Solvent in biological systems

Hansen CM, Andersen BH. 1988. The affinities of organic solvents in biological systems. Am Ind Hyg Assoc J 49 301-308. 

Other uses of the solubility parameter theory include pigment-solvent interactions in terms of suspension behavior, the compatibility of plasticizers and polymers, the critical strain behavior of commercial plastics in the presence of solvents, the effects of solvents on other mechanical properties of the polymers and the affinities of organic solvents in biological systems. Equation 1.3, which uses the three partial Hansen solubility parameters, can be used to estimate the surface tension of a liquid. 

Oil A broad range of inflammable and often volatile organic compounds insoluble in water but soluble in organic solvents. In biological systems, a fat that is liquid at room temperature (20°C). 

Hansen and Anderson (1988) examined the affinities of organic solvents in biological systems using solubility paramter methods, and introduced the term realtive energy difference (RED) as a measure of the relative affinity of a solvent for a material. The RED is defined as the ratio of R/ R or the radius of interaction of the solvent-substance divided by the characteristic radius of the substance. If RED < 1.0, the solvent would be expected to be compatible with the substance, whereas for RED > 1.0, poorer solvents would be expected. An RED value of zero would indicate a perfect match between solubility values of the solvent and the substance. 

For mixture.s the picture is different. Unless the mixture is to be examined by MS/MS methods, usually it will be necessary to separate it into its individual components. This separation is most often done by gas or liquid chromatography. In the latter, small quantities of emerging mixture components dissolved in elution solvent would be laborious to deal with if each component had to be first isolated by evaporation of solvent before its introduction into the mass spectrometer. In such circumstances, the direct introduction, removal of solvent, and ionization provided by electrospray is a boon and puts LC/MS on a level with GC/MS for mixture analysis. Further, GC is normally concerned with volatile, relatively low-molecular-weight compounds and is of little or no use for the many polar, water soluble, high-molecular-mass substances such as the peptides, proteins, carbohydrates, nucleotides, and similar substances found in biological systems. LC/MS with an electrospray interface is frequently used in biochemical research and medical analysis. 

The concentration of salt in physiological systems is on the order of 150 mM, which corresponds to approximately 350 water molecules for each cation-anion pair. Eor this reason, investigations of salt effects in biological systems using detailed atomic models and molecular dynamic simulations become rapidly prohibitive, and mean-field treatments based on continuum electrostatics are advantageous. Such approximations, which were pioneered by Debye and Huckel [11], are valid at moderately low ionic concentration when core-core interactions between the mobile ions can be neglected. Briefly, the spatial density throughout the solvent is assumed to depend only on the local electrostatic poten-... 

The treatment of electrostatics and dielectric effects in molecular mechanics calculations necessary for redox property calculations can be divided into two issues electronic polarization contributions to the dielectric response and reorientational polarization contributions to the dielectric response. Without reorientation, the electronic polarization contribution to e is 2 for the types of atoms found in biological systems. The reorientational contribution is due to the reorientation of polar groups by charges. In the protein, the reorientation is restricted by the bonding between the polar groups, whereas in water the reorientation is enhanced owing to cooperative effects of the freely rotating solvent molecules. 

Similar behavior can occur when a crystalline network is disassembled by adding a solvent rather than by heating. These mesogens are called lyotropic liquid crystals and the mesophase formation shows temperature and concentration dependence. They are very important in biological systems, but have been much less studied in materials science. 

Low affinity to polar solvents and fullerenes aggregation in water limit their use in biologic systems. To increase water solubility of fullerenes, few ways are used solubilization with the use of some water-soluble polymers like PDT or polyvin-ilpyrrolidone, generation of complexes with cyclodextrines or calixarenes, and... 

One may wonder whether a purely harmonic model is always realistic in biological systems, since strongly unharmonic motions are expected at room temperature in proteins [30,31,32] and in the solvent. Marcus has demonstrated that it is possible to go beyond the harmonic approximation for the nuclear motions if the temperature is high enough so that they can be treated classically. More specifically, he has examined the situation in which the motions coupled to the electron transfer process include quantum modes, as well as classical modes which describe the reorientations of the medium dipoles. Marcus has shown that the rate expression is then identical to that obtained when these reorientations are represented by harmonic oscillators in the high temperature limit, provided that AU° is replaced by the free energy variation AG [33]. In practice, tractable expressions can be derived only in special cases, and we will summarize below the formulae that are more commonly used in the applications. 

Solvent extraction deals with the transport of chemical substances from one phase into another one, the chemical kinetics of this process, and the final equilibrium distribution of the substances between the two phases. Such transport and distribution processes are the motors that make life in biological systems possible. Fundamental studies of such solvent extraction processes contribute to the better understanding of all processes in nature. Here, only the lack of imagination stands in the way of important new scientific discoveries. 

Molecular conformation is highly related to functional properties. Since the conformation of the crystalline solids can be precisely determined by diffraction methods, molecular modeling is most important for interpreting molecular structures in solution. This is, however, even more difficult for theoreticians. While carbohydrates dissolve in a variety of solvents, the important solvent for biological systems is water and this solvent deserves special emphasis. 

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