Hydrogen Bonding: From Biological Systems To Interstellar Medium

Intermolecular interactions are very vital in the elucidation of structure and properties of many biological molecules like water, DNA, proteins, etc. They are embedded in chemical and biological systems by nature.

Due to their importance, they are well-studied and understood to an extent. Some of these interactions are critical in maintaining the 3-dimensional structure of large molecules such as proteins and nucleic acids. They enable one large molecule to bind specifically but transiently to another, thus making them the basis of many dynamic biological processes.

Among the different intermolecular interactions, the hydrogen bond is well-recognized, studied, and understood to a large extent because of its overwhelming impact in different systems and phenomena. The hydrogen bond underlines the chemical and biological properties of water. It acts as a stabilizing force in macromolecules. The IUPAC task group defined the hydrogen bond as “ The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X–H in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation.

Other intermolecular interactions that have been identified include halogen bonding, lithium bonding, chalcogen bonding, agostic bonding, pnicogen bonding, and carbon bonding. These interactions also influence drug design, crystallinity, and design of materials, particularly with self-assembly, and the synthesis of many organic molecules. Whether hydrogen bonding also exists and plays a role in interstellar chemistry will be seen in this work

The significance of chemistry in the interstellar medium (ISM) cannot be overemphasized. The ISM is not just an open vacuum dotted with stars, planets and other celestial formations as it is considered to be in popular perception; rather it consists of a bizarre mixture of both familiar molecules such as water, ammonia etc., and a large number of exotic ones such as radicals, acetylenic carbon chains, highly reactive cationic and anionic species, carbenes, and high molecular isomers that are so unfamiliar in the terrestrial laboratory that chemists and astronomers have termed them “non-terrestrial.” These molecules serve as probes of astrophysical phenomena.

As important as these molecules are, not much is known about how they are formed under the conditions (low temperature and low density) in the interstellar medium. As a result of this challenge, there is hardly a consensus as to how most of these molecules are formed.  Some common chemical characteristics exist among the known interstellar molecular species, these include isomerism, successive hydrogen addition, the dominance of carbon-containing species, and periodic trends. The characteristics serve ad pointers as to how these molecules are formed in ISM. Of these characteristics, isomerism appears to e the most pronounced as about 40% of all the known interstellar molecules have isomeric counterparts (with the exception of the diatomic species, a number of hydrogen saturated species and other special species like the C3, C5, which cannot form isomers).

In our previous studies while trying to address the question, “Why are some related molecular species observed in the interstellar space and others not?” we showed that there exists a relationship between the energy, stability, and abundance (ESA) which influences the astronomical observation of some related molecular species at the expense of others. This ESA relationship was shown to exist in isomers species, linear interstellar carbon chains among others as shown in the figures below. However, a few deviations we observed in the ESA relationship.

The reactions that occur on the surface of interstellar dust grains are the dominant processes that help form interstellar molecules. Water molecules make up roughly 70% of interstellar ice. These water molecules also serve as the platform for hydrogen bonding.

The present study reports the first extensive study of the existence and effects of interstellar hydrogen bonding. The binding energies of the hydrogen-bonded complexes of interstellar molecules with water monomer obtained from high-level quantum chemical simulations show a direct relationship between the binding energies of some species (whose formation is mainly controlled by the ice phase reactions) and the interstellar abundances of the molecules. From the relationship, the higher the binding energy of the interstellar molecule bonded with water, the lower its interstellar abundance as compared to its counterparts with lower binding energies. This is because the stronger the molecule is being bonded to the surface of the interstellar dust grains; the more a greater portion of it is being attached to the surface of the interstellar dust grains, thereby reducing its gas phase abundance. Available interstellar observations data confirms this. Interstellar hydrogen bonding accounts for the deviations from thermodynamically controlled processes (ESA relationship), the delay in detecting the most stable isomers whose less stable counterparts have been detected, the difficulty in observing amino acids (e.g. glycine).

Optimized structures of the hydrogen-bonded C2H4O2 and C2H6O isomer complexes with water.

These findings are described in the article entitled Interstellar hydrogen bonding, recently published in the journal Advances in Space ResearchThis work was conducted by Emmanuel E. Etim from the Federal University WukariPrasanta Gorai, Ankan Das, and Sandip K. Chakrabarti from the Indian Centre for Space Physics, and Elangannan Arunan from the Indian Institute of Science Bangalore.

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