8. Properties of HMC (High Molecular weight Compounds) in water solutions

The substances with a huge value of molecular mass (Mr~10⁴) belong to HMC (High Molecular weight Compounds). Usually, they contain repeating elementary units. Some examples include both synthetic or artificial (polyethylene, viscose), and natural polymers (polysaccharides, proteins, nucleic acids), called biopolymers.

These compounds show some specific properties: ability to swell, formation of gels, liquid-crystal state, anomalous viscosity and other rheological (flow) characteristics, and so on.

Viscosity (η, eta) is defined as the resistance to movement. There are different types of viscosity (dynamic, relative, specific), dynamic viscosity has units like Pascal-second [Pa ∙ s] or Poise [P], 1 Pa ∙ s = 10 P.

The value of viscosity in polymer solutions is very high and decreases under pressure (anomalies of η), as the molecules of HMC change their conformation and tend to stretch along the stream.

The dynamic viscosity of blood plasma is about 0.015 Poise (1.5 cP), viscosity of the whole blood is about 3.2 cP, or 3.2 ∙ 10^–3 Pa∙s.

Blood viscosity is 3 times greater than viscosity of water due to the presence of the huge amount of RBC, which create extra rubbing forces. Viscosity of blood depends on Hematocrit (Ht or HCT), which is defined as the ratio of erythrocytes volume to blood volume, %. Separation of blood in a centrifuge gives two main layers: plasma (~55% of the total volume) and RBC (~45% normally). If Ht = 60–70% (polycythemia), viscosity of blood becomes 10 times greater than viscosity of water, and its movement in blood vessels delays, vascular blood flow decreases. Anemia leads to a decrease in RBC volume (Ht <30%).

There are many diseases accompanied by Ht <N (excess of fluid in blood) or Ht >N (dense blood), which helps in diagnostics.

Hydrophilicity of most biopolymers appears due to polar organic functional groups, as well as to the formation of the highest structures: hydrophobic fragments are hidden inside of molecule, hydrophilic ones are situated on the outside, this position leads to the interaction with water dipoles and formation of hydrated covers. Consequently, as distinct from hydrophobic micelle, hydrophilic HMC molecule has two protection factors: hydrated shell and charge, that is why HMC colloids are not only more stable but can also protect sols from coagulation, caused by addition of electrolytes. Golden number serves as a characteristic of protective action.

Proteins are HMC, which exist in biological fluids as polycations or polyanions. As any protein contains amino acids, there are both carboxylic and amino groups, in ionized state. Isoelectric state (IES) is such a state, when the number of positive charges of molecule is equal to the number of negative ones (look at the middle of scheme presented below). In this neutral state, point of “zero charge”, electrophoretic movement of protein particles is absent, and that may be proved experimentally.

The value of рН, when the surface of protein has no charge, is called isoelectric point (рI). This рI depends on the number of ionogenic groups and their K_d, dissociation constants, which may change slightly according to surroundings (presence of donor or acceptor substitutes, inductive and mesomeric effects of neighbouring groups). Isoelectric point (рI) at standard temperature is an individual characteristic of protein or amino acid and may be found in the special reference tables (handbooks of chemistry). If the protein molecule contains equal numbers of carboxylic and amino groups, рI belongs to the following interval: 4.6–6.6, which corresponds to the simple mean of рК_d of these groups. Any additional –СООН, –SН and another acidic group shifts рI of protein to acidic area. Presence of additional nitrogen-containing groups contributes to basic shift of рI.

Both decrease and increase of рН relatively рI leads to the charge of protein molecule (look at the scheme). So, if рН <рI , excess H⁺ ions react with ionized carboxylic groups –COO^–, and uncompensated –NH₃⁺ groups create positive charge. And vice versa, when рН >рI, surplus hydroxyl ions react with ionized amino groups, and uncompensated carboxylic groups create negative charge.

Proteins of blood create oncotic pressure, which normally changes within 0.03–0.04 atm. This is a small part of osmotic pressure (compare with π_osm = 7.7 atm), responsible for protein-water balance, homeostasis. If a decrease of oncotic pressure happens (starvation, renal diseases), water moves into intercellular space, and oncotic edema of subcutaneous cellular tissue appears. The rise of protein content in blood increases oncotic pressure. Another factor with the same result is water loss.

To get the protein sediments, denaturation and salting-out are used. The main information about denaturation:

1) all the structures of protein, excepting primary structure, will be destroyed during this process (further exposure may destroy the primary structure, the sequence of amino acids, as well, but these processes do not belong to denaturation, they include decomposition, oxidation, hydrolysis, and so on);

2) this process is irreversible in general (or may be reversible at the beginning of influence, if exposure is weak or short);

3) some factors, which may cause denaturation, include: heating, change in external pressure, beaming, irradiation, the addition of concentrated solutions of acids or alkali, some organic solvents, salts of heavy metals (reactions with ions of heavy metals are used in the clinic for the detection of protein in biological fluids).

Salting-out is characterized by the following features:

1) all the structures, including the primary structure, are intact; sedimentation of protein happens due to the loss of hydrated covers (unlike coagulation, there is no agglomeration during salting-out);