Physical Properties of Proteins
- Colour and Taste
Proteins are colourless and usually tasteless. These are homogeneous and crystalline.
- Shape and Size
The proteins range in shape from simple crystalloid spherical structures to long fibrillar structures. Two distinct patterns of shape
have been recognized :
A. Globular proteins- These are spherical in shape and occur mainly in plants, esp., in seeds and in leaf cells. These are bundles formed by folding and crumpling of protein chains. e.g., pepsin, edestin, insulin, ribonuclease etc.
B. Fibrillar proteins- These are thread-like or ellipsoidal in shape and occur generally in animal muscles. Most of the studies regarding protein structure have been conducted using these proteins. e.g., fibrinogen, myosin etc.
- Molecular Weight
The proteins generally have large molecular weights ranging between 5 × 103 and 1 × 106. It might be noted that the values of molecular weights of many proteins lie close to or multiples of 35,000 and 70,000.
- Colloidal Nature
Because of their giant size, the proteins exhibit many colloidal properties, such as; Their diffusion rates are extremely slow and they may produce considerable light-scattering in solution, thus resulting in visible turbidity (Tyndall effect).
Denaturation refers to the changes in the properties of a protein. In other words, it is the loss of biologic activity. In many instances the process of denaturation is followed by coagulation— a process where denatured protein molecules tend to form large aggregates and to precipitate from solution.
- Amphoteric Nature
Like amino acids, the proteins are amphoteric, i.e., they act as acids and alkalies both. These migrate in an electric field and the direction of migration depends upon the net charge possessed by the molecule. The net charge is influenced by the pH value. Each protein has a fixed value of isoelectric point (pl) at which it will move in an electric field.
- Ion Binding Capacity
The proteins can form salts with both cations and anions based on their net charge.
The solubility of proteins is influenced by pH. Solubility is lowest at isoelectric point and increases with increasing acidity or alkalinity. This is because when the protein molecules exist as either cations or anions, repulsive forces between ions are high, since all the molecules possess excess charges of the same sign. Thus, they will be more soluble than in the isoelectric state.
- Optical Activity
All protein solutions rotate the plane of polarized light to the left, i.e., these are levoratotory.
Chemical Properties of Proteins
Proteins are hydrolyzed by a variety of hydrolytic agents.
A. By acidic agents: Proteins, upon hydrolysis with conc. HCl (6–12N) at 100–110°C for 6 to 20 hrs, yield amino acids in the form of their hydrochlorides.
B. By alkaline agents: Proteins may also be hydrolyzed with 2N NaOH.
- Reactions involving COOH Group
A. Reaction with alkalies (Salt formation)
B. Reaction with alcohols (Esterification)
C. Reaction with amines
- Reactions involving NH2 Group
A. Reaction with mineral acids (Salt formation): When either free amino acids or proteins are treated with mineral acids like HCl, the acid salts are formed.
B. Reaction with formaldehyde: With formaldehyde, the hydroxy-methyl derivatives are formed.
C. Reaction with benzaldehyde: Schiff ‘s bases are formed
D. Reaction with nitrous acid (Van Slyke reaction): The amino acids react with HNO2 to liberate N2 gas and to produce the corresponding α-hydroxy acids.
E. Reaction with acylating agents (Acylation)
F. Reaction with FDNB or Sanger’s reagent
G. Reaction with dansyl chloride
- Reactions involving both COOH AND NH2 Group
A. Reaction with triketohydrindene hydrate (Ninhydrin reaction)
B. Reaction with phenyl isocyanate: With phenyl isocyanate, hydantoic acid is formed which in turn can be converted to hydantoin.
C. Reaction with phenyl isothiocyanate or Edman reagent
D. Reaction with phosgene: With phosgene, N-carboxyanhydride is formed
E. Reaction with carbon disulfide: With carbon disulfide, 2-thio-5-thiozolidone is produced
- Reactions involving R Group or Side Chain
A. Biuret test
B. Xanthoproteic test
C. Millon’s test
D. Folin’s test
E. Sakaguchi test
F. Pauly test
G. Ehrlich test
- Reactions involving SH Group
A. Nitroprusside test: Red colour develops with sodium nitroprusside in dilute NH4.OH. The test is specific for cysteine.
B. Sullivan test: Cysteine develops red colour in the presence of sodium 1, 2-naphthoquinone- 4-sulfonate and sodium hydrosulfite.