RESUMEN
Displacement of particles from the purified light-harvesting chlorophyll a/b protein aggregate (LHC) was studied in magnetic fields of various strengths (0 to 1.6 T) by polarized fluorescence measurements. Macromolecular aggregates of LHC have a considerable magnetic susceptibility which enables the particles to rotate and align with their nematic axes parallel with H. As LHC is embedded in a transmembrane direction thylakoids should align perpendicular to H, the mode of alignment experimentally observed in thylakoids. The value of the magnetic susceptibility could be estimated by relating it to the integral susceptibility of the chlorophyll molecules in LHC. The fitting of this value with the field strength dependency of the fluorescence polarization ratio (FP) revealed a relationship between the LHC content of various photosynthetic membranes and their capacity for alignment, which suggested that LHC might be the torque ordering chloroplasts in a magnetic field.
RESUMEN
Pigmented vesicular membranes embedded in polyacrylamide gel exhibit linear dichroism when the gel sample is squeezed [Abdourakhmanov, I.A., Ganago, A.O., Erokhin, Yu.E., Solov'ev, A.A. and Chugunov, V.A. (1979) Biochim. Biophys. Acta 546, 183-186]. The orientation technique of gel-squeezing was modified to enhance polarization effects in membrane vesicles of spherical symmetry. Model calculations were carried out to provide a tool for the quantitative evaluation of the dichroism of squeezed gel samples. The orientation angles of the dipoles can be calculated with reasonable precision by measuring two quantities: (i) the macroscopic deformation parameter of the gel sample, and (ii) a parameter (e.g. the polarization ratio of the fluorescence emission) characterizing the orientation of the transition dipoles in the membranes embedded in the squeezed gel. The validity of the model was confirmed through a series of polarization measurements relating to the fluorescence of chlorophyll a in membranes of osmotically shocked chloroplasts, 'blebs'.
Asunto(s)
Membranas/análisis , Resinas Acrílicas , Cloroplastos/análisis , Electroquímica , Polarización de Fluorescencia , Geles , Matemática , Modelos Teóricos , Análisis Espectral/métodosRESUMEN
Orientation angles of five emitting dipoles of chlorophyll a in thylakoids were estimated from low temperature fluorescence polarization ratio spectra of magnetically oriented chloroplasts. A simple expression is given also for the evaluation of data from linear dichroism measurements. It is shown that the Qy dipoles of chlorophylls lie more in the plane of the membranes and span a larger angular interval than was previously thought. Values for the orientation factor are calculated using various models corresponding to different degrees of local order of the Qy dipoles of chlorophylls in the thylakoid. We show that the characteristic orientation pattern of the Qy dipoles of chlorophylls in the membrane, i.e., increasing dichroism toward longer wavelengths, may favour energy transfer between the antenna chlorophylls as well as funnel the excitation energy into the reaction centers.
Asunto(s)
Clorofila/metabolismo , Cloroplastos/metabolismo , Polarización de Fluorescencia , Magnetismo , Modelos Químicos , Zea maysRESUMEN
Flash-induced transients in light scattering were shown to occur with Chlorella cells. The kinetic and spectral patterns of the scattering transients and their relation to the absorption changes studied in the 10 microseconds--5 s time range, between 450 and 540 nm. 1. The kinetics of the fast changes (less than 500 ms) in scattering and absorbance were identical. From about 500 ms divergence of the two signals was observed. 2. The transient spectrum characterizing the fast scattering changes exhibited a large double band between 480 and 500 nm. Transients corresponding to the slower changes resembled the steady scattering spectrum (Latimer, P. and Rabinowitch, E. (1959) Arch. Biochem. Biophys. 84, 428--441) with a maximum at about 515 nm. 3. From theoretical considerations it is suggested that fast transients in scattering and absorbance are physically interrelated, and as has been shown for absorption changes (Witt, H.T. (1971) Q. Rev. Biophys. 4, 365--477) fast scattering transients can also be interpreted as an electrochromic phenomenon. Slower changes are accounted for by alterations in the microenvironment and conformation of the particles responsible for scattering.
Asunto(s)
Chlorella/metabolismo , Cinética , Luz , Matemática , Dispersión de Radiación , EspectrofotometríaRESUMEN
Selective scattering spectra of granal and agranal chloroplasts were measured in the red spectral region and compared with calculations based on the Mie theory. The spectra were influenced considerably by the intactness and ultrastructural pattern of the chloroplasts. It was demonstrated that the spectra consist of two components: one attributable to grana and the other, to single lamellae. The dependence of the selective scattering spectra on the ultrastructural characteristics offers a convenient method for monitoring the quality of chloroplast preparations by a procedure much faster than electron microscopy.