Pt dissolution under potentiostatic and potential cycling conditions have been investigated in a 0.5 M H 2 SO 4 solution at 298 K using a channel flow double electrode (CFDE) combined with EPMA and ICP-MS to clarify a dissolution mechanism of a Pt cathode catalyst in PEMFC. Under potentiostatic conditions, dissolved Pt ions are detected at a potential higher than 0.8 V with ICP-MS. The dissolution shows a maximum at E = 1.1 V and is suppressed by formation of 1-2.5 monolayers (ML) of Pt-O at E ≥ 1.2 V. Under potential cycling conditions, Pt dissolution is enhanced when the upper potential limit is higher than 0.8 V and the lower potential limit is less than 0.6 V, where the Pt-O is completely reduced. Dissolution is more enhanced as the upper potential limit shifts more positively. The CFDE study revealed that the dissolution is accelerated in the anodic scan under potential cycling even though more than 1 monolayer of Pt-O is formed, which is different from the potentiostatic conditions, and that the reductive dissolution of PtO 2 occurs in the cathodic scan. A Pt dissolution mechanism is proposed in simulated PEMFC cathode conditions.

A microelectrochemical system for in situ high-resolution optical microscopy was fabricated and applied to the real-time observation of pit initiation at MnS inclusion in type 304 stainless steel in NaCl solutions. It was directly observed that the metastable and stable pits were initiated at the MnS/steel boundaries, and that deep trenches were generated at these boundaries during anodic polarization. The initial rounded form of metastable and stable pits became polygonal in shape within 1 s. After that, the dissolution proceeded in the depth direction with no change in the appearance of the pit as observed externally. In the case of the metastable pitting, the duration of this stage was ca. 1.5 s, and then the pit repassivated, and the polygonal metastable pit remained on the electrode surface. The in-depth growth stage for stable pitting was relatively longer (ca. 3.5 s), and the pit grew deeply into the steel matrix and wrapped beneath the inclusion, leading to the formation of a large occluded cavity, in which the corrosivity considerably exceeded the critical conditions for autocatalytic pit growth. Chloride ions were shown to increase the probability of metastable pit initiation and affected the surface and cross-sectional morphology of stable pits.

Microscopic polarization, scanning transmission electron microscopy, and Raman spectroscopy were performed to ascertain the pit initiation mechanism at MnS inclusions in stainless steels. While the inclusion surfaces dissolved under anodic polarization in 0.1 M Na 2 SO 4 as well as 0.1 and 3 M NaCl solutions, the boundaries between the inclusions and the steel matrix dissolved selectively only in the NaCl solutions. This selective dissolution resulted in the formation of trenches, in which metastable and stable pits were initiated. The trenches were shown to be formed by the active dissolution of the steel sides of the boundaries, where no anomalous phase and no compositionally altered zone was observed. It was found that elemental sulfur was deposited on the inclusions and at the boundaries after anodic polarization in 3 M NaCl. The active dissolution of the steel matrix occurred in solutions in which chloride ions and elemental sulfur coexist. The synergistic effect of the elemental sulfur produced by MnS inclusion and chloride ions is likely to cause the trenches, and the decrease in both pH and potential inside the trenches results in pit initiation.Stainless steels suffer from pitting corrosion in chloride-containing environments. 1 Sulfide inclusions, such as MnS, are known to act as the initiation sites of pitting. 2-7 It is known that chromium in sulfide inclusions provides higher pitting potentials of the steels. 8 The sensitivity to pit initiation seems to be directly related to the solubility of sulfide inclusions; 9,10 however, how the dissolution of the inclusions affect the pit initiation is not well understood. In order for stainless steels to be used safely in severe environments, it is necessary that the mechanism of the pitting at MnS inclusions in stainless steels be clarified.Muto et al. demonstrated that the pitting was initiated at the boundaries between MnS inclusions and steel matrix. 11 It has been proposed that the synergistic effect of MnS dissolution products and chloride ions plays an important role in the initiation of pitting. 12,13 Under anodic polarization, the passivation of the steel matrix proceeds in near-neutral environments; however, MnS inclusions dissolve electrochemically in the passive region of the steels, which causes pitting corrosion in chloride-containing solutions. 12,14,15 Among the many types of sulfur-containing species that the electrochemical dissolution of MnS inclusions can produce are thiosulfate ion (S 2 O 3 2− ), solidstate elemental sulfur (S), hydrogen sulfide (H 2 S), and hydrogen sulfide ion (HS − ). This diversity is possible because sulfur can present itself in a range of states from the negative bivalent state to positive sexivalent state. Webb et al. and Lott et al. detected thiosulfate ions released from MnS inclusions under anodic polarization. 16,17 Krawiec et al., Ke et al., and Castle et al. demonstrated that the dissolution of MnS inclusions produces sulfur deposition on stainless steel matrix near the MnS inclusions. 18-20 Brossia et al. re...

