Some amino acids are 'essential'; they cannot be made inside the body and therefore need to come from food. An amino acid that forms a common part of animal feed is methionine; however its high solubility makes it difficult to use in underwater systems.
An alternative to methionine that has been developed by Evonik Nutrition & Care GmbH is the dipeptide methionyl-methionine, which can then be broken down in the digestive system of water organisms such as shellfish or salmon, or bottom feeders like shrimps or prawns, to form the desired amino acid.
Amino acid structure has an additional layer of complexity as, inside each amino acid, there is a carbon atom around which the bonded atoms can be in a clockwise or anticlockwise order. They can therefore be left or right handed, and this property is known as chirality.
Although biological systems are very good at telling the difference between the two forms, industrial synthesis techniques are often difficult to tailor to producing one of the other. This makes it crucial to understand the behaviour of the different forms, or stereoisomers.
The chirality of an amino acid is shown by the use of an L- or D- proceeding the name. For methionine, the L- form is biologically useful, although the D- form can be converted to the desired form inside the body. When forming a dipeptide, this chirality occurs in both halves, leading to four possible structures.
This study, published in Scientific Reports, looked at crystals of methionyl-methionine where the component peptides had different stereochemistry, forming a DD-LL crystal, and a DL-LD crystal. The scientists used electron microscopy, X-ray photoelectron spectroscopy, X-ray crystallography and vibrational spectroscopy including inelastic neutron scattering on Tosca to study the crystals, building a full picture of their structures.
They discovered one area where the DD-LL and DL-LD racemates are distinctly different. Their electron microscopy showed that the DL-LD form is markedly less crystalline than the DD-LL form, and that they have very different morphologies.
In addition, the two forms showed very different solubilities, which is important when considering how the nutrient is released when used underwater.
They also investigated how the two would interact with each other in an equal mixture, which would likely be formed during industrial synthesis. Their experiments showed that there is no evidence that the DL-LD and DD-LL stereoisomers interact with one another. Instead, they behave independently on all length scales from the macroscopic to the atomic.
The difference in solubility will be a focus for future studies and computational work, to understand its cause and any implication on feeding techniques.
The full paper can be found onine at DOI: 10.1038/s41598-020-80385-z