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.

Fluorinated compounds, including alkanes, alkenes, aromatics, amines, and cyclic and heterocyclic fluorochemicals, are studied as solvents of oxygen, using for this purpose the paramagnetic relaxation induced by the presence of molecular oxygen onto the I3C nuclei of neighboring solvent molecules in the solution. Relaxation data are expressed by the variation rates qx of relaxation rates TI-' per mole fraction of dissolved oxygen. A general correlation is found between the solubility of oxygen (in mole fraction) and the relaxation coefficients qx, the lower qx values being associated to the higher solubilities. This was accounted for by the existence of large cavities in the liquid for the better solvents, which can thus accommodate small gaseous molecules more easily, while simultaneously the magnetic dipolar interaction between oxygen and solvent nuclei is decreased. The sequence found for compounds of decreasing solubility, aliphatic > cyclic > aromatic, can be rationalized on the basis of cavities of decreasing size from one class of solvents to the other. The order of solubility seems to depend mainly upon the shape of solvent molecules and not upon the details of molecular structure, thus discarding the alternative assumption of specific fluorine-oxygen forces and of a possible charge-transfer complex. The higher relaxation coefficients obtained for some typical analogous hydrocarbons can be rationalized by using the same concepts.

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The macro- and microprotonations of glycylglycylhistamine (GGHA) have been determined by combined potentiometric and 1H-NMR methods. The complexation of GGHA with Co(II), Ni(II), and Cu(II) has been studied by potentiometric, EPR, and 1H-NMR methods. In the pH range 3−11.2, more or less deprotonated monomeric complexes (MLH, ML, MLH- 1, MLH- 2, MLH- 3) formed in all systems. In the case of Ni(II) and Cu(II) at physiological pH, the MLH- 2 species is predominant with four nitrogen coordination sites (one amino, two peptide, and one imidazole-N3 nitrogens) in square planar arrangement. In Co(II) containing systems however, CoL is the predominant complex near pH 7 with a macrochelate coordination of terminal amino and imidazole nitrogens, while CoLH- 2 species forms at much higher pH. In accordance with NMR measurements, the formation of MLH- 3 species can be assigned to the further deprotonation of the N1-pyrrolic nitrogen in the imidazole ring without metal coordination. The formation constants determined were compared with those of the analogous histidine derivatives. Single-crystal X-ray analysis of CuLH- 2·3H2O verified the expected four nitrogen coordination in the equatorial plane of Cu(II).

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