K.C. Cho

Ultrasonic absorption in myoglobin and other globular proteins.

The ultrasonic absorption of myoglobin has been measured by the resonance and pulse-echo techniques as a function of pH. The absorption at a given frequency can be separated into pH-dependent and pH-independent contributions. Like other proteins, two peaks at pH 3 and 11.5 are observed which can be accounted for a proton-transfer reactions of the side-groups. In addition, the absorption undergoes a large increase within a small range of 0.2 pH unit at pH around 4, when denaturation of myoglobin occurs, indicating that the absorption is sensitive to the overall protein conformation. To elucidate the origin of the pH-independent component, the absorptions of several other globular proteins at neutral pH are also measured. The absorptions of these proteins exhibit a linear correlation with their isothermal compressibilities, suggesting that the pH-independent component is related to volume fluctuations of protein molecules. The activation energy of 4 kcal/mol found for this relaxation is consistent with such an interpretation.

Kinetics of the pretransition of synthetic phospholipids a calorimetric study

The pretransition in aqueous dispersions of two synthetic phospholipids (dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine) has been examined in detail by differential scanning calorimetry. The transition from the high-temperature state (state above pretransition) to the low-temperature state (state below pretransition) is complex and appears to occur via some metastable states. In contrast, the kinetics of the transition from the low- to the high-temperature state is consistent with an activated two-state model. The observed hysteresis is shown to arise mainly from the kinetic nature of the pretransition.

Measurement of the protein-ligand bond energy of carboxymyoglobin by pulsed photoacoustic calorimetry

Abstract Pulsed photoacoustic spectroscopy has been used to study two horse heart myoglobin derivatives, deoxymyoglobin and carboxymyoglobin, in the temperature range of 0–25° C. The myoglobin-CO bond energy has been measured to be 13.4±0.5 kcal/mole. The volume change associated with the photodissociation of carboxymyoglobin into deoxymyoglobin and CO has also been observed.

Adiabatic compressibility of myoglobin. Effect of axial ligand and denaturation

An ultrasonic technique has been employed to study the adiabatic compressibility of three metmyoglobin derivatives (aquomet-, fluoromet- and azidometmyoglobin) at neutral pH, and aquometmyoglobin as a function of pH in the frequency range of 1-10 MHz at 20 degrees C. No difference was observed in the adiabatic compressibility of the various derivatives. This indicates that the binding of different axial ligands to myoglobin does not affect significantly the conformational fluctuations of the protein. The finding is consistent with the results of the hydrogen exchange rate experiment, indicating that both types of measurements are useful for the study of protein dynamics. Upon acid-induced denaturation, the adiabatic compressibility of myoglobin drops from 5.3 X 10(-12) cm2/dyn to 0.5 X 10(-12) cm2/dyn. Plausible reasons for such a decrease are discussed.

Metal–oxo photo-oxidants. Photochemistry and photophysics of trans-[OsVI(tmc)(O)2]2+(tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane) and trans-[OsVI(CN)4(O)2]2–

trans-[OsVI(tmc)(O)2]2+(tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane) and trans-[OsVI(CN)4(O)2]2– are emissive in the solid state and in fluid solutions at room temperature (τ∼ 1.0–1.5 µs); the 3Eg state of trans-[OsVI(tmc)(O)2]2+ is a powerful one-electron oxidant {[OsVI(O)2]2++ e–→[OsV(O)2]+, Ef0 > 2.0 V vs. normal hydrogen electrode} in aqueous solution, reacting with PPh3 and (PhCH2)2S to give OPPh3 and (PhCH2)2SO, respectively.

Thermal stability of hemoglobin and myoglobin: Effect of spin states

The thermal stability of various methemoglobin and metmyoglobin derivatives at different ligand concentrations and pH were studied by the method of differential scanning calorimetry. Comparing to water as a ligand, cyanide and azide are most effective in protecting the protein; fluoride stabilizes the protein slightly whereas imidazole and thiocyanate have little effect. Assuming thermal denaturation can be described by an activated two-state process, the activation parameters of all the derivatives were determined. We have also found that the stability of hemoglobin and myoglobin does not correlate with the iron atom spin state but depends instead on steric interactions with the ligands. On the reasonable assumption that structure and stability are related, this implies a definite dependence of structure on steric interactions.

Spectroscopic properties and redox chemistry of the phosphorescent excited state of [Au2(dppm)2]2+[dppm = bis(diphenylphosphino)methane]

The complex [Au2(dppm)2]2+[dppm = bis(diphenylphosphino)methane] exhibits a long-lived room temperature photoluminescence in fluid solutions (emission maximum, 565 nm; τ0, 17.0 µs; ϕem, 0.15); the phosphorescent excited state of [Au2(dppm)2]2+ undergoes oxidative and reductive quenching with methyl viologen and N,N,N′,N′-tetramethyl-p-phenylenediamine, respectively, and can be quenched by organic halides.

Kinetics of cold-induced denaturation of metmyoglobin

Using a slow temperature-jump spectrophotometer, we have studied the kinetics of cold-induced denaturation of metmyoglobin between 0 degrees C and 20 degrees C at acidic pH. The time-scale of the transition is slow and is of the order of minutes. The results are consistent with the transitions involving a total of three states, native (N), transient intermediate (I) and denatured (D), which are converted from one to the other in that order.

Photoionization of tetrakis(pyrophosphito)diplatinum(II)

Abstract Hydrated electrons are produced by photoexcitation of aqueous solutions of a binuclear platinum(II) complex, Pt2(POP)4−4. Studies of yields of e−aq as a function of laser energy indicate that the electrons are liberated by a two-photon process. The efficiency of production of e−aq relative to that of the Pt(II) triplet can be as high as 0.15.

Electron transfer between cytochrome c and metal hexacyanide complexes effect of thermodynamic driving force on the electron transfer rate

The electron transfer reactions between ferrocytochrome c and three isomorphic hexacyanide complexes, [Fe(CN)6]3-, [Os(CN)6]3- and [Ru(CN)6]3-, have been studied using the method of photoexcitation. The transfer rates for [Os(CN)6]3- and [Ru(CN)6]3- are, respectively, about 45- and 200-times higher than that of [Fe(CN)6]3-. A reorganization energy of approx. 0.8 eV was found for the cytochrome c-hexacyanide system when the data were analyzed according to the theory of Marcus.