RESUMO
Acid-base properties of lyophilized powders of L-histidine have been systematically investigated using parent solutions at pH varying from 1.8 to 10. For the first time, high-resolution solid-state 13C NMR was shown to allow separate observation of all three acid-base pairs in the successive deprotonations of the carboxylic end, the imidazolium cation, and the terminal ammonio group of histidine. 1H CRAMPS NMR spectra directly visualize the absence of the N3-H(pi) tautomer in neutral and anionic species. Solid-state titration shifts are enlarged by approximately 1-4 ppm with respect to those measured in solution, permitting unambiguous observation of conjugate acid-base pairs. Calculated pK's from solid-state acid-to-base ratios r are found equal to those classically measured in solution at 0 degrees C with a similar ionic strength of 0.1 mol x dm(-3). This proves that natural-abundance 13C solid-state determinations of r can be used to measure pK's in parent solutions without recourse to full titration curves and subsequent curve-fitting procedures. Such an approach also leads to noninvasive characterizations of the acidity of lyophilized powders, i.e., to the prediction of in situ pH of products obtained after rehydration and solubilization of powders. These results show the possibility of measuring the pK of nonvolatile acidic substrates dissolved in any sublimable solvent through lyophilization of the investigated solutions; this leads the way to pH and pK determinations when electrochemical or spectrophotometric measurements are impossible or ambiguous, e.g., for concentrated solutions, polyacids, or mixtures of acidic solutes, and possibly to the establishment of pK scales in nonaqueous solvents and in melts.
RESUMO
The complex formation equilibria between pH 2 and 11.2 in Co(II)-L-O(2) ternary systems (L = histamine, glycylhistamine, and sarcosylhistamine) have been studied by pH-metric, spectrophotometric, and (1)H-NMR spectroscopic methods. In contrast to earlier findings, we detected several protonation states of oxygen-carrying complexes in the pH range 7-11.2. The active species in oxygen uptake is the CoL(2) complex already suggested in the Co(II)-histamine-O(2) system; however, in the case of pseudopeptides, both CoLH(-)(1) and CoL(2)H(-)(1) complexes can take up oxygen. The (1)H-NMR study revealed stereoisomerism of oxygenated species in the same protonation states, undergoing slow ligand exchange. In the case of the Co(2)(Hist)(4)(OH)(O(2)) complex, among the 20 possible geometrical isomers, at least six can be distinguished by their imidazole proton signals. The most abundant isomers have axial-equatorial imidazole coordination, presumably on steric grounds. The stability constants determined by pH studies for the above systems proved that {4N}-coordinated species have a greater affinity for oxygen uptake than the {3N}-coordinated ones. They also showed the stabilizing effect of the deprotonated amide nitrogen in the oxygenated complexes of pseudopeptides.
