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H Frauenfelder

Publications and source records attributed to H Frauenfelder.

68 records · Page 4Linked to original sources
PubMed1978

Binding of carbon monoxide to isolated hemoglobin chains.

Binding of carbon monoxide to the separated alpha and beta chains of hemoglobin, with and without bound p-mercuribenzoate, has been measured at temperatures from 5 to 340 K for times 2 mus to 1 ks using flash photolysis. All four proteins exhibit three different rebinding processes. The data are interpreted by a model in which the carbon monoxide, moving from the solvent to the binding site at the ferrous heme iron, encounters three barriers. The temperature dependences of the three processes yield activation enthalpies and entropies for the three barriers for all four proteins. Binding at temperatures below about 200 K is nonexponential, implying that the innermost barrier has a distribution of activation enthalpies. The distributions for the four proteins have been determined. At temperatures below 30 K, the CO binding rates approach finite low-temperature limits; binding thus proceeds by quantum-mechanical tunneling. Invoking a simple model, the widths of the innermost barriers are extracted from the measured tunneling rates. The experimental parameters are correlated with structural features of the hemoglobin chains and compared with previously published data on myoglobin and protoheme. A correlation is established between the height of the innermost barrier and the equilibrium CO pressure.

Carbon Monoxide↗
PubMed1978

Fast reactions in carbon monoxide binding to heme proteins.

Using fast flash photolysis, we have measured the binding of CO to carboxymethylated cytochrome c and to heme c octapeptide as a function of temperature (5 degrees-350 degreesK) over an extended time range (100 ns(-1) ks). Experiments used a microsecond dye laser (lambda = 540 nm), and a mode-locked frequency-doubled Nd-glass laser (lambda = 530 nm). At low temperatures (5 degrees-120 degreesK) the rebinding exhibits two components. The slower component (I) is nonexponential in time and has an optical spectrum corresponding to rebiding from an S = 2, CO-free deoxy state. The fast component (I*) is exponential in time with a lifetime shorter than 10 mus and an optical spectrum different from the slow component. In myoglobin and the separated alpha and beta chains of hemoglobin, only process I is visible. The optical absorption spectrum of I* and its time dependence suggest that it may correspond to recombination from an excited state in which the iron has not yet moved out of the heme plane. The temperature dependences of both processes have been measured. Both occur via quantum mechanical tunneling at the lowest temperatures and via over-the-barrier motion at higher temperatures.

Binding Sites↗
PubMed1978

Stochastic theory of ligand migration in biomolecules.

When ligand binding to proteins involves the presence of more than one ligand inside a given biomolecule, linear deterministic rate equations become useless. A stochastic approach, however, permits a treatment of the migration and binding of small molecules to proteins even at high ligand concentrations. An appropriate linear master equation and its analytic solution are given. As an example, the binding of carbon monoxide to myoglobin at partial pressures from 1 to 10(3) bars (0.1 to 100 MPa) is treated.

Binding Sites↗
PubMed1976

Tunneling in ligand binding to heme proteins.

Rebinding of carbon monoxide to the beta chain of hemoglobin after photodissociation by a laser flash is intramolecular below about 200 K. Above 25 K, rebinding occurs via classical over-the-barrier motion; below, quantum-mechanical tunneling dominates. Both are described by an energy spectrum peaked at Epeak=4.0 kilojoules per mole. The barrier width d(E), determined from the energy dependence of the tunneling rate, depends on barrier height, d(E) approximately 0.05 nanometer X (E/Epeak) 1.5.

Carbon Monoxide↗
PubMed1975

Dynamics of ligand binding to myoglobin.

