= Solution
For hydrogen-bond acceptance, the three strongest groups are \b[the terminal <carboxylate>], \b[the Cys–CO–N(Pro) tertiary-amide <carbonyl>], and \b[the Pro–CO–NH(Tyr) secondary-amide <carbonyl>], in that qualitative order. Negative charge makes <carboxylate> a particularly strong acceptor. Both <amide> <oxygens> accept strongly; greater <nitrogen> alkyl donation generally makes a tertiary <amide> <oxygen> more basic than the comparable secondary-amide <oxygen>. This last ordering is a functional-group estimate, not a precise calculation of a folded <tripeptide>'s site parameters.
For donation, the order is \b[terminal ammonium $>$ <tyrosine> phenolic OH $>$ the peptide NH between Pro and Tyr]. The <thiol> is a weaker donor. Protonated ammonium has no available <lone pair> to accept; the <proline> <amide> <nitrogen> has no N–H bond to donate, and <amide> <nitrogen> <lone pairs> are delocalized into <carbonyl groups>. The <carboxylate> has no O–H donor in the drawn protonation state.
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