The role of deuterium in molecular evolution - статья

Secondary effects may still be of importance in biological systems sensitive to kinetic distortions. Deuterium also affects equilibrium constants, particularly the ionization constants of weak acids and bases in composition of macromolecules dissolved in heavy water (see a Table below). Acid strength of macromolecules in ²H₂O is decreased by factors of 2 to 5, and consequently, the rates of acid-base catalyzed reactions may be greatly different in ²H₂O as compared to H₂O. Such reactions frequently may be a faster in ²H₂O than H₂O solution (Covington A. K., Robinson R. A., and Bates R. G., 1966; Glasoe P. K., and Long F. A., 1960).

4.2. The chemical isotopic effect of ²H₂O.

The effect of isotopic replacement that has particularly attracted the attention of chemists is the kinetic isotope effect (Thomson J. F., 1963). The substitution of deuterium for hydrogen in a chemical bond of macromolecules can markedly affect the rate of scission of this bond, and so exert pronounced effects on the relative rates of chemical reactions going in ²H₂O with participation of macromolecules. This change in rate of scission of a bond resulting from the substitution of deuterium for hydrogen is a primary isotopic effect. The direction and magnitude of the isotope effect will depend on the kind of transition state involved in the activated reaction complex, but in general, deuterium depresses reaction rates. The usual terminology of the chemist to describe the primary kinetic effect is in terms of the ratio of the specific rate constants kh/kd. The maximum positive primary kinetic isotopic effect which can be expected at ordinary temperatures in a chemical reaction leading to rupture of bonds involving hydrogen can be readily calculated, and the maximum ratio kh/kd in macromolecules is in the range of 7 to 10 for C-H versus C-²H, N-H versus N-²H, and O-H versus O-²H bonds. However, maximum ratios are seldom observed for a variety of reasons, but values of kh/kd in the range of 2 to 5 are common (Wiberg K. B., 1955). Deuterium located at positions in a macromolecule other than at the reaction locus can also affect the rate of a reaction. Such an effect is a secondary isotope effect and is usually much smaller than a primary isotope effect.

In general, when the macromolecules transfer to deuterated medium not only water due to the reaction of an exchange (Н₂О -²Н₂О) dilutes with deuterium, but also occurs a very fast isotopic (¹Н-²Н)-exchange in hydroxylic (-OH), carboxilic (-COOH), sulfurhydrilic (-SH) and nitrogen (-NH; -NH₂) groups of all organic compounds including the nucleic acids and proteins. It is known, that in these conditions only С-²Н bond is not exposed to isotopic exchange and thereof only the species of macromolecules with С-²H type of bonds can be synthesized de novo. This is very probably, that the most effects, observed at adaptation to ²Н₂О are connected with the formation in ²Н₂О [U -²H]labeled molecules with conformations having the other structural and dynamic properties, than conformations, formed with participation of hydrogen, and consequently having other activity and biophysical properties.

So, according to the theory of absolute speeds the break of С-¹H-bonds can occur faster, than С-²H-bonds (C-²H-bonds are more durable than C-¹ , mobility of an ion ²H+ is less, than mobility of ¹Н+, the constant of ionization ²Н₂О is a little bit less than ionization constant of ²Н₂О. Thus, in principle, the structures of [U -²H]labeled macromolecules may to be more friable that those are forming in ordinary H₂O. But, nevertheless, the stability of [U -²H]labeled macromolecules probably depending on what particular bond is labeled with deuterium (covalent bonds -C²H that causing the instability or hydrogen bonds causing the stabilization of conformation of macromolecules via forming the three-dimentional netwok of hydrogen(deuterum) bonds in macromolecule) and what precise position of the macromolecule was labeled with deuterium. For example, the very valuable and sensitive for deuterium substitution position in macromolecule is the reactive center (primary isotopic effects). The non-essential positions in macromolecule are those ones that situated far away from the reactive center of macromolecule (secondary isotopic effects). It is also possible to make a conclusion, that the sensitivity of various macromolecules to substitution on ²Н bears the individual character and depending on the structure of macromolecule itself, and thus, can be varried. From the point of view of physical chemistry, the most sensitive to replacement of ¹Н+ on ²H+ can appear the apparatus of macromolecular biosyntesis and respiration system, those ones, which use high mobility of protons (deuterons) and high speed of break of hydrogen (deuterium) bonds. From that it is posible to assume, that the macromolecules should realize a special mechanisms (both at a level of primary structure and a folding of macromolecules) which could promote the stabilizition of the macromolecular structure in ²H₂O and somewhat the functional reorganization of their work in ²H₂O.

A principal feature of the structure of such biologically important compounds as proteins and nucleic acids is the maintenance of their structure by virtue of the participation of many hydrogen bonds in macromolecule. One may expect that the hydrogen bonds formed by of many deuterium will be different in their energy from those formed by proton. The differences in the nuclear mass of hydrogen and deuterium may possibly cause disturbances in the DNA-synthesis, leading to permanent changes in its structure and consequently in the cells genotype. The multiplication which would occur in macromolecules of even a small difference between a proton and a deuteron bond would certainly have the effect upon its structure.

The sensitivity of enzyme function to structure and the presumed sensitivity of nucleic acids function (genetic and mitotic) to its structure would lead one to expect a noticeable effect on the metabolic pattern and reproductive behavior of the organism. And next, the changes in dissociation constants of DNA and protein ionizable groups when transfer the macromolecule from water to ²H₂O may perturb the charge state of the DNA and protein. Substitution of ¹H for deuterium also affects the stability and geometry of hydrogen bonds in apparently rather complex way and may, through the changes in the hydrogen bond zero-point vibrational energies, alter the conformational dynamics of hydrogen (deuterium)-bonded structures within the DNA and protein in ²H₂O.

5. CONCLUSION

The successful adaptation of organisms to high concentration of ²H₂O will open a new avenues of investigation with using [U- ²H]labeled macromolecules could be isolated from these organisms. For example, fully deuterated essential macromolecules as proteins and nucleic acids will give promise of important biological, medical and diagnostical uses. Modern physical methods of study the structure of [U- ²H]labeled macromolecules, particularly three-dimentional NMR in a combination with crystallography methods, X-ray diffraction, IR-, and CD- spectroscopy should cast new light on many obscure problems concerning with the biological introduction of deuterium into molecules of DNA and proteins as well as the structure and the function of macromolecules in the presence of ²H₂O. The variety of these and other aspects of biophysical properties of fully deuterated macromolecules in the presence of ²H₂O remain an interesting task for the future.

First, I hope that the structural and the functional studies of [U- ²H]labeled macromolecules can provide us to the useful information about a many aspects of the synthesis of fully deuterated macromolecules and their biophysical behaviour in ²H₂O.

Second, the extensive body of available structural data about a cell protection system (at the level of the structure and the functioning of [U- ²H]labeled DNA and enzymes) will also form the basis for a particularly useful model for the study of biological adaptation to ²H₂O in aspect of molecular evolution of macromolecules with difined isotopic structures.

Finally, we also believe, the research can make a favour the medicine and biotechnology, especially for creating a fully deuterated analogues of enzymes and DNA having something different properties then the protonated species and working in the presence of ²H₂O.

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