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The changes in vibrational absorption and vibrational circular dichroism (VCD) spectral features with structural changes of polytiboadenylic acid (polyA) as a function of pH are reported. The infrared absorption and VCD spectral changes are reported in both solution and film states.

We apply the photon counting histogram (PCH) model, a fluorescence technique with single-molecule sensitivity, to study pH-induced conformational changes of cytochrome c. PCH is able to distinguish different protein conformations based on the brightness of a fluorophore sensitive to its local environment. We label cytochrome c through its single free cysteine with tetramethylrhodamine-5-maleimide (TMR), a fluorophore with specific brightnesses that we associate with specific protein conformations. Ensemble measurements demonstrate two different fluorescence responses with increasing pH: (i) a decrease in fluorescence intensity caused by the alkaline transition of cytochrome c (pH 7.0-9.5), and (ii) an increase in intensity when the protein unfolds (pH 9.5-10.8). The magnitudes of these two responses depend strongly on the molar ratio of TMR used to label cytochrome c. Using PCH we determine that this effect arises from the proportion of a nonfunctional conformation in the sample, which can be differentiated from the functional conformation. We further determine the causes of each ensemble fluorescence response: (i) during the alkaline transition, the fluorophore enters a dark state and discrete conformations are observed, and (ii) as cytochrome c unfolds, the fluorophore incrementally brightens, but discrete conformations are no longer resolved. Moreover, we also show that functional TMR-cytochrome c undergoes a response of identical magnitude regardless of the proportion of nonfunctional protein in the sample. As expected for a technique with single-molecule sensitivity, we demonstrate that PCH can directly observe the most relevant conformation, unlike ensemble fluorometry. single-molecule fluorescence spectroscopy | protein conformations protein labeling | confocal microscopy | metalloprotein

A study on chemomechanical mechanism leading to morphogenetic instabilities is demonstrated experimentally. The results reveal that in constant and uniform nonequilibrium chemical environment the responsive gel undergoes large amplitude dynamical deformations under the form of traveling contraction waves and complex spatio-temporal volume oscillations.

Abstract -- It was found that the correction for organic acid anions to the alkalinity of water depends on the concentration of these anions, their dissociation constant, and pH; for [...]

Oocytes grow within ovarian follicles, and only gain the ability to complete meiosis when they are nearly fully grown. We have found that both of the major types of intracellular pH regulatory mechanisms in the mammal--the [Na.sup.+]/[H.sup.+] and HC[O.sup.-.sub.3]/[C1.sup.-] exchangers--were essentially inactive in mouse oocytes over most of the course of their growth. However, as oocytes approached full size, [Na.sup.+]/[H.sup.+] and HC[O.sup.-.sub.3]/[C1.sup.-] exchangers became simultaneously active, and, at the same time, the intracellular pH of isolated oocytes increased sharply by about 0.25 pH unit. This activation of intracellular pH regulatory mechanisms and increase in pH occurred coincident with the acquisition of meiotic competence. The activation of pH regulatory mechanisms during oocyte growth represents a previously unknown milestone in the development of the capacity of the oocyte to function independently upon ovulation. Keywords: Follicle; HC[O.sup.-.sub.3]/[Cl.sup.-] exchange; Meiotic competence; Oocyte; Ovary; [Na.sup.+]/[H.sup.+] exchange; pH regulation

The gas-phase free energy difference between nine tetrazole derivatives and their anions are calculated using the CBS-QB3 method. The calculations are performed on both gas-phase and solvent-phase optimized structures.

Byline: Alexandre Carlos Camacho Rodrigues (1) Keywords: hydrotalcite; basicity; copper; cobalt; iron Abstract: An intuitive and computationally non-intensive model for the classification of Hydrotalcite-like compounds (HTLCs) based simply on the chemical composition using the Sanderson Method led to good prediction of basicity and different basic sites (oxygen atoms with different charge). That model was evaluated at different M.sup.3+ /(M.sup.2++M.sup.3+) ratio and with different divalent and trivalent metallic cations. Author Affiliation: (1) NUCAT/PEQ/COPPE/UFRJ, Ilha do Fundao, Universidade Federal do Rio de Janeiro, P.O. Box 68502, 21945-970, Rio de Janeiro, Brazil Article History: Registration Date: 03/12/2004 Received Date: 28/06/2004 Article note: AMS subject classification: 92E10

Abstract: An extraction and sensitive differential pulse anodic stripping voltammetric (DPASV) method at a hanging mercury drop electrode (HMDE) is described for the determination of Zn, Cu, Pb ([micro]g [g.sup.1]) [...]

