Why are viruses afraid of heat and not cold? What is the reason?

In fact, all viruses like cold and fear heat. However, few people talk about why viruses like cold and fear heat. This paper will explain it from a chemical point of view, starting with the structure of the virus.

What is the structure of the virus? In the middle of the virus is genetic material, that is, nucleic acid molecules-DNA or RNA, and outside are some protein molecules. Only when the virus finds a host can it replicate itself in the host cell; Without a host, the virus can't replicate itself at all, because it doesn't have its own metabolic mechanism and enzyme system. In other words, the virus leaving the host is not a complete life form, but just some molecules in the chemical sense.

So far, it seems to chemists that it is a very simple problem: rising temperature is not conducive to the stability of molecules.

As we all know, molecules are made up of atoms. Atoms can form molecules because there are interactions between atoms. Some interactions are so strong that valence bonds can be formed between atoms. For example, a hydrogen molecule is a system consisting of two hydrogen atoms. Two hydrogen nuclei (positively charged protons) repel each other, and two electrons (negatively charged) repel each other, but protons and electrons attract each other. According to quantum mechanics, it can be calculated that the energy of this system is related to the distance between two nuclei (nuclear spacing), as shown in the following figure.

We set the relative energy of the system when the two nuclei are far apart (that is, two independent atoms) as the zero point of energy. As can be seen from the figure, the energy of the system decreases when the two nuclei are close; But when the two nuclei are close together, the energy of the system increases rapidly. At the "balanced nuclear spacing" shown in the figure, the energy of the system is the lowest.

If the nuclear spacing is greater than or less than the equilibrium nuclear spacing, the energy of the system will increase, so it will be in an unstable state. It's like a hollow ball, which can only be balanced at the hollow bottom. If you leave the bottom, it will not be balanced and will automatically roll back to the bottom.

The equilibrium nuclear spacing of hydrogen molecules is about 0.074 nm, when the energy of hydrogen molecules is the lowest and the molecular system is the most stable. The length of the dotted line in the figure indicates the energy difference between a hydrogen molecule and two independent hydrogen atoms, that is, the bond energy of two hydrogen atoms forming valence bonds. This is the stable energy of hydrogen molecules. It is precisely because hydrogen molecules have lower energy than two separate hydrogen atoms that hydrogen molecules can exist stably.

But the atom itself has kinetic energy, so it should move freely and leave this equilibrium nuclear spacing. The existence of stable energy will pull the hydrogen atoms leaving the equilibrium nuclear spacing back to the equilibrium nuclear spacing. So under normal circumstances, hydrogen atoms vibrate near the equilibrium nuclear spacing. It's like a small ball falling into a pit and shaking at the bottom. If the pit is shallow and the kinetic energy of the ball is large, the ball may run out of the pit. Similarly, if the stable energy of the molecule is relatively small, but the kinetic energy of the atom is relatively large, the atom may break away from the stable energy and leave the molecule, leading to the dissociation of the molecule.

Obviously, the greater the stability energy of a molecule, the more stable it is. The greater the kinetic energy of atoms, the more it can destroy the stability of molecules. These are two contradictory aspects.

What does it mean that atoms have great kinetic energy?

From a macroscopic point of view, the system composed of these atoms has a high temperature. Temperature represents the average kinetic energy of the atoms that make up the system. As mentioned above, the greater the kinetic energy of atoms, the more it can destroy the stability of molecules, that is, the higher the system temperature, the worse the stability of molecules. At higher temperatures, molecules are easily dissociated.

Q 1

Since the higher the temperature, the worse the stability of molecules, so why don't we feel that oxygen, water, stones and other common substances decompose or deteriorate because of the temperature rise?

Answer 1

This is because the molecules that make up these substances are very stable molecules, that is, the valence bonds formed between the atoms that make up these molecules are very strong.

Taking the hydrogen molecule mentioned above as an example, its bond energy (that is, the energy required for bond breaking) is 2 17 kJ/mol. The "pit" in the above picture is simply a deep "well", and it takes great kinetic energy to jump out of this "well".

That is to say, it is very difficult to dissociate hydrogen molecules by heating to increase the temperature. At 2000K, only about 1‰ of hydrogen molecules will dissociate, and at 3000K, less than 10% will dissociate. Common water, oxygen and stones around us are all bonded by similar valence bonds, so they are all very stable molecules.

However, molecules like protein are different. Protein molecules are composed of hundreds of amino acid molecules, each of which has a dozen to dozens of atoms, and these thousands of atoms are also bound together by valence bonds. The arrangement order of amino acid molecules is different, so is protein molecule, which is the primary structure of protein molecule. This long string of atoms is not arranged in a long straight line. Due to the ability of each atom to attract electrons, the positive and negative charges around these atoms are also different. There is electrostatic interaction between these positive and negative charges, and the electrostatic interaction of parity bonds is much weaker, and some larger interactions are called "hydrogen bonds".

Under these electrostatic interactions, especially hydrogen bonds, the "lines" of the atomic arrangement that make up the protein curl and fold, forming the secondary structure of the protein. There is weak electrostatic interaction between the secondary structures, which constitutes the tertiary or even quaternary structure of protein. Protein has these very fine and ingenious advanced structures. Because the forces that form these advanced structures are weak electrostatic interactions, their stabilization energy, that is, the "pits" in the above figure, is very shallow. When the temperature is slightly higher, these advanced structures will be destroyed and protein will "deteriorate". When protein deteriorated, the virus was inactivated. So viruses like cold and fear heat.

Question 2

Since protein is so unstable, why can we and other creatures exist stably?

Answer 2

This is because all protein in organisms exist in living cells, and they are protected by the cellular environment, which increases their stability. More importantly, they are in the process of metabolism.

In other words, protein is constantly decomposing, and at the same time it is constantly generating, keeping the overall balance. The cells containing these protein are constantly metabolized. Once a living thing dies, protein in the body will deteriorate rapidly.

Along this line, we can also explain why bacteria don't like cold and heat like viruses, but prefer warm environment. The reason is that bacteria are living cells. When the temperature is low, although the protein is relatively stable, the growth and reproduction of cells will become slow or even dormant. In a warm environment, although protein is unstable and easy to decompose, its formation is fast, the growth and reproduction of cells are greatly accelerated, and the number of bacteria will still increase dramatically.

Viruses that invade our organisms depend on biological cells for survival and reproduction. Although the temperature rise makes them unstable, because they can replicate themselves quickly in biological cells, these viruses spread rapidly in the body and even endanger the life of organisms.

Because viruses like cold and fear heat, bacteria can grow and multiply rapidly in a relatively warm environment, so we can see that respiratory diseases caused by viruses such as influenza virus, SARS virus and novel coronavirus are easy to spread in cold weather conditions in winter and spring; In the warm environment in summer, digestive tract diseases caused by bacteria such as dysentery and diarrhea are prone to high incidence.