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Chemical properties of amino acids
chemical property
Reaction of amino group: acylation reaction; Reaction with nitrite; React with aldehyde; Sulfonation reaction; React with DNFB; Salifying reaction.
Carboxyl reaction the carboxyl group of amino acids, like other carboxylic acids, can undergo acylation, esterification, decarboxylation and salifying reactions under certain conditions.
Reaction with ninhydrin hydrate: α- Amino acids and hydrated ninhydrin are co heated in weak acidic solution, and the corresponding compounds are formed by oxidative deamination α- Keto acid is further decarboxylated to form aldehyde, and ninhydrin hydrate is reduced to reduced ninhydrin. In weak acidic solution, reduced ninhydrin and ammonia removed from amino acid react and condense with another ninhydrin hydrate to form blue purple complex. Proline and hydroxyproline react with ninhydrin to produce yellow substance, and the rest α- Amino acids react with ninhydrin to produce blue purple substances. This color response is often used α- Colorimetric determination of amino acids and chromogenic analysis.
acid-base
1. Bisexual dissociation and isoelectric point
Amino acids basically exist in the form of facultative ions or dipole ions in aqueous solutions or crystals. The so-called zwitterions refer to nh3+ valine ions that can release protons and coo negative ions that can accept protons on the same amino acid molecule. Therefore, amino acids are zwitterionic electrolytes.
Isoelectric point of amino acids: the charged state of amino acids depends on the pH value of the environment. Changing the pH value can make amino acids positively or negatively charged, or make them in the zwitterion state with equal number of positive and negative charges, that is, the net charge is zero. The pH value of the solution when the net charge of the amino acid is zero is called the isoelectric point of the amino acid.
Amino acid molecules contain both acidic and basic groups, so amino acids can react with strong acids. It can also react with strong bases to form stable salts with the characteristics of amphoteric compounds.
When the pH of an amino acid solution is adjusted to a certain value, the amino acid just exists in the form of dipole ions. In the electric field, it moves neither to the negative nor to the positive, that is, at this time, the number of positive and negative charges it carries is equal, the net charge is zero, and it is electrically neutral. At this time, the pH of the solution is called the isoelectric point of the amino acid, which is usually expressed by PI. At the isoelectric point, amino acids exist mainly in the form of dipole ions. When the pH of the amino acid solution is greater than PI (such as adding alkali), an nh3+ in the amino acid gives protons and shifts to the right in equilibrium. At this time, the amino acid mainly exists in the form of anions. If in the electric field, it moves to the positive pole. On the contrary, when the pH of the solution is less than PI (such as adding acid), a coo binding proton in the amino acid shifts the balance to the left. At this time, the amino acid mainly exists in the form of cation, and if in the electric field, it moves to the negative pole.
Various amino acids have different isoelectric points due to their different composition and structure. The isoelectric point of neutral amino acids is less than 7, generally 5.0 ~ 6.5. The isoelectric point of acid amino acids is about 3. The isoelectric of basic amino acids is 7.58 ~ 10.8. Under the action of electric field, charged particles move towards the electrode opposite to its electricity, which is called electrophoresis. Because the relative molecular weight and Pi of various amino acids are different, in the buffer solution of the same pH, different amino acids not only have different charge conditions, but also have different swimming directions and rates in the electric field. Therefore, based on this difference, the mixture of amino acids can be separated by electrophoresis. For example, when the mixture of aspartic acid and arginine is placed in the center of the electrophoresis support medium (filter paper or gel) and the pH of the solution is adjusted to 6.02 (buffer solution), aspartic acid (pi=2.98) is negatively charged and moves to the positive pole in the electric field, while arginine (pi=10.76) is positively charged and moves to the negative pole
Reaction
What foods contain amino acids?
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