Partition coefficients polar groups

Mathai and Singh have estimated the permeability coefficient P, using the formula P = kD where k is the partition coefficient and D is the diffusivity. They have used both parallel and series models to calculate P. The experimental values are always greater than measured values. The poor agreement between the experimental and calculated values is attributed to the polar-polar interaction between the epoxy group and nitrile group. 

In this model, one can argue that a peptide must have both an affinity for the interface (favorable n-octanol partition coefficient) and small desolvation energy (favorable A log PC) in order to efficiently cross a cell membrane. On the other hand, this model also predicts that a peptide with a large n-octanol/water partition coefficient and large desolvation energy, due to a significant number of polar groups, should adsorb and remain at the membrane interface. Both of these predicted events have been observed in the laboratory. 

Adsorption Partition Coefficients. Experimental KQC values were used when available otherwise, the K values were estimted. We used a correlation between aqueous solubility (Cs t) and Koc that contained data for pesticides and a group of polar and nonpolar organic chemicals collected by Kenega and Goring (18) and Smith et al. (3) ... 

Measurements of aqueous solubility and partition coefficient between cellulose acetate and water were compared for thirty disperse dyes and an approximate inverse relationship was postulated [60]. This can only be valid to a limited extent, however, because the partition ratio also depends on the saturation solubility of the dye in cellulose acetate. This property varies from dye to dye and is not directly related to aqueous solubility. The solubilities of four dyes in a range of solvents were compared with their saturation values on cellulose acetate. Solubilities in benzene showed no significant correlation. With the other solvents the degree of correlation increased in the order ethanol < ethyl acetate < 20% aqueous diethylene glycol diacetate (CH3COOCH2CH2OCH2CH2OCOCH3). The last-named compound was suggested as a model with polar groups similar to those in cellulose acetate [86]. 

The measurement of the solubility of drugs in polar and non-polar media is very important in the pharmaceutical field. One method proposed to describe this solubility is the partition coefficient between octanol and water. The mathematical calculation of an octanol-water partition coefficient from values for functional groups was first proposed by Hansch et al. as Hansch s n constants,1 and was later developed by Rekker as hydrophobic fragmental constants (logP).2 This method was further improved by the use of molecular connectivities.17 The prediction of logP values can be performed by either a computer program or by manual calculation. For example, approximate partition coefficients (log P) have been calculated by Rekker s method ... 

Richards, N., Williams, P. B., and Tute, M. (1991) Empirical methods for computing molecular partition coefficients. I. Upon the need to model the specific hydration of polar groups in fragment based approaches. Int. J. Quant. Chem. 18, 299-316. 

Leo et al. indicated that the van der Waals volume is linearly related to hydro-phobicity for non-polar compounds expressed in terms of log P (octanol/water)66). Moriguchi et al. showed that the log P value is generally factored into two components attributable to hydrophilic effect of polar group and hydrophobic effect due to the net molar volume 67). Thus, the van der Waals volume could be a parameter related to solute-solvent interactions and partition coefficient. 

Furthermore, for most compounds of interest to us, the octanol molecules present as cosolutes in the aqueous phase will have only a minor effect on the other organic compounds activity coefficients. Also, the activity coefficients of a series of apolar, monopolar, and bipolar compounds in wet versus dry octanol shows that, in most cases, Yu values changes by less than a factor of 2 to 3 when water is present in wet octanol (Dallas and Carr, 1992 Sherman et al., 1996 Komp and McLachlan, 1997a). Hence, as a first approximation, for nonpolar solvents, for w-octanol, and possibly for other solvents exhibiting polar groups, we may use Eq. 6-11 as a first approximation to estimate air- dry organic solvent partition constants for organic compounds as illustrated in Fig. 6.2. Conversely, experimental KM data may be used to estimate K,aw or Kitvi, if one or the other of these two constants is known. 

The structure of silica gel tends to change with time and this creates problems of irreproducibility in the separations. To remedy this situation and reduce the gel s polarity, the reactivity of silanol groups can be used to covalently bind organic molecules. Bonded stationary phases behave like liquids. However, the separation mechanism now depends on the partition coefficient instead of adsorption (Fig. 3.9). Bonded phases, whose polarity can be easily adjusted, constitute the basis of reversed phase partition chromatography, which is used in the majority of analyses by HPLC. 

For neutral organic compounds, in soils having a low clay/organic carbon ratio, sorption coefficients tend to increase as the hydrophobicity of the compound increases. Aqueous solubility or octanol/ water partition coefficients often are used as indicators of a compound s hydrophobicity. An increase in polarity, number of functional groups, and ionic nature of the chemical will increase the number of potential sorption mechanisms for a given chemical (Garbarini and Lion, 1985). For ionizable compounds, pKa is of particular importance because it determines the dominant form of a chemical at the specific environmental pH. 

The physico-chemical parameters of the chemical stimuli which have been shown to have relevance and to be interrelated to the sensory response it elicits as specific odor or taste, are the factors controlling concentration at the receptor areas (solubility, hydrophilicity, lipophilicity, volatility, and partition coefficients), molecular features (size, shape, stereochemical and chirality factors and functional groups), and electronic features (polarity and dipoles) controlling positioning and contact at receptor surfaces (53). Many of these physico-chemical data are not available for many of the chemical stimulants, and till they are gathered, structure-response studies will be much restricted. The effects of interactions of the above parameters appear to a larger degree in the perception of odor, the dimensions of which are many and complex viz. nuances, composite... 

Various aspects of structure activity relationships in 4,5-epoxymorphinans have been reviewed,(201,46 U467) and specific consideration has been given to quantum chemical studies with particular reference to polar group variation,(468) stereoisomeric ligands as receptor probes, 469 partition and distribution coefficients, 470 471 and antagonists. 145 ... 

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