The pH and temperature dependences of tyrosine oxidation were measured in reactions centers from mutants of Rhodobacter spheroids containing a tyrosine residue near a highly oxidizing bacteriocholorphyll. Details about the yield of tyrosyl formation measured using electron paramagnetic resonance spectroscopy are discussed. Also regarding the variation in temperature, which affects the rate of formation of tyrosyl, is mentioned.

The mechanism for controlling the optimum pH of beta-amylase is explored by constructing five mutants of Bacillus cereus beta-amylase (BCB) and characterizing with respect to enzymatic properties and X-ray structural analysis. The findings suggest that a water-mediated hydrogen bond network (Glu 367...W1...Tyr 164... Thr 328) is the primary requisite for the increased pH optimum of wild-type BCB.

Byline: A. A. Fedorov (1), T. S. Shmata (1) Abstract: The mathematical tools, a universal algorithm, and a program for the calculation and construction of titration curves in titrimetric analysis are presented. Author Affiliation: (1) Fed'kovich State University, ul. Kotsyubinskogo 2, Chernovtsy, 58012, Ukraine Article History: Registration Date: 06/10/2004

The electrochemical behaviour of a cobalt rotating electrode in [H.sub.2]C[O.sub.3]/HC[O.sub.3]C[O.sup.2-.sub.3] aqueous solutions was investigated in the pH region from 7 to 9. The effects of [H.sub.2]C[O.sub.3]/HC[O.sub.3]-/C[O.sup.2-.sub.3] concentration, pH, and the presence of phosphates as inhibitors was explored using a rotating disc electrode at 1000 rpm. Some potentiodynamic experiments indicate that for pH 8.5 and higher, carbonate and bicarbonate species play a key role on the rate of electrooxidation of cobalt. For pH lower than 8.5, the electrochemical behaviour of cobalt changes drastically and very aggressive corrosion is observed. The involvement of carbonic acid must be considered in the corrosion process of cobalt in this pH region. The study of passive film potential decay under open circuit potential and galvanostatic reduction was performed on preanodized cobalt electrodes in solutions of various compositions. These experiments indicated the phenomena occurring at the electrode-solution interface during the corrosion process. A mechanism involving competitive adsorption of different species is suggested on the basis of the experimental evidence. Key words: cobalt, bicarbonate, phosphate, carbonic acid, corrosion.

The freeze-quenching technique is extremely useful for trapping meta-stable intermediates populated during fast chemical or biochemical reactions. The application of this technique, however, is limited by the long mixing time of conventional solution mixers and the slow freezing time of cryogenic fluids. To overcome these problems, we have designed and tested a novel microfluidic silicon mixer equipped with a new freeze-quenching device, with which reactions can be followed down to 50 [micro]s. In the microfluidic silicon mixer, seven 10-[micro]m-diameter vertical pillars are arranged perpendicular to the flow direction and in a staggered fashion in the 450-pL mixing chamber to enhance turbulent mixing. The mixed-solution jet, with a cross section of 10 [micro]m x 100 [micro]m, exits from the microfluidic silicon mixer with a linear flow velocity of 20 m/s. It instantaneously freezes on one of two rotating copper wheels maintained at 77 K and is subsequently ground into an ultrafine powder. The ultrafine frozen powder exhibits excellent spectral quality and high packing factor and can be readily transferred between spectroscopic observation cells. The microfluidic mixer was tested by the reaction between azide and myoglobin at pH 5.0. It was found that complete mixing was achieved within the mixing dead time of the mixer (20 [micro]s), and the first observable point for this coupled device was determined to be 50 [micro]s, which is ~2 orders of magnitude faster than commercially available instruments.

Periodic ab initio QM calculations are applied to study the structure and acidity of Mg, Ca, Cr, Mn, Fe, Co, Ni, Zn, and Sr divalent metal ions in the A1PO-34 framework, charge compensated by an acid proton on a neighboring oxygen. The results show that the local environment of the divalent dopants is a distorted tetrahedron, in which the Me-O(sub H) bond of the dopant to the protonated oxygen is ~0.15 degree angstron longer than the other three Me-O bonds.

