Aqueous solutions of buthidazole {3-[5-(1,1-dimethylethyl)-1,3,4-thiadazol-2-yl]-4-hydroxy-1-methyl-2-imidazolidinone}, VEL 3510 {1-β,β-dimethoxyethyl-1-methyl-3-[5-(1,1-dimethylethyl)-1,3,4-thiadazol-2-yl] urea}, tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadazol-2-yl]-N,N′-dimethylurea}, fluridone 1-methyl-3-phenyl-5-[3-trifluoromethyl)phenyl]-4(1H)-pyridinone, metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one], and prometryn [2,4-bis(isopropylamino)-6-(methylthio)-s-triazine] were spectrophotometrically titrated to obtain pKA values. Ionizing properties of the chemicals were compared with other related compounds reported in the literature. The pKA values for the six herbicides were 0.6 and 11.3, 0.9 and 9.0, 1.2, 1.7, 1.1, and 4.1, respectively.

Electrospray ionization mass spectrometry (ESI-MS) was used to measure the binding of Cu2+ ions to synthetic peptides corresponding to sections of the sequence of the mature prion protein (PrP). ESI-MS demonstrates that Cu2+ is unique among divalent metal ions in binding to PrP and defines the location of the major Cu2+ binding site as the octarepeat region in the N-terminal domain, containing multiple copies of the repeat ProHisGlyGlyGlyTrpGlyGln. The stoichiometries of the complexes measured directly by ESI-MS are pH dependent: a peptide containing four octarepeats chelates two Cu2+ ions at pH 6 but four at pH 7.4. At the higher pH, the binding of multiple Cu2+ ions occurs with a high degree of cooperativity for peptides C-terminally extended to incorporate a fifth histidine. Dissociation constants for each Cu2+ ion binding to the octarepeat peptides, reported here for the first time, are mostly in the low micromolar range; for the addition of the third and fourth Cu2+ ions to the extended peptides at pH 7.4, KD's are <100 nM. N-terminal acetylation of the peptides caused some reduction in the stoichiometry of binding at both pH's. Cu2+ also binds to a peptide corresponding to the extreme N-terminus of PrP that precedes the octarepeats, arguing that this region of the sequence may also make a contribution to the Cu2+ complexation. Although the structure of the four-octarepeat peptide is not affected by pH changes in the absence of Cu2+, as judged by circular dichroism, Cu2+ binding induces a modest change at pH 6 and a major structural perturbation at pH 7.4. It is possible that PrP functions as a Cu2+ transporter by binding Cu2+ ions from the extracellular medium under physiologic conditions and then releasing some or all of this metal upon exposure to acidic pH in endosomes or secondary lysosomes.