Human oxyhemoglobin showed a biphasic autoxidation curve containing two rate constants, i.e. k f for the fast autoxidation due to the ␣ chains, and k s for the slow autoxidation of the ␤ chains, respectively. Consequently, the autoxidation of the HbO 2 tetramer produces two different curves from the pH dependence of k f and k s . The analysis of these curves revealed that the ␤ chain of the HbO 2 tetramer does not exhibit any protoncatalyzed autoxidation, unlike the ␣ chain, where a proton-catalyzed process involving the distal histidine residue can play a dominant role in the autoxidation rate. When the ␣ and ␤ chains were separated from the HbO 2 tetramer, however, each chain was oxidized much more rapidly than in the tetrameric parent. Moreover, the separated ␤ chain was recovered completely to strong acid catalysis in its autoxidation rate. These new findings lead us to conclude that the formation of the ␣ 1 ␤ 1 contact produces in the ␤ chain a conformational constraint whereby the distal histidine at position 63 is tilted away slightly from the bound dioxygen, preventing the proton-catalyzed displacement of O 2. by a solvent water molecule. The ␤ chains have thus acquired a delayed autoxidation in the HbO 2 tetramer.The reversible and stable binding of molecular oxygen to the heme iron(II) is the basis of hemoglobin function. Consequently, much attention has been directed to the elucidation of the molecular mechanism of cooperative oxygen binding to the hemoglobin tetramer (1). However, the oxygenated form of hemoglobin, as well as of myoglobin, is known to be oxidized easily to the ferric(III) met form, which cannot bind molecular oxygen and is therefore physiologically inactive, with generation of the superoxide anion (2-5).To this autoxidation reaction, it has been widely accepted that hemoglobin is much more resistant as compared with myoglobin. Unlike myoglobin, Mansouri and Winterhalter (6) reported that oxyhemoglobin (HbO 2 ) showed a biphasic autoxidation reaction with a fast and a slow component. They also demonstrated that the ␣ chain was oxidized more rapidly than the ␤ chain in hemoglobin tetramer. At the same time, however, there have been a number of reports that such a rate difference was not observed between the ␣ and ␤ chains in the autoxidation reaction of HbA (7-9). Rather, Zhang et al. (9) showed that the rate of autoxidation was markedly enhanced when the HbO 2 tetramer dissociates into ␣␤ dimers. To clarify these discrepancies, we have recently examined systematically the effect of hemoglobin concentration on the autoxidation rate at several different values of pH, and found that human HbO 2 exhibits a biphasic autoxidation curve only in the pH range from neutral to acidic (5). By dissociation of tetramers into ␣␤ dimers, the rate of autoxidation for the fast component (due to the ␣ chain) was also found to increase markedly at the acidic pH, but the addition of 2,3-diphosphoglyceric acid offered no significant effect on the increment of the autoxidation rate (5).In the present pape...

In order to investigate ligand binding sites in alpha-thrombin that interact with nonpolymerized fibrin, fibrinogen was conjugated (with CNBr) to Sepharose 4B and converted to the nonpolymerized fibrin resin with alpha-thrombin. Human alpha-thrombin was bound to the resin at 22 degrees C and eluted with a linear NaCl gradient [50-300 mM in 50 mM tris(hydroxymethyl)aminomethane hydrochloride, pH 7.6] with midpeak elution occurring at an ionic strength that corresponds to 170 +/- 5 mM NaCl. Among various ligands examined, ATP and its analogues caused alpha-thrombin to elute with 125 mM or less salt. Apparent dissociation constants were estimated by the dependence of elution volume on ligand concentration. The most potent ligands for desorption from the column were anionic (e.g., adenine nucleotides), which also inhibit thrombin esterolytic/amidolytic and clotting activity [Conery, B. G., & Berliner, L. J. (1983) Biochemistry 22, 369-375]. The desorption series was at 10 mM concentrations: ATP = ADP greater than pyrophosphate greater than citrate greater than oxalate greater than PO4(3-). Contrastingly, serotonin and related apolar compounds did not cause dissociation of alpha-thrombin from the fibrin resin, even though several of these substances inhibit fibrinogen clotting and esterolytic/amidolytic activities of the enzyme. These data imply that independent sites for apolar and anionic binding in alpha-thrombin are required for converting fibrinogen into clottable fibrin and that alpha-thrombin-fibrin binding involves an anionic site.

A positive correlation between age and the occurrence of thrombosis has been suggested. We studied the relationship between age and fibrinolytic activities; namely levels of tissue plasminogen activator (t-PA) antigen, plasminogen activator inhibitor (PA inhibitor) activity, and plasminogen activator activity (PA activity). A dramatic increase in both t-PA antigen and PA inhibitor was shown in persons with increasing age. PA activity decreased with age. Therefore it is suggested that the tendency of decreased PA activity with increasing age may be related to the high incidence of thrombosis in older persons.

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These findings provide a clue on the crucial role of haemoglobin molecule for senescent cell recognition or homeostasis in the blood circulation.