This paper describes a novel method that applies field-amplified sample injection (FASI) in micellar electrokinetic chromatography (MEKC) with a low pH background electrolyte (BGE). Six phenolic compounds prepared in water or NaOH solution were used as the test analytes. Sample was injected electrokinetically after the introduction of a plug of water. During the injection, the water plug was pumped out of the capillary inlet by the electroosmotic flow, and the phenolic anions migrated very quickly in the direction of the outlet. When the anions reached the boundary between the water plug and BGE, they were neutralized and ceased moving. Thereafter, MEKC was initiated for the separation. This on-line preconcentration method could be conveniently coupled with a liquid-liquid-liquid microextraction procedure, in which a hollow fiber was used as an extraction solvent support to extract the analytes from the water sample. The acceptor phase consisted of 8 mM NaOH. After extraction, the extract was analyzed directly by MEKC, as described.

The kinetic experiments, calculations of electrostatic properties and a new theoretical treatment of pH effects are combined together to study the effects of pH on protein association. The anomalous pK(sub a) values of certain amino acid residues in both cytochrone c and plastocyanin are seen to be significant for their biological functions.

Research has been conducted on the beta2-microglobulin-related (A(beta)2M) amyloidosis. The results of the fluorescence spectroscopic analysis with thioflavin T and electron microscopic study demonstrate that A(beta)2M amyloid fibrils depolymerize into the monomeric beta2-m at a neutral to basic pH.

The structure and energetic of protonation site, aromaticity, charge distribution and NMR properties of 3-aminothiophene, 3,4-diaminothiophene, aniline and 1,2-benzenediamine are examined. Data based on Guassian-3 theory via reduced Moller-Plesset order (G-3(MP2)) and the Isodensity Polarizable Continuum Method (IPCM), suggest similar stability and aromacity for C-protonated and N-protonated ring forms in aminothiophene with larger nucleophilicity of the C-2 atom, while for aniline N-protonated form is strongly preferred.

Research has been conducted on the tetranuclear lanthanide-hydroxo complexes. The preparation of these complexes via lanthanide ion hydrolysis controlled by alpha-amino acids has been carried out under near physiological pH conditions.

The decarboxylation rates of the malonate species as a function of pH at hydrothermal conditions appear to be controlled mainly by two factors. The first is the entropy loss in the cyclic intermediate due to greater rigidity brought on by the greater negative charge on the anion and secondly the higher negative charge also decreases the total entropy by organizing the surrounding water molecules.

Research has been conducted on the holomyoglobin. The equilibrium methanol-induced conformation changes of this holomyoglobin have been investigated at pH 4.0 via dichroism, tryptophan fluorescence and by electrospray ionization mass spectrometry.

An examination is done of the change in the relative acidities of the halocetic acids on going from the gas phase to aqueous solution which has been studied via ab initio calculations for the gas phase, the SCIPCM reaction field model for an aprotic polar solvent, and Monte Carlo statistical mechanics for aqueous solution. Since the remainder of the solvent effect was probably due to hydrogen bonding, Monte Carlo calculations were carried out and they were in satisfactory accord with the aqueous solution pK(sub a)'s.

Research is done on ab initio estimates of the gas-phase acidities deltaH(sub acid) of the second-row metal hydrides NaH, MgH2, and AlH3. The results suggest that the current NIST-JANAF recommended bond dissociation energy for NaH is too high by roughly 2kcal/mol.

Stable free radicals derived from 2,2.'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) ([ABTS.sup..+]) have been extensively employed to monitor the antioxidant capacity of biological fluids and beverages. Besides reacting with typical antioxidants (such as phenols or thiols) these radicals react with a variety of hydrogen or electron donors. The present work reports on the kinetics and mechanism of these radical reactions with several amino acids. Reaction rates notably increase when the pH of the media increases and, when measured under similar conditions, follows the order cysteine > tryptophan > tyrosine > histidine > cystine The kinetics of the process is interpreted in terms of a mechanism comprising an initial pH dependent reversible step, followed by secondary reactions of the substrate derived radical with itself or with another [ABTS.sup..+]; this simple three-step mechanism leads to very complex kinetic expressions. The specific rate constants of several of the elementary steps were determined by working under a wide range of substrate, radical, and ABTS concentrations. The values obtained for the initial interaction between the ABTS derived radical and the substrate range from 0.5 [M.sup.-1] [s.sup.-1] to 1.9 x [10.sup.6] [M.sup.-1] [s.sup.-1] for histidine and cysteine, respectively. Key words: ABTS radical cation, 2,2.'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid), amino acids, kinetics.

