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. Author manuscript; available in PMC: 2014 Jan 21.

Published in final edited form as: Chem Soc Rev. 2013 Jan 21;42(2):485–496. doi: 10.1039/c2cs35348b

(A), Proportion of compound categories for ionisable oral drugs.⁷ (B), pK_a distributions of oral drugs containing a single acidic functional group, or (C) single basic functional group. Compounds with the following criteria were classified as always ionised: acids with pK_a values < 0 and bases with pK_a values > 12, plus compounds with permanently charged groups (e.g., quaternary nitrogen atoms). Acids with pK_a values above 10 or bases with pK_a values below 0.0 were considered neutral. The remaining ionisable compounds (acid pK_a range 0-10 and base pK_a range 0-12) were divided into the following groups: single acid-containing substances, single base-containing substances, compounds with two acidic groups, compounds with two basic groups, simple ampholytes (one acidic and one basic group) and other complex combinations of acidic and basic groups (complex ampholytes). pK_a values were binned into single log unit ranges (i.e. 0.0 < X ≤ 1.0, 1.0 < X ≤ 2.0, etc.). The histogram column heights are expressed as a percentage.

(A), Proportion of compound categories for ionisable oral drugs.⁷ (B), pK_a distributions of oral drugs containing a single acidic functional group, or (C) single basic functional group. Compounds with the following criteria were classified as always ionised: acids with pK_a values < 0 and bases with pK_a values > 12, plus compounds with permanently charged groups (e.g., quaternary nitrogen atoms). Acids with pK_a values above 10 or bases with pK_a values below 0.0 were considered neutral. The remaining ionisable compounds (acid pK_a range 0-10 and base pK_a range 0-12) were divided into the following groups: single acid-containing substances, single base-containing substances, compounds with two acidic groups, compounds with two basic groups, simple ampholytes (one acidic and one basic group) and other complex combinations of acidic and basic groups (complex ampholytes). pK_a values were binned into single log unit ranges (i.e. 0.0 < X ≤ 1.0, 1.0 < X ≤ 2.0, etc.). The histogram column heights are expressed as a percentage.