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W Doster

Publications and source records attributed to W Doster.

29 records · Page 2Linked to original sources
PubMed1986

Thermal properties of water in myoglobin crystals and solutions at subzero temperatures.

The water of hydration in myoglobin crystals and solutions was studied at subzero temperatures by calorimetry and infrared spectroscopy (ir). For comparison we also investigated glycine, DL-alanine and DL-valine solutions. The hydration water remains amorphous at low temperatures. We find a broad glass transition between 180 and 270 K depending on the degree of hydration. The ice component shows a noncolligative melting point depression that is attributed to a finite conformational flexibility. The ir spectrum and the specific heat of water in myoglobin crystals was determined for the first time between 180 and 290 K. The glass transition in crystals is qualitatively similar to what is found in amorphous samples at the same water content. These data are compared with Mössbauer experiments and dielectric relaxation of water in myoglobin crystals. The similar temperature dependencies suggest a cross correlation between structural fluctuations and the thermal motion of crystal water. A hydrogen bond network model is proposed to explain these features. The essential ingredients are cooperativity and a distribution of hydrogen-bonded clusters.

Animals↗
PubMed1985

Dynamics of dioxygen and carbon monoxide binding to soybean leghemoglobin.

The association of dioxygen and carbon monoxide to soybean leghemoglobin (Lb) has been studied by laser flash photolysis at temperatures from 10 to 320 K and times from 50 ns to 100 s. Infrared spectra of the bound and the photodissociated state were investigated between 10 and 20 K. The general features of the binding process in leghemoglobin are similar to the ones found in myoglobin. Below about 200 K, the photodissociated ligands stay in the heme pocket and rebinding is not exponential in time, implying a distributed enthalpy barrier between pocket and heme. At around 300 K, ligands migrate from the solvent through the protein to the heme pocket, and a steady state is set up between the ligands in the solvent and in the heme pocket. The association rate, lambda on, is mainly controlled by the final binding step at the heme, the bond formation with the heme iron. Differences between Lb and other heme proteins show up in the details of the various steps. The faster association rate in Lb compared to sperm whale myoglobin (Mb) is due to a faster bond formation. The migration from the solvent to the heme pocket is much faster in Lb than in Mb. The low-temperature binding (B----A) and the infrared spectra of CO in the bound state A and the photodissociated state B are essentially solvent-independent in Mb, but depend strongly on solvent in Lb. These features can be correlated with the x-ray structure.

Carbon Monoxide↗
PubMed1983

Viscosity scaling and protein dynamics.

The rates of molecular motions in the interior of some proteins were found to scale with an inverse power of the external solvent viscosity. The data were explained by a flexible protein structure whose dynamics is partially controlled by the solvent. Reaction dynamics in the presence of structural fluctuations with finite lifetimes lead to a dynamic friction coefficient defined by a generalized Langevin equation and a fluctuation-dissipation theorem. A model for the dynamic friction is derived assuming that the fluctuation spectrum at the reaction site involves two components: solvent-independent diffusion of local structural defects in the protein matrix and global fluctuations coupled to the solvent. The theory is applied to the viscosity dependence of molecular oxygen-binding rates in sperm whale myoglobin.

Mathematics↗
PubMed1982

Control and pH dependence of ligand binding to heme proteins.

The recombination after flash photolysis of dioxygen and carbon monoxide with sperm whale myoglobin (Mb), and separated beta chains of human hemoglobin (beta A) and hemoglobin Zürich (beta ZH), has been studied as a function of pH and temperature from 300 to 60 K. At physiological temperatures, a preequilibrium is established between the ligand molecules in the solvent and in the heme pocket. The ligand in the pocket binds to the heme iron by overcoming a barrier at the heme. The association rate is controlled by this final binding step. The association rate of CO to Mb and beta A is modulated by a single titratable group with a pK at 300 K of 5.7. The binding of CO to beta ZH, in which the distal histidine is replaced by arginine, does not depend on pH. Oxygen recombination is independent of pH in all three proteins. Comparison of the binding of CO at 300 K and at low temperatures shows that pH does not affect the preequilibrium but changes the barrier height at the heme. The pH dependence and the difference between O2 and CO binding can be explained by a charge-dipole interaction between the distal histidine and CO.

Adult↗
PubMed1982

Infrared spectroscopy of photodissociated carboxymyoglobin at low temperatures.

We have studied the infrared spectra of the bound and photodissociated states of Mb-12CO and Mb-13CO from 5.2 to 300 K. The absorbance peaks seen between 1800 and 2200 cm-1 correspond to CO stretching vibrations. In the bound state of Mb-12CO, the known lines A0 at 1969, A1 at 1945, and A2 at 1927 cm-1, have center frequencies, widths, and absorbances that are independent of temperature between 5.2 and 160 K. Above 160 K, A2 gradually shifts to 1933 cm-1. The low-temperature photodissociated state (Mb) shows three lines (B0, B1, B2) at 2144, 2131, and 2119 cm-1 for 12CO. The absorbances of the three lines depend on temperature. B0 is tentatively assigned to free CO in the heme pocket and B1 and B2, to CO weakly bound to the heme or heme pocket wall. The data are consistent with a model in which photodissociation of MbCO leads to B1 and B2. B2 decays thermally to B1 above 13 K; rebinding to A occurs from B1. The barriers between B2 and B1 and between B1 and A are described by activation enthalpy spectra. Heme and the central metal atom in state Mb have near-infrared, EPR, and Mössbauer spectra that differ slightly from those of deoxyMb. The observation of essentially free CO in state B implies that the difference between Mb and deoxyMb is not due to an interaction of the flashed-off ligand with the protein but is caused by an incomplete relaxation of the protein structure at low temperatures.

Animals↗
PubMed1981

Reversible solvent denaturation of rabbit muscle pyruvate kinase.

The structural transitions of the tetrameric rabbit muscle pyruvate kinase induced by guanidine hydrochloride and urea are characterized by elastic and quasi-elastic light-scattering, sedimentation velocity, and intrinsic viscosity experiments as well as by protein fluorescence, circular dichroism, and enzymic activity measurements. The transition curves are shown to be reversible. We find a new pathway of unfolding which is different from that described in the literature: The first intermediate with increasing concentration of denaturant is a less compact and inactive tetramer which can be renatured if substrates are added. Dissociation of the tetramer results in an expanded dimer with a partial loss of the secondary structure. The final state is a completely disordered monomer. These intermediates are consistent with a domain structure of pyruvate kinase, as it was suggested by Stammers & Muirhead [Stammers, D. K., & Muirhead, H. (1975) J. Mol. Biol. 95, 213--225] on the basis of their X-ray data. Using Schellman's solvent denaturation model [Schellman, J. A. (1978) Biopolymers 17, 1305--1322], we calculate the free energies of stabilization of the folding--unfolding equilibrium.

Animals↗
PubMed1978

Photoselection and aggregation in purple membrane of Halobacterium halobium.

Preparations of the purple membrane of Halobacterium halobium suspended in dilute buffer and basal salt have been examined by circular dichroism spectroscopy and correlation analysis of scattered laser light. Dark adapted samples suspended in basal salt show photoselection when examined by circular dichroism. This was confirmed by irradiation with plane polarized light. Light-adapted samples or dark-adapted samples suspended in dilute buffer did not show this phenomenon. The reaction responsible for photoselection was shown to be the light induced cis-trans isomerization of bacteriorhodopsin. The stability of the induced anisotropy was due to aggregation in the basal salt suspensions which occurred despite little or no visual indication. This aggregation was confirmed by correlation analysis of scattered laser light.

Bacteriorhodopsins↗