The reactivity of aminoboranes is researched theoretically and the Lewis acidity of the boron center and the Lewis basicity of the nitrogen center are examined. The results suggest a view to be applicable to other types of reactions of aminoboranes.

The oxidation of iodide, bromide, and chloride by peroxynitric acid (HOONO2) was examined by spectroidomietry. The results indicate that in polluted coastal regions or at low temperatures, it might become marginally significant compared to other known reactions leading to halogen release from sea-salt particles.

The reversible partitioning of molecules is examined between immiscible phases, such as in solvent extraction, which is usually controlled by chemical composition of one or both phases, temperature or pH. In conclusion, the fit of theory to the data confirms the importance of interface speciation.

Optical nanosensors, or PEBBLEs (probes encapsulated by biologically localized embedding), have been produced for intracellular measurements of pH and calcium. Five varieties of pH-sensitive sensors and three different calcium-selective sensors are presented and discussed. Each sensor combines an ion-selective fluorescent indicator and an ion-insensitive internal standard entrapped within an acrylamide polymeric matrix. Calibrations and linear ranges are presented for each sensor. The photo-bleaching of dyes incorporated into PEBBLEs is comparable to that of the respective free dye that is incorporated within the matrix. These PEBBLE sensors are fully reversible over many measurements. The leaching of fluorescent indicator from the polymer is less than 50% over a 48-h period (note that a typical application time is only a few hours). The PEBBLE sensors have also been applied to intracellular analysis of the calcium flux in the cytoplasm of neural cells during the mitochondrial permeability transition. Specifically, a distinct difference is noted between cells of different types (astrocyte vs neuronderived cells) with respect to their response to the toxicant m-dinitrobenzene (DNB). Use of PEBBLE sensors permits the quantitative discrimination of subtle differences between the ability of human SY5Y neuroblastoma and C6 glioma to respond to challenge with DNB. Specifically, measurement of intracellular calcium, the precursor to cell death, has been achieved.

Control of electroosmosis by an applied external voltage field in capillary electrophoresis has been limited to buffer pH approximately 5 or below. This poor control at higher pH is caused by a high density of surface charge induced by chemical equilibrium overwhelming the influence of the external voltage-induced charges within the electric double layer. A tert-butyldiphenylchlorosilane treatment was used on fused-silica capillaries to minimize chemically generated [Zeta]-potential where this treatment allowed for control of electroosmosis over a large pH range (210). Blocking the surface with traditional polymer-based surface treatments does not work in this application since the polymers increase the viscosity within the electric double layer and impede electroosmosis. The surface created by this reaction is demonstrated in extremely narrow capillaries, down to 2-[[micro]meter] internal diameter. The treated surface is sterically hindered against acid- and base-catalyzed degradation reactions typically associated with organosilanes. This results in a surface that was stable to experimental buffer pH extremes, from pH 3 to pH 10, and was stable for at least 8 weeks exposed to both solution and air.

The digestibility of D-tagatose, its effect on the digestibility of macronutrients and the metabolic response of the microbiota of the gastrointestinal tract to the ingestion of this carbohydrate were studied in pigs. Eight pigs were fed a low fiber diet comprising 15% sucrose (control group). Another eight pigs were fed a similar diet except that 100 g sucrose per kg diet was replaced by D-tagatose (test group). After 18 d, the pigs were killed and the gastrointestinal contents removed for analysis. The digestibility of D-tagatose was 25.8 [+ or -] 5.6% in the distal third of the small intestine. The small intestinal digestibilities of dry matter (86.9 [+ or -] 1.3 vs. 92.9 [+ or -] 0.9%), gross energy (74.4 [+ or -] 1.6 vs. 80.7 [+ or -] 1.8%) and sucrose (90.4 [+ or -] 2.5 vs. 98.0 [+ or -] 0.5%) were lower (P < 0.05) in the pigs fed D-tagatose. Digestibilities of starch, protein and fat did not differ between groups. D-Tagatose, sucrose and starch were fully digested in the large intestine. The fecal digestibilities of energy, dry matter and fat did not differ between the two groups, whereas D-tagatose reduced the fecal digestibility of protein (91.1 [+ or -] 0.6 vs. 93.5 [+ or -] 0.7%, P < 0.05). D-Tagatose served as a substrate for the microbiota in the cecum and proximal colon as indicated by a reduced pH, and a greater ATP concentration, adenylate energy charge (AEC) ratio and concentration of short-chain fatty acids. In particular, the increase in the concentrations of propionate, butyrate and valerate suggests possible health benefits of this monosaccharide. KEY WORDS: sweetener; sugar; digestibility; fermentation; pigs; D-tagatose

