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In drugs and biomolecules, there are quite a few groups that are negatively charged around neutral pH (pH 5 to 9): carboxylates, sulfates, sulfonates, phosphates, phosphonates, tetrazoles, certain polycyclic phenolates.

There seem to be fewer options for positively charged groups: protonated amines (primary, secondary, tertiary), quaternary ammonium, and guanidinium. Notably, all these involve nitrogen. Imidazole doesn't stay positive in this entire pH range (pKa = 6.9). Do medicinal chemists have any other options for positively charged groups available?

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    $\begingroup$ Proteins binds to cations like magnesium, calcium or zinc when "they need a positive charge". A crown ether with potassium would also work. However, why not just use a nitrogen compound? $\endgroup$
    – Karsten
    Commented Oct 26, 2021 at 19:50
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    $\begingroup$ @KarstenTheis Crown ethers are an interesting idea. So is binding divalent metal ions. I’ve tried amines and guanidinium but they don’t quite have the right geometry and charge distribution for my application. Just trying to find some new ideas. $\endgroup$ Commented Oct 27, 2021 at 1:27

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