Relationship between Microlog p Macrolog P and log D

The distribution coefficient, D, is the same whether the species are described with microconstants or macroconstants. However, two types of log P are possible. Eq. (19) was derived with macroconstants. The second term in the numerator and denominator of Eq. (19) can be modified to utilize microconstants, drawing on Eqs. (34-40). From the substitutions, one can derive the following useful relation between micro- and macro-log P. We will use lowercase p to denote the micro partition constant.

where

The micro log pn° refers to the partitioning of just the unionized species between water and octanol. The micro log p^ refers to just the zwitterion partitioning. Fig. 15 illustrates the distinction between the two simultaneously occurring processes. If both the unionized microspecies and the zwitterion partition into the organic phase, Eq. (19) reduces to r, = Pnh ki° + Pnh (aa\

If Pn»Pi^ the equation can be further rearranged to log p°u - 6 + log D + log (l + kz + h (45)

where 6 = kz pn ±/(2.303 Pu ). If the zwitterion does not partition into the organic phase (<5=0), then Eq. (45) reduces to Eq. (9) reported by Takacs-Novak et al. [36].

Pm"

OCTANOL

m/cro-log p

MACRO-

Oct /ogP

Figure 15. Microconstant log p scheme.

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