Uranium sorption experiments were carried out at [approximately]25 [degrees] C using natural samples of the lichen Peltigera membranacea. Thalli were incubated in solutions containing 100 ppm U for up to 24 h at pH values from 2 to 10. Equilibrium sorption was not observed at less than [approximately]6 h under any pH condition. U sorption was strongest in the pH range 4-5, with maximum sorption occurring at a pH of 4.5 and an incubation time of 24 h. Maximum U uptake by P. membranacea averaged [approximately]42 000 ppm, or [approximately]4.2 wt% U. This appears to represent the highest concentration of biosorbed U, relative to solution U activity, of any lichen reported to date. Investigation of post-experimental lichen tissues using electron probe microanalysis (EPM) reveals that U uptake is spatially heterogeneous within the lichen body, and that U attains very high local concentrations on scattered areas of the upper cortex. Energy dispersive spectroscopic (EDS) analysis reveals that strong U uptake correlates with P signal intensity, suggesting involvement of biomass-derived phosphate ligands or surface functional groups in the uptake process.

Uranium sorption experiments were carried out at [approximately]25 [degrees] C using natural samples of the lichen Peltigera membranacea. Thalli were incubated in solutions containing 100 ppm U for up to 24 h at pH values from 2 to 10. Equilibrium sorption was not observed at less than [approximately]6 h under any pH condition. U sorption was strongest in the pH range 4-5, with maximum sorption occurring at a pH of 4.5 and an incubation time of 24 h. Maximum U uptake by P. membranacea averaged [approximately]42 000 ppm, or [approximately]4.2 wt% U. This appears to represent the highest concentration of biosorbed U, relative to solution U activity, of any lichen reported to date. Investigation of post-experimental lichen tissues using electron probe microanalysis (EPM) reveals that U uptake is spatially heterogeneous within the lichen body, and that U attains very high local concentrations on scattered areas of the upper cortex. Energy dispersive spectroscopic (EDS) analysis reveals that strong U uptake correlates with P signal intensity, suggesting involvement of biomass-derived phosphate ligands or surface functional groups in the uptake process.

An on-column sample concentration method for capillary-based DNA sequencing was studied. This base-stacking method allows direct injection of unpurified products of dye-primer sequencing reactions onto capillaries without any pretreatment. On-column concentration of DNA fragments is achieved simply by electrokinetic injection of hydroxide ions. A neutralization reaction between these O[H.sup.-] ions and the cationic buffer component [Tris.sup.+] results in a zone of lower conductivity, within which field focusing occurs. DNA fragments are concentrated at the front of this low-conductivity zone. With sample injection times as long as 360 s at 50 V/cm, resolution could still be restored by the stacking process. Using a 36/47-cm-long uncoated capillary, with poly(dimethylacrylamide) as the separation matrix, and an electric field of 160 V/cm, a resolution of at least 0.5 could be generated for fragments up to 650 nucleotides long. Both resolution and signal strength are excellent relative to conventional injection of highly purified samples. No significant degradation of the capillary performance was observed over at least 20 sequencing runs using this new sample injection method.

Byline: M. Hekking (1), H. J. L. M. Ulenkate (2), B. Speelberg (3), M. J. E. Van Puyenbroek (3), H. M. J. Goldschmidt (2), E. S. Gelsema (1) Keywords: Key words Monitoring; Acid-base chart; pH; PaCO2; HCO3; Intensive care unit Abstract: Objective: To demonstrate the practicability of a tri-axial chart for the graphical and quantitative monitoring of arterial pH, arterial carbon dioxide partial pressure (PaCO.sub.2) and actual arterial bicarbonate-ion concentration (a[HCO.sub.3 .sup.--]) in intensive care patients. Design: Case report. Setting: A general intensive care unit (ICU). Methods: Using a standard mathematical transformation, a data set of pH, log PaCO.sub.2 and log a[HCO.sub.3 .sup.--] values can be transformed in such a way that a graphical display of all three variables is possible while being faithful to their linear relationship. Remarkably, the graphical display closely resembles the tri-axial chart that Hastings and Steinhaus described in 1931 for studying displacements of the acid-base balance. Two new monitoring parameters based on the chart and the transformation are described. One monitors the abnormality of the acid-base status while the other monitors the rate of acid-base changes. Conclusions: With the tri-axial acid-base chart, the complete acid-base status can be faithfully monitored. Moreover, the proposed monitoring parameters provide extra information about the arterial acid-base status that, otherwise, would remain hidden. Author Affiliation: (1) Department of Medical Informatics, Erasmus University Rotterdam, P. O. Box 1738, 3000 DR, Rotterdam, The Netherlands Tel.: + 31 10 4 08 81 16 Fax: + 31 10 4 36 28 82 email: hekking@mi.fgg.eur.nl, NL (2) Department of Clinical Chemistry and Hematology, St. Elisabeth Hospital, Hilvarenbeekseweg 60, 5022 GC, Tilburg, The Netherlands, NL (3) Intensive Care Unit, St. Elisabeth Hospital, Hilvarenbeekseweg 60, 5022 GC, Tilburg, The Netherlands, NL Article note: Received: 28 October 1997 Accepted: 4 June 1998

