The values of the electron self-exchange rate constants, kl I (~~) , for the copper(II/I) complexes formed with the cyclic tetrathiaethers [ 131aneS4 and [ 1 51aneS4 have been determined using IH-NMR line-broadening measurements in D20 at several different temperatures to yield the following results for 25 OC, corrected to p = 0.1 M (NO3-): for CuIV1 ([13]aneS4), kll(ex) = 3.2 X 105 M-1 s-1, AH* = 10 f 1 kJ mol-', AS* = -106 f 7 J K-1 mol-1; for CuII/1- ([15]aneS,), kll(cx) = 1.2 X 104 M-1 s-1, AH* = 21 f 1 kJ mol-1, AS* = -97 f 7 J K-1 mol-'. The cross-reaction rate constants have also been determined in aqueous solution by means of stopped-flow methods for these complexes reacting with a series of selected oxidants and reductants: CoII(Med[ 141 tetraeneN4)(H20)2, Ru1I(NHs)4bpy, RuI1(NH3)5py, RuI1(NH3)6, CoI1l(Me4[ 141 tetraeneN4)( H20)2, Ru1I1(NHd4bpy, NilI1( [ 14]aneN4)(H20)2, Ru1lI(NH3)2(bpy)2, and FeI1I(4,7-Me2phen)3. The self-exchange rate constants calculated by applying the Marcus relationship to the rate constants for reactions involving CuIIL reduction (kl I ( R~) ) are within experimental error of each other and agree with the kl values determined by NMR. However, as in earlier studies, the self-exchange rate constant values calculated from CuIL oxidation reactions (kl '(ox)) are generally smaller, except for very slow cross reactions. This pattern of behavior is in agreement with our previously proposed dual-pathway square scheme in which conformational change and the electron-transfer step occur in a sequential, rather than a concerted, manner. For the Cul1/1([ 131aneS4) system, a lower limit of k~p 1 200 s-I is estimated for the rate constant representing the conformational change from the ground state Cu(1) species to the metastable intermediate which precedes the electron transfer step via the preferred pathway. For the CuII/I( [ 1 51aneS4) system, a rough limit of k~p I 5 s-I is suggested by the data. Moreover, for this latter system, the secondary pathway was found to be more competitive than is the case for either the CulI/I([l3]aneS4) system or the previously studied Cu1I/1( [ 14laneS4) system, so that gated behavior is anticipated to occur only within a very narrow set of conditions. IntroductionStudies on CuII/IL systems (L = coordinated ligand) have often

The authors investigated early human focal ischemia with phosphorus-31 nuclear magnetic resonance spectroscopy at 1.89 T to characterize the temporal evolution and relationship of brain pH and phosphate energy metabolism. Data from 65 symptomatic patients were prospectively studied; none of the patients had had ischemic stroke in the internal carotid artery territory before. Twenty-eight neurologically normal individuals served as control subjects. Serial ischemic brain pH levels indicated a progression from early acidosis to subacute alkalosis. When acidosis was present there was a significant elevation in the relative signal intensity of inorganic phosphate (Pi) and significant reductions in signal intensities of alpha-adenosine triphosphate (ATP) and gamma-ATP compared with those of control subjects. Ischemic brain pH values directly correlated with the relative signal intensity of phosphocreatine (PCr) and the PCr index and inversely correlated with the signal intensity of Pi. There was a general lack of correlation between either ischemic brain pH or phosphate energy metabolism and the initial clinical stroke severity. The data suggest a link between high-energy phosphate metabolism and brain pH, especially during the period of ischemic brain acidosis, and the authors propose that effective acute stroke therapy should be instituted during this period.

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31P NMR spectra obtained in vivo reveal the presence of a few reasonably well defined chemical species, namely, ATP, orthophosphate (Pi), and, in brain, phosphocreatine. The chemical shifts of these resonances respond to changes in concentrations of ions such as H+ and Mg2+ in a manner that depends on both the chemical shifts intrinsic to individual complexes and the formation or binding constants for the several complexes. Values of the appropriate formation constants are well established in the literature. We have derived estimates of the chemical shifts intrinsic to the individual complexes by analyzing high resolution spectra of solutions whose composition brackets the domain of physiological relevance. This provides information sufficient to estimate intracellular concentrations of H+ and Mg2+ from chemical shifts seen with in vivo spectra. The primary finding is an estimate of 0.3 mM for the concentration of free magnesium in human brain. Differing values are obtained from other tissues.

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