There is a tremendous amount of matter on Earth that is not just difficult, but impossible to call homogeneous. We are talking about biological structures and living matter in general.
The National Nanotechnology Initiative cites as one of the reasons for the special interest in the nanoscale field Since the systemic organization of matter at the nanoscale is a key feature of biological systems, nanoscience and technology will make it possible to incorporate artificial components and ensembles into cells, thereby creating new structurally organized materials based on imitation of self-assembly methods in nature.
Now let’s try to understand the meaning of the concept “nanoscale” as applied to biology, keeping in mind that the properties should change fundamentally or drastically during transition to this dimensional interval. But first, let’s remember that the nanoscale can be approached in two ways: “top-down” (fragmentation) or “bottom-up” (synthesis). So, the “bottom-up” movement for biology is nothing but the formation of biologically active complexes from individual molecules.
Let’s take a brief look at the chemical bonds that determine the structure and shape of a molecule. The first and strongest is the covalent bond, which is characterized by a strict orientation (only from one atom to another) and a certain length, which depends on the type of bond (single, double, triple, etc.). It is the covalent bonds between atoms that determine the “primary structure” of any molecule, i.e., which atoms and in what order they are connected to each other.
But there are other types of bonds that determine what is called the secondary structure of a molecule, its form. First of all, there is the hydrogen bond, a bond between a polar atom and a hydrogen atom. It is closest to the covalent bond, as it is also characterized by a certain length and directionality. However, this bond is weak, its energy is an order of magnitude lower than the energy of the covalent bond. The other types of interactions are non-directional and are characterized not by the length of the bonds formed, but by the rate of bonding energy decrease with the increase in the distance between the interacting atoms (long-range action). Ionic bonding is a long-range interaction, van der Waals interactions are short-range. Thus, if the distance between two particles increases by a factor r, the attraction will decrease to 1/r2 of the initial value in the case of ionic bonding, and to 1/r3 or more (up to 1/r12) in the case of the van der Waals interaction mentioned more than once. All these interactions can generally be defined as intermolecular interactions.
Let us now consider such a concept as “biologically active molecule. It should be recognized that the molecule of a substance by itself is of interest only to chemists and physicists. They are interested in its structure (“primary structure”), its form (“secondary structure”), such macroscopic parameters as, for example, aggregate state, solubility, melting and boiling points, etc., and microscopic ones (electronic effects and mutual influence of atoms in this molecule, spectral properties as manifestation of these interactions). In other words, we are talking about the study of the properties manifested in principle by a single molecule. Recall that, by definition, a molecule is the smallest particle of a substance that carries its chemical properties.
From the point of view of biology, an “isolated” molecule (in this case, it does not matter whether it is a single molecule or a number of identical molecules) is not capable of exhibiting any biological properties. This thesis sounds quite paradoxical, but let us try to substantiate it.
Let’s look at the example of enzymes, protein molecules that are biochemical catalysts. For example, the enzyme hemoglobin, which ensures the transport of oxygen in tissues, consists of four protein molecules (subunits) and one so-called prosthetic group – a heme containing an iron atom that is noncovalently bound to the protein subunits of hemoglobin.
The forces, sometimes referred to as hydrophobic interactions, which are the forces of intermolecular interaction, make the main, or rather, the determining contribution to the interaction of protein subunits and heme, the interaction leading to the formation and stability of the supramolecular complex, which is called hemoglobin. The bonds formed by these forces are much weaker than covalent bonds. But in complementary interaction, when two surfaces are very close to each other, the number of these weak bonds is large, and therefore the total interaction energy of the molecules is quite high and the resulting complex is quite stable. But until these bonds between the four subunits are formed, until a prosthetic group (gemm) is attached (again, due to non-covalent bonds), under no circumstances can individual parts of hemoglobin bind oxygen, much less carry it anywhere. And, consequently, they do not have this biological activity. (The same reasoning can be extended to all enzymes in general).
At the same time, the process of catalysis itself implies the formation during the reaction of a complex of at least two components – the catalyst itself and molecule(s), called substrate(s), undergoing some chemical transformations under the action of the catalyst. In other words, at least two molecules should form a complex, i.e. a supramolecular (supramolecular) complex.
The idea of complementary interaction was first proposed by E. Fischer to explain the interaction of drugs with their targets in the body and called the “key to lock” interaction. Although drugs (and other biological substances) are far from being enzymes in all cases, they can cause any biological effect only after interacting with the corresponding biological target. And this interaction, again, is nothing other than the formation of a supramolecular complex.
Consequently, if “common” molecules exhibit principally new properties (in the considered case – biological activity), they form supramolecular (supramolecular) complexes with other molecules due to intermolecular interaction forces. This is the way most enzymes and systems in the body (receptors, membranes, etc.) are arranged, including such complex structures which are sometimes called biological “machines” (ribosomes, ATPase, etc.). And it happens exactly at the level of nanometer dimensions – from one to several tens of nanometers.
Further increasing in complexity and size (over 100 nm), i.e. during the transition to another dimensional level (microlevel), considerably more complex systems emerge, capable not only of independent existence and interaction (in particular, of energy exchange) with their environment, but also of self-reproduction. That is, the properties of the whole system change again – it becomes so complex that it is already capable of self-reproduction, and what we call living structures emerges.
Many thinkers have repeatedly tried to define Life. Without going into philosophical discussions, let us note that, in our opinion, life is the existence of self-reproducing structures, and living structures begin with a single cell. Life is a microscopic and macroscopic phenomenon, while the main processes ensuring living systems functioning occur at the level of nanoscopic dimensions.
Functioning of a living cell as an integrated self-regulating device with distinct structural hierarchy is provided by miniaturization at nanoscopic level. It is obvious that miniaturization at the nanoscale level is a fundamental attribute of biochemistry and, therefore, the evolution of life consists of the appearance and integration of various forms of nanostructured objects.13 It is the nanoscale section of the structural hierarchy, limited in size both above and below (!), that is critical for the appearance and ability to exist of cells. That is, it is the nanoscale level that represents the transition from the molecular level to the Living level.
However, because miniaturization at the nanoscale is a fundamental attribute of biochemistry, we cannot still consider any biochemical manipulation as nanotechnology – nanotechnology implies construction, rather than the trivial application of molecules and particles.
