Glycine Structure

Glycine is an organic compound which has the formula HO₂CCH₂NH₂. It belongs to the category amino acid and is prevalent often in small quantities in proteins both animal and plant proteins. m Glycine is an important element in both myoglobin and hemoglobin. It is a sweet-tasting crystalline amino acid. This falls under the category of the principal amino acid which is mostly found in sugar cane. It can be produced from the alkaline hydrolysis of gelatin. It has multiple uses. It is used extensively in biochemical research and medicine. But this organic compound is not essential to make proteins and it is represented as a 3 letter abbreviation, Gly. Its structure is composed of an H, COOH, and NH₂ bound to CH. It is one of those twenty amino acids found in animal proteins. It’s an inhibitory neurotransmitter of the central nervous system, with its maximum presence in the spinal cord, brainstem, and retina.

The simplest structure of glycine can be written as 

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Formation of Glycine

Although glycine can be isolated by the hydrolysis of protein, it can never be used for industrial production since the amount produced is less. But it can be manufactured more in a more convenient manner by chemical synthesis. The two main processes involved in the production of glycine are amination of chloroacetic acid with ammonia, that produces glycine and ammonium chloride as the end products, and the Strecker amino acid synthesis, the main synthetic method used in the United States and Japan. About 15 thousand tonnes of glycine are produced per annum in this way.

Glycine is also produced as by-product though considered as an impurity in the synthesis of EDTA, arising from reactions of the ammonia coproduct.

The structure of glycine can be studied in mainly two forms. One form being the anionic form and the other one is the neutral form. Here we will discuss in detail the structure of these two forms: Anionic form and neutral form. Again the anionic form is of two types viz. zwitterionic form and canonical form.

GLY–(H₂O)₃ ANION

The dipole-bound excess electron in amino acid or amino acid–solvent cluster stabilizes the zwitterionic form of the amino acid relative to the canonical form. Bowen and his coworkers once observed that arginine along with an excess electron exists as a zwitterionic form in an environment with the low-temperature gas phase, as predicted by Gutowski and co-workers.

Three zwitterionic conformers (a-zw-1), (a-zw-2), and (a-zw-3) are calculated to have the lowest energy, which indicates that an excess electron may indeed stabilize the zwitterion as compared to the canonical form, in contrast with Gly–(H₂O)n(n=2). This is the reason why canonical forms have been much more thermodynamically stable. The energies of these three conformers are very close to each other and are calculated to be around 1 kcal/mol. Three water molecules are grouped together in the vicinity of the carboxylate group, which indicates that interactions between the water molecules are way far stronger than those with the functional groups in the Gly moiety. In the fourth lowest energy conformer (a-zw-4), a water molecule reacts with the ammonium group, on the other hand, the other two water molecules get bound to the carboxylate side. 

The lowest-energy Gly–(H₂O)₃ conformer which has canonical Gly is (a-c-1) and it is calculated to be 1.89 kcal/mol, is higher in energy than the lowest-energy zwitterionic conformer (azw-1). In this conformer, three water molecules form a ring-like structure, that interacts with the amino group. In other conformers, at least a water molecule results in the formation of a hydrogen bond with the carboxyl group.

But these Gly–(H₂O)₃ cluster anions are extremely unstable, and will not be detected in any experiment. These low barriers of isomerization need to be understood by the fact that the water molecules are not there to bridge the amino and the carboxyl groups in these clusters. This allows the proton to transfer almost freely. The zwitterionic conformer (azw-12) is higher in energy than (a-c-1), the canonical conformer. Thus the canonical form is stable against the isomerization when it is formed in experiments. Keeping in mind the fact, that the energy of the canonical conformer (a-c-1) is higher than the lower-energy zwitterionic conformers, we thus predict that it may exist in a small amount in gas phase when the temperature is considerably low.

Zwitterionic form of glycine

NEUTRAL GLY–(H₂O)₃ CLUSTER

While conducting photoelectron spectroscopy experiments, it is found that the excess electron gets detached to produce the neutral species. For the Gly–water clusters, the neutral Gly–(H₂O)₃ cluster is produced with Gly either in zwitterionic or canonical form, depending on multiple factors. If the incumbent cluster anion Gly–(H₂O)₃ produced is zwitterionic, the neutral cluster formed would also be zwitterionic and if the cluster ion is canonical then the neutral cluster will also be canonical in nature. This is because the photodetachment processes do not imbibe or inculcate much change in the nuclear configuration.

The neutral species thus formed may subsequently stay long enough for experimental observation, or may even evolve to the other form following the process of the zwitterion 4,3 canonical transformation. Both the thermodynamic (relative Gibbs function) and the kinetic (magnitude of a barrier) factors need to be considered in order to determine which resultant forms (zwitterionic or canonical) would be more stable under what experimental circumstances. 

Now we need to highlight certain observations. Firstly, the energy and the Gibbs function associated with the conformers of zwitterionic Gly are much more (by 9 kcal/mol) than those with canonical Gly. This is high, which is in contrast with the case of a few types of Gly–(H₂O)₃ anion Therefore, once the neutral zwitterionic Gly– (H₂O)₃ cluster is generated by the method of photodetachment the stable zwitterionic cluster anion, it may result in isomerization to its canonical form, unless the barrier is significantly high. 

Secondly, one of the canonical conformers, (n-c-1), has lower energy as compared to the others. In this lowest-energy state, the neutral canonical conformer depicted has three water molecules that act as a bridge among the carboxyl group, whereas in all the other conformers three water molecules result in the formation of a chain connecting the amino and the carboxyl group. In the second lowest-energy conformer (n-c-2), a water molecule on reacts with the hydrogen atom in the OH group of the Gly moiety, whereas in another form of conformers [with the only exception of (n-c-6)] it forms a hydrogen bond either with the carbonyl oxygen present or with the oxygen atom in the hydroxyl group of the amino acid. But one thing to be noted that in various Gly–(H₂O)₃ conformers with zwitterionic Gly water molecules forms the bridge between the ammonium and the carboxylate group. When the three water molecules result in the formation of a chain in the conformer, the ammonium group becomes more close to the carboxylate. This is in contrast to the fact that these two functional groups are often bridged by a water molecule(s) in some of the conformers.