Transportation of a substance in a biological system is a very complicated process.Absorption and transport of a substance often involve its solubility in the medium. Substances prefer to dissolve in water type fluid are said to be hydrophilic, whereas those prefer to dissolve in oily fluids are lipophilic. Absorption and transport of substances depend on their solubility in water and lipid media. The ultimate effect on cells, tissues, and organs must take place at the molecular level. However, effects at the molecular level are often not observable, and the symptoms of these effects may appear to be unrelated to the material in question.
Passive diffusion of a chemical is based on the difference in concentration of the chemical between the outside of the cell as compared to inside the cell. The greater the difference in concentration between the outside and the inside, the greater the diffusion of the chemical to the inside of the cell.
the ability of the chemical to diffuse across the membrane will be dependent on the lipophilic (fat loving) properties of the chemical. but that doesnt mean that molecule would be able to cross brain barrier if they cross cell mebrane easily, but most lipophilic substancec cross brain barier. molecular weight shoud be less than 5000 daltons in this case.
here is study suggest relation between substance lipophilic nature and crossing BBB.
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Blood-brain barrier permeation: molecular parameters governing passive diffusion.
Fischer H, Gottschlich R, Seelig A.
Department of Biophysical Chemistry, Biocenter of the University of Basel, Klingelbergstr, 70, CH-4056 Basel, Switzerland.
53 compounds with clinically established ability to cross or not to cross the blood-brain barrier by passive diffusion were characterized by means of surface activity measurements in terms of three parameters, i.e., the air-water partition coefficient, Kaw, the critical micelle concentration, CMCD, and the cross-sectional area, AD. A three-dimensional plot in which the surface area, AD, is plotted as a function of K-1aw and CMCD shows essentially three groups of compounds: (i) very hydrophobic compounds with large air-water partition coefficients and large cross-sectional areas, AD > 80 A2 which do not cross the blood-brain barrier, (ii) compounds with lower air-water partition coefficients and an average cross-sectional area, AD congruent with 50 A2 which easily cross the blood-brain barrier, and (iii) hydrophilic compounds with low air-water partition coefficients (AD < 50 A2) which cross the blood-brain barrier only if applied at high concentrations. It was shown that the lipid membrane-water partition coefficient, Klw, measured previously, can be correlated with the air-water partition coefficient if the additional work against the internal lateral bilayer pressure, pibi = 34 +/- 4 mN/m is taken into account. The partitioning into anisotropic lipid membranes decreases exponentially with increasing cross-sectional areas, AD, according to Klw = const. Kaw exp(-ADpibi/kT) where kT is the thermal energy. The cross-sectional area of the molecule oriented at a hydrophilic-hydrophobic interface is thus the main determinant for membrane permeation provided the molecule is surface active and has a pKa > 4 for acids and a pKa < 10 for bases
Increased permeability of the BBB is a feature of many diseases of the central nervous system (CNS)
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