A new and simple chemical method for deposition of thin blue films composed of Prussian Blue and N-substituted polypyrroles on nonconductive supports is presented. It is found that only pyrroles which are difficult to polymerize can be used for the preparation of such films. The resulting composite films were examined by SEM-EDAX, vis-NIR, and IR spectroscopy. The films are stable, thin, homogeneous, and optically transparent The absorption maxima are at 720 nm, and spectral changes can be monitored using semiconductor light sources and detectors. The composite films are shown to be suitable for optical determination of pH over the pH 5-9 range because their absorbance strongly depends on pH in the physiological pH range. However, they undergo irreversible spectral changes if exposed to pH's >9. The films represent an alternative to indicator-based pH sensor materials because they do not require a dye to be immobilized. The pH measurements are highly reproducible, reversible in the physiological range, and not affected by ionic strength, alkaline cations, and typical oxidants and reductants.

Proton pumps ([H.sup.+]-ATPases) are the primary active transport systems in the plasma membrane of higher plant cells. These enzymes are encoded by a large gene family expressed throughout the plant, with specific isoforms directed to various specialized cells. While their involvement in membrane energetics has been suggested by a large body of biochemical and physiological studies, a genetic analysis of their role in plants has not yet been performed. We report here that mutant Arabidopsis thaliana plants containing a phloem-specific transgene encoding a plasma membrane [H.sub.+]-ATPase with an altered carboxy terminus show improved growth at low pH during seedling development. These observations provide the first genetic evidence for a role of the plasma membrane [H.sup.+]-ATPase in cytoplasmic pH homeostasis in plants.

The infectious agent of transmissible spongiform encephalopathies is believed to consist of an oligomeric isoform, Pr[P.sup.Sc], of the monomeric cellular prion protein, Pr[P.sup.C]. The conversion of Pr[P.sup.C] to Pr[P.sup.Sc] is characterized by a decrease in [Alpha]-helical structure, an increase in [Beta]-sheet content, and the formation of Pr[P.sup.Sc] amyloid. Whereas the N-terminal part of Pr[P.sup.C] comprising residues 23-120 is flexibly disordered, its C-terminal part, PrP(121-231), forms a globular domain with three [Alpha]-helices and a small [Beta]-sheet. Because the segment of residues 90-231 is protease-resistant in Pr[P.sup.Sc], it is most likely structured in the Pr[P.sup.Sc] form. The conformational change of the segment containing residues 90-120 thus constitutes the minimal structural difference between Pr[P.sup.C] and a Pr[P.sup.Sc] monomer. To test whether PrP(121-231) is also capable to undergo conformational transitions, we analyzed its urea-dependent unfolding transitions at neutral and acidic pH. We identified an equilibrium unfolding intermediate of PrP(121-231) that is exclusively populated at acidic pH and shows spectral characteristics of a [Beta]-sheet protein. The intermediate is in rapid equilibrium with native PrP(121-231), significantly populated in the absence of urea at pH 4.0, and may have important implications for the presumed formation of Pr[P.sup.Sc] during endocytosis.

Research was conducted to examine the stability of Mn(II) binding to manganese peroxidase (MnP) as a function of pH by spectrophotometric and potentiometric titrations. A Radiometer ABU93 Triburet was utilized to study the binding of Mn(II) to MnP while titrations were determined under interactive computer control. Results suggested that the melting temperature of MnP can be increased during a 50-fold molar excess of Mn(II).