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Proteins - Organic Chemistry Tutor

Proteins, also known as holoproteids, represent a class of complex substances that have a macromolecular character and a high level of structural organization. They are formed only by aminoacids and have molecular masses that vary between 10 3 and 10 5 or more. Animal proteins define themselves through an enormous structural diversity which can be reflected on their functional area.

Proteins have a complex structure and this complexity results from the fact they are macromolecules and from the way the atoms and atom groups are placed in their configurationally spatial arrangement.

Based on their form and aspect and on the differences that can be identified in their three-dimensional structure and water solubility, proteins can be divided in : fibrillar proteins and globular proteins.

Fibrillar proteins have a filliform aspect and are being formed by long individual polypeptidic chains that are laterally bound in order to form a stabile structure. This morphology offers this category of proteins mechanical resistance water insolubility. As examples from this class of fibrillar proteins we have keratins, silk fibroin, collagen, ellastins, etc. These proteins play different biochemical roles, participating in mechanic resistance and protection of different tissues, blood coagulation, muscular contraction and others.

Globular proteins posses a three-dimensional structure, they have a compact aspect and usually are water soluble. These proteins play a part, in different proportion, in alpha-helix, myoglobin and hemoglobin. They include a variety of proteins and have multiple roles in the animal body.

Chemical and physical properties

Aggregation property: Protids, when pure, are solid and incolor substances that can be obtained as crystals or amorphous precipitates.

Denaturizing: When chemical or physical agents act on proteins, these tend to lose their spatial structure and the physical, chemical and biologic properties. When denaturized, a substance's molecular mass remains unchanged and nor does its primary structure (aminoacid sequence) and this is explained by the fact that denaturizing process does not involve breaking polypeptide chains but a reorganizing process of its structure. At usual temperature they are stabile and they are being destroyed with little exception when temperature rises.

Molecular mass: They are macromolecular substances with high molecular masses that reach 10 7 Da.

Optical properties: In UV they posses a maximal absorption capacity at 280nm due to the existence in their molecule of aminoacids that contain conjugated double links in cyclic cores.

Ionic properties: Proteins are polyelectrolytes that posses an amphiionic or an amphoteric character. The second property is provided by the existence of several polar groups like -COOH, -NH2, -OH. In an acid atmosphere proteins act like bases while in basic atmosphere they become acids.

Solubility: Proteins have different solubilities, most of them are soluble in cells and the buffer solutions. A protein's solubility is conditioned by the existence, in the macromolecular chain, of the polar hydrophilic groups that bind water molecules in hydrogen links. Fibrillar proteins are long and tend to organize in order to form fibers and in some cases they are bonded through hydrogen links resulting strong intermolecular forces that need a strong solvent to be broken. Some fibrillar proteins like alpha and beta-keratins are insoluble. Globular proteins have a compact structure that is frequently similar to a spherical aspect. The hydrogen links are internal and the contact areas between the molecules are little and this is why the intermolecular forces are not strong and globular proteins are water soluble.

Salting-out effect: This effect refers to partial dehydration of proteins, due to the competition between electrolytes' ions used in precipitance and the polar or ionic groups of the protein for water molecules. In this way protein molecules that are partially dehydrated start to precipitate.

Colloidal aspect: Proteins act like hydrophilic colloids in watery solutions due to their macromolecular configuration. Colloids are heterogeneous systems that are formed by a dispersed phase and a dispersant phase represented by water, buffer solution or the cellular cytoplasm.

Denaturizing: When chemical or physical agents act on proteins, these tend to lose their spatial structure and the physical, chemical and biologic properties. When denaturized, a substance's molecular mass remains unchanged and nor does its primary structure (aminoacid sequence) and this is explained by the fact that denaturizing process does not involve breaking polypeptide chains but a reorganizing process of its structure.

Hydrolysis: When proteins are acid, alkaline or enzymatic hydrolyzed the result of these reactions are aminoacids. Proteins' hydrolysis can be total or partial and only the first category of hydrolysis, the total one, has as a final result aminoacids.

Color reactions: Various aminoacids that form proteins react with substances and the result are color specific reactions that are used in order to identify certain proteins. In this category of reactions are: Millon reaction (characteristic for cyclic aminoacids that contain hydroxylic groups and it results a red precipitance), Sakaguchi reaction (arginin-specific-red or violet precipitance), biuret reaction-it results a blue or violet precipitance, xantoproteic reaction-yellow or orange precipitance, Amamkievicz-Hopkins reaction-violet precipitance, Pandy reaction-dark red precipitance, etc.


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