G. M. Mrevlishvili

Mechanisms of the Influence of UV Irradiation on Collagen and Collagen-Ascorbic Acid Solutions

The study of the influence of UV irradiation on collagen solutions has shown the destabilization of the collagen molecule by calorimetric method. It is reflected both in changes of thermodynamic parameters of transition (Tm, ΔH, Cp=f(t)) and in the appearance of a low temperature peak, that is practically irreversible against rescanning. All these indicate that the important defects in the molecule occur. The ESR measurements have shown that the above-mentioned thermal changes are connected with the occurrence of free radicals in solution under UV irradiation. They interact with proline (Pro) residues of the protein with the appearance of secondary free radicals, with following migration to glycine (Gly) residues. The emergence of the free radicals at the Pro and then at the Gly residues may cause the dramatic structural defect resulting from the UV irradiation, which significantly alters the network of hydrogen bonds in the triple helix of the collagen molecule. All this is connected with destabilization of the collagen molecule, because the defects in amino acid residues probably lead to cleavage of covalent bonds near the damaged sites maintaining the triple helical structure. The presence of ascorbic acid in collagen solution protects the collagen molecule from occurring of secondary free radicals.

Low-temperature heat capacity of biomacromolecules and the entropic cost of bound water in proteins and nucleic acids (DNA)

Abstract On the basis of the heat capacity data for proteins and DNA, obtained in the wide temperature range (2–300 K), the amount of the entropic cost of bound water in biomacromolecules is determined. The entropic cost of transferring a single water molecule from the liquid to a site of biopolymers is: 66.9, 58.1, 10.4 and 15.5 J mol −1 K −1 for fibrous protein (collagen), nucleic acid (double helical DNA), globular protein (Ribonuclease A), desoxynucleotides (d(AMP), d(TMP), d(GMP), d(CMP)) mechanical mixture and DNA polynucleotide chains in the state of statistical coils, respectively. These correspond to transfer-free energy costs as follows (at 298 K): 19.15, 17.5, 3.7 and 4.6 kJ mol −1 , respectively. We emphasize that the transfer entropy values determined here are not to be confused with the “entropy of hydration” of polar and nonpolar groups in biopolymers, which are relevant to the thermodynamics of protein folding or DNA double helix winding-unwinding.

Partial heat capacity change — Fundamental characteristic of the process of thermal denaturation of biological macromolecules (proteins and nucleic acids)☆

Abstract We prove experimentally that for any biopolymers (proteins and nucleic acids), the process of thermal denaturation in aqueous medium is characterized by change of the heat capacity regardless a unique spatial structures of different conformations. Absolute values and sign of ΔCP=Cp, denatured−Cp, native, depends primarily on conformational peculiarities of macromolecules and concrete mechanisms of interaction of water molecules with biopolymers chains — interaction is very clearly found to be of critical importance for the stability of the spatial structure of any biopolymers in aqueous medium.

Calorimetric investigation of DNA in the native and denatured states

Abstract The results of determination of apparent heat capacities of naturally occurring DNA in native (helix) and denatured (coils) states in dilute aqueous solutions have been reported. These results, supplemented by direct heat capacity measurements as a function of temperature, using a capillary scanning calorimeter, clearly show that the apparent heat capacity of polynucleotide chains of DNA in aqueous solution is strongly dependent upon the conformational state of the macromolecule. Comparison of data obtained for DNA in the “solid state”, at different levels of hydration and for DNA in H 2 O and D 2 O solutions reveals the possibility that the heat capacity increment ( Δc p = 0.36 ± 0.04 J g −1 K −1 ) is determined by an increase in the number of vibrational degrees of freedom, by hydrophobic effects and as a result of hydrogen bond destruction in water (including the ordered water clusters in the hydration shells of double helix of DNA).

Heat capacity peculiarities of DNA at low temperatures (2–25 K)

Abstract This study deals with the low-temperature heat capacity (2–30 K) of DNA native fibres measured at different moisture contents with respect to the specificity of DNA hydration caused by its chemical composition (GC contents). Some peculiarities have been found for the heat capacity dependence of DNA on temperature (Cp = f(T)) at 2–4 K (the maximum heat capacity). The Cp for DNA reflects the redundant low-energy density of vibrational states (DVS) contributions as well as the ordinary Debye contribution. It was concluded that whereas the peculiarities of the DNA heat capacity at very low energy, below, 1 K, are well explained by the common two-level tunnelling system (TLS) model, the nature of the redundant DVS at 3–10 K is connected with the location of the vibration on the heterogeneous parts of the structure; these areas in the hydrated fibres of DNA may represent clusters of hydrate water “grown” on DNA matrix with a specific size of 1–2 nm.

A microcalorimetric and electron spin resonance study of the influence of UV radiation on collagen

It was demonstrated microcalorimetrically that UV radiation cardinally changed the behavioral pattern of the temperature dependence of heat capacity of a collagen solution and decreased the enthalpy of collagen denaturation. It was discovered by electron spin resonance (ESR) that the primary products of UV-irradiated acid-soluble collagen were atomic hydrogen and the anion radical of acetic acid. The latter, when exposed to long-wavelength UV light, converted into the methyl radical, which interacted with acetic acid to produce the acetic acid radical. These free radicals interacted with the collagen molecule, leading to the appearance of seven superfine components with the split ΔH = 1.13 mT in the ESR spectrum. It was assumed that this spectrum is related to the free radical that occurred in the proline residue of the collagen molecule, which, in this particular case, was the major defect in the triple helix of collagen that caused instability of the macromolecule.

Hydration of DNA and Possible Role of Water in the Transition between Right- and Left-Handed Double Helix

As we already know from the classical X-ray studies, the conformation of DNA double helix depends to a great extent on water and different cation content[1,2]. Unfortunately, until now we have been forced to consider structural parameters of DNA molecules only in terms of spatial molecular models[3]. Only recently a noticeable progress has been made in this field[4–10]. The obtained data show[5] that structural and dynamic properties of polynucleotide chains with definite nucleotide sequence lead to the forms of double helix radically differing from B-DNA conformation. We are considering so-called Z-DNA, or left handed DNA with the structure determined on the basis of X-ray analysis of monocrystals of oligonucleotides with alternating pur-pyr sequences[4–7].