength and application. In general, unconventional hydrogen bonds were
not explored earlier in detail, probably due to the conventional wisdom that these
hydrogen bonds are very weak in nature. However, the study of these unconventional hydrogen bonds, considering various elements (C, P, S, Se, etc.) apart from
the conventional electronegative elements in the periodic table as hydrogen bond
donors and acceptors, has been increased to a great extent after the re-definition of
the hydrogen bond by the IUPAC committee in 2011 [16, 50, 80, 86–93].
In general, various experimental techniques such as solution-phase FT-IR spectroscopy, Nuclear magnetic resonance (NMR) spectroscopy, and X-ray crystallography are used to characterize different types of hydrogen bonds as well as other
non-covalent interactions present in small molecules to large supramolecular assemblies and biomolecules [89, 92, 94–98]. Solution-phase FT-IR spectroscopy is a
prevalent technique, which exploits, in general, the red-shift in the frequency as well
as the intensity of the vibrational transition of the hydrogen bond donor as a characteristic signature of the strength of the hydrogen bond present in a molecular system
or complex [99]. The presence of the hydrogen bond in a system is also characterized
by the chemical-shift of the NMR of the proton of the hydrogen bond donor as well
as the hydrogen bond acceptor atom [100]. Temperature-dependent NMR, as well
as 2D-NMR spectroscopy, render additional confirmation in support of the presence
of the hydrogen bond in a system [101]. X-ray crystallography is another powerful
method to determine the presence of the hydrogen bonding and other non-covalent
interactions by measuring the distances and angles of the atoms in consideration for
the attractive interaction [96–98].
Despite the versatility of the X-ray crystallography technique on determining
the positions of the atoms in the crystal structure with atomic resolution in small
molecules as well as large macromolecules such as proteins, nucleic acids, etc.,
this technique suffers some limitation in terms of accurate identification of the noncovalent interactions, especially, the hydrogen bonding. In the crystal structure, two
atoms can come close to the distance within the sum of their van der Waals (vdW)
radii due to the crystal packing forces, as well as the optimization of some other interactions between neighboring units. Consequently, parameters of these non-covalent
interactions in terms of distances and angles vary widely from one protein to another.
Further, it is quite uncertain to confirm the presence of the hydrogen bond in the X-ray
crystal structure as the position of the hydrogen atom is not known there. Similarly,
in solution phase FT-IR and NMR spectroscopy, the non-covalent interactions under
investigation are perturbed by the intermolecular interaction with the solute as well