Myoglobin rebinding of carbon monoxide and dioxygen after photodissociation has been observed in the temperature range between 40 and 350 K. A system was constructed that records the change in optical absorption at 436 nm smoothly and without break between 2 musec and 1 ksec. Four different rebinding processes have been found. Between 40 and 160 K, a single process is observed. It is not exponential in time, but approximately given by N(t) = (1 + t/to)-n, where to and n are temperature-dependent, ligand-concentration independent, parameters. At about 170 K, a second and at 200 K, a third concentration-independent process emerge. At 210 K, a concentration-dependent process sets in. If myoglobin is embedded in a solid, only the first three can be seen, and they are all nonexponential. In a liquid glycerol-water solvent, rebinding is exponential. To interpret the data, a model is proposed in which the ligand molecule, on its way from the solvent to the binding site at the ferrous heme iron, encounters four barriers in succession. The barriers are tentatively identified with known features of myoglobin. By computer-solving the differential equation for the motion of a ligand molecule over four barriers, the rates for all important steps are obtained. The temperature dependences of the rates yield enthalpy, entropy, and free-energy changes at all barriers. The free-energy barriers at 310 K indicate how myoglobin achieves specificity and order. For carbon monoxide, the heights of these barriers increase toward the inside; carbon monoxide consequently is partially rejected at each of the four barriers. Dioxygen, in contrast, sees barriers of about equal height and moves smoothly toward the binding site. The entropy increases over the first two barriers, indicating a rupturing of bonds or displacement of residues, and then smoothly decreases, reaching a minimum at the binding site. The magnitude of the decrease over the innermost barrier implies participation of heme and/or protein. The nonexponential rebinding observed at low temperatures and in solid samples implies that the innermost barrier has a spectrum of activation energies. The shape of the spectrum has been determined; its existence can be explained by assuming the presence of many conformational states for myoglobin. In a liquid at temperatures above about 230 K, relaxation among conformational states occurs and rebinding becomes exponential.

Binding Sites↗
PubMed1973

Dynamics of carbon monoxide binding by heme proteins.

Rebinding of carbon monoxide to myoglobin and to cytochrome P-450 after removal by a light flash occurs down to 50 degrees K for myoglobin and 25 degrees K for cytochrome P-450 in glycerol-water solution. Above 240 degrees K the reaction is second order; between 240 degrees and 200 degrees K the rebinding becomes exponential and independent of the carbon monoxide concentration. Below 150 degrees K the reaction follows a power law and is approximately 10(3) times faster for cytochrome P-450 than for myoglobin.

Carbon Monoxide↗
PubMed1972

Photoreduction of NADP + sensitized by synthetic pigment systems.

Two synthetic pigment systems capable of enzymatically photoreducing NADP(+) are described. One system contains proflavine; the other, acridine. The complete system consists of ethylene diamine tetraacetic acid (EDTA), proflavine (or acridine), ferredoxin-NADP reductase (EC 1.6.99.4), and NADP(+). The two pigments initiate the photoreduction of NADP(+) in different portions of the electromagnetic spectrum. Proflavine photosensitizes in the visible portion; acridine, in the ultraviolet. Neither proflavine nor acridine is structurally related to chlorophyll. The acridine system has the attractive property that the enzyme, ferredoxin-NADP reductase, is the only component of the system that absorbs appreciably in the visible region of the spectrum.

Acridines↗
PubMedDate not supplied

Relaxations and fluctuations in myoglobin.

One major goal of biological physics is the discovery and understanding of the concepts and laws that govern biomolecules, in particular proteins. Since there exist at least 10(5) different proteins, the choice of a suitable prototype is necessary. Myoglobin (Mb) has for many years played the role of such a prototype. It appears to be simple enough so that many of its properties can be understood, yet it is complex enough to display many of the fascinating characteristics of biomolecules. One major achievement in the study of any protein would be the establishment of convincing connections among structure, kinetics, energy landscape, dynamics, and function. We believe that this goal has not yet been reached in any protein, but the present knowledge of Mb gives some hope that the end is near in this case. Here, we sketch some of the results that have been obtained in the past 50 or more years in the research on Mb, obtained by an army of investigators.

Kinetics↗