Drug-binding to biological macromolecules. A kinetic study of the system chlorodiazepoxide (librium) and bovine serum albumin

The Kinetics of the binding of librium to bovine serum albumin were investigated at an ionic strength of 0.15 M, in the pH-range between 5.5 and 10.5, using the temperature-jump method. Two relaxation times were observed. The first, rapid one is assigned to the equilibrium between reactants and complex, whereas the second, slower relaxation time measures a conformational change of this complex. Both relaxation times are independent of pH up to about 8.0 when they increase sharply to a new plateau at a pH of about 9.3. A possible parallelism between this behavior and the N ? B transition of albumin is discussed. The overall equilibrium constant K, as well as the equilibrium constant between the two conformers, K₂, were estimated from our kinetic results. Whereas the change of K with changing pH was within the experimental error, the equilibrium between the two conformers of the complex is somewhat shifted by an increase of the pH. At all values of pH, however, they are present in comparable concentrations. The value of K was also measured spectrophotometrically. The results of the two methods showed reasonable agreement. The relaxation times were affected very little by a change in temperature between 12 and 25°C from which it is concluded that the isomerization has a high negative value of ΔS?.     

Global energy-momentum conservation in general relativity

It is shown that there exists a family of coordinate systems in which the energy-momentum tensor is globally conserved. Furthermore, this preferred class of frames includes geodesic systems with respect to any arbitrary point or timelike geodesic line. This implies a physically satisfactory conservation law with no need to introduce an extraneous pseudotensor.     

Bovine serum albumin and alizarin yellow G — a model system for positive cooperative binding to macromolecules

The kinetics of the binding of alizarin yellow G to six nonequivalent binding sites on bovine serum albumin were investigated, using the stopped flow and T-jump techniques. The first step in the reaction sequence is very fast, and is followed by a slower step which is suggested to be an internal rearrangement of the collision complex formed in the first step. This temporal sequence shows that the strong cooperativity between the first and second binding sites is not due to the existence of different conformations in equilibrium in free albumin, but rather to a ligand-induced change. Next, an additional, still slower transformation is observed. From the fact that this transformation is not rate-determining for the formation of higher complexes we conclude that it corresponds to a branch reaction. The higher complexes are again formed with very high rate constants. The first complex has a high negative value of both ΔH and ΔS° of formation. For the second and higher complexes, the heat of formation is much lower, and the standard entropy of formation is positive. The formation of these complexes is thus largely entropy-driven. The difference between the thermodynamic parameters of the first and subsequent ligands is in accordance with the assumption that only the binding of the first ligand entails significant structural changes.     

Gravitational energy and momentum: A tensorial approach

A tensorial expression for localized gravitational energy-momentum is delineated as an integral part of the energy-momentum tensor. A bona fide conservation law of the total energy-momentum tensor is obtained in the geodesic-nonrotating coordinates, in which the covariant divergencelessness of the energy-momentum tensor reads, globally, as ordinary divergencelessness. The integral gravitational energy in the exterior of a spherically symmetric source is calculated based on this tensorial relativistic expression. For an ordinary star, such as the sun, it coincides with the Newtonian value up to six digits.     

Experimental facts and gravitational energy in general relativity

It is shown that the nonrotating coordinates wherein the energy-momentum is globally conserved share the experimental features of the inertial frames. The falling of matter in a spherically symmetric gravitational field is studied in the light of the energy-momentum conservation valid in these coordinates.Deceased.     

SL(2,C) gravitational conserved current and Noether's theorem

Noether's theorem is applied to Hilbert's Lagrangian written as a functional of spinorial variables. The associatedSL(2,C) conserved current is obtained, and its expression for the Tolman metric is given explicitly.     

SL(2,C) gauge theory of gravitation: Conservation laws

Conservation laws in the SL(2,C) gauge theory of gravitation are reviewed and their relation to the ordinary conservation laws in general relativity theory is discussed. The vector currents that were proposed by different authors along the lines of the Yang-Mills conserved vector current are discussed and their interrelation is given. Likewise Lagrangian densitiies, from which one obtains the SL(2,C) gauge theory gravitational field equations, are discussed and related to the conservation laws through Noether's theorem.     

THE PHOTOEXCITED TRIPLET STATE OF TETRAPHENYL CHLORIN, MAGNESIUM TETRAPHENYL PORPHYRIN AND WHOLE CELLS OF CHLAMYDOMONAS REINHARDI. A LIGHT MODULATION-EPR STUDY

Abstract— The photoexcited triplet states of frozen solutions of tetraphenyl chlorin (TPC), magnesium tetraphenyl porphyrin (MgTPP) and whole cells of Chlamydomonas reinhardi have been studied by light modulation-EPR spectroscopy. The porphyrins were chosen to be studied as model compounds for chlorophyll molecules, From EPR spectra the zero field splitting parameters (ZFS) were calculated. For TPC, |D| = 0.0364 ± 0.0002 cm^-1, |E| = 0.0063 ± 0.0002 cm^-1. For MgTPP, |D| = 0.0310 ± 0.0002 cm^-1. For chloroplasts, |D| = 0.0280 ± 0.0004 cm^-1, |E| = 0.0032 ± 0.0004 cm^-1. In all compounds studied, except MgTPP, electron spin polarization (ESP) was observed. From the analysis of the kinetic curves at each canonical orientation we evaluated the spin lattice relaxation rate W, the depopulation rate constants k_p, and the ratio between the population rate constants, A_p, at zero magnetic field. For TPC in ethanol-toluene (5:1) k_x= (0.70 ± 0.10) × 10³ s^-1, k_y= (0.40 ± 0.07) × 10³ s^-1, k_x= (0.24 ± 0.05) × 10³ s^-1; A_x:A_y:A_z? 1.0:0.6:0.4; W= (2.60 ± 0.40) × 10³ s^-1. For MgTPP, only the total decay rate constant, k_T, was calculated: (1.5 ± 0.2) × 10 s^-1 in n-octane and (4.8 ± 0.8) × 10 s^-1 in ethanol. The results for TPC and MgTPP are compared to those reported previously for chlorophyll. It is concluded that the dynamics of the photoexcited triplet state in chlorophylls are mainly governed by the chlorin macrocycle. From the EPR spectrum and ZFS parameters of chloroplasts, we propose that both chlorophyll a and chlorophyll b are the main constituents of the EPR spectrum. From the analysis of the kinetic curves we obtain separately the kinetic parameters for chlorophylls a and b, k^a_x= (1.30 ± 0.20) × 10³ s^-1, k^a_y;= (0.85 ± 0.15) × 10³ s^-1k^a_x= (0.32 ± 0.05) × 10³ s^-1; A^a_x:A^a_y:A^a_z? 1.0:0.7:0.2; W^a= (1.20 ± 0.20) × 10³ s^-1; k^b_x= (0.56 ± 0.09) × 10³ s^-1, k^b_y= (0.30 ± 0.04) × 10³ s^-1, k^b_z= (0.06 ± 0.01) × 10³ s^-1; A^b_x:A^b_y:A^b_x? 1.0:0.6:0.1; W^b= (5.00 ± 0.80) × 10³ s^-1. These results are very close to those found separately for chlorophyll a and chlorophyll b oligomers in vitro.