Photoselection Effects Research Articles

Compass magnetoreception in birds arising from photo-induced radical pairs in rotationally disordered cryptochromes.

According to the radical pair model, the magnetic compass sense of migratory birds relies on photochemical transformations in the eye to detect the direction of the geomagnetic field. Magnetically sensitive radical pairs are thought to be generated in cryptochrome proteins contained in magnetoreceptor cells in the retina. A prerequisite of the current model is for some degree of rotational ordering of both the cryptochromes within the cells and of the cells within the retina so that the directional responses of individual molecules do not average to zero. Here, it is argued that anisotropic distributions of radical pairs can be generated by the photoselection effects that arise from the directionality of the light entering the eye. Light-induced rotational order among the transient radical pairs rather than intrinsic ordering of their molecular precursors is seen as the fundamental condition for a magnetoreceptor cell to exhibit an anisotropic response. A theoretical analysis shows that a viable compass magnetoreceptor could result from randomly oriented cryptochromes contained in randomly oriented cells distributed around the retina.

A method for analysis of lipid vesicle domain structure from confocal image data

Quantitative characterization of the lateral structure of curved membranes based on fluorescence microscopy requires knowledge of the fluorophore distribution on the surface. We present an image analysis approach for extraction of the fluorophore distribution on a spherical lipid vesicle from confocal imaging stacks. The technique involves projection of volumetric image data onto a triangulated surface mesh representation of the membrane, correction of photoselection effects and global motion of the vesicle during image acquisition and segmentation of the surface into domains using histograms. The analysis allows for investigation of the morphology and size distribution of domains on the surface.

The effect of pea chloroplast alignment and variation of excitation wavelength on the circularly polarized chlorophyll luminescence.

Circularly polarized luminescence (CPL) is a powerful technique to study the macroorganization of photosynthetic light-harvesting apparatus in vivo and in vitro. It is particularly useful for monitoring environmental stress induced molecular re-organization of thylakoid membranes in green leaves. The current study focuses on two questions which are important to perform and interpret such experiments: how does CPL depend on the excitation wavelength and how on the orientation of the granal thylakoids. CPL and circular dichroism (CD) of pea chloroplasts were complementarily applied when chloroplasts were either in suspension or trapped in a polyacrylamide gel (PAAG) after alignment in a magnetic field. In contrast to the CD spectrum, the CPL signal was found to be independent of the excitation wavelength in both the Soret and the Qy absorption region for chloroplasts in both suspension and PAAG. The improved resolution of luminescence measurements revealed a relatively small negative CPL band in addition to the previously described large positive band. No effect of photoselection upon excitation on the CPL spectra was detected. The CPL intensity at 690 nm at the edge of the granal thylakoids was found to be higher than at the face of the grana suggesting the CPL anisotropy.

Photoselection effects in laser flash-induced spin-polarised EPR spectra of the radical pair state [P +Q A−] in Rhodobacter sphaeroides reaction centers

The spin-polarised EPR signal of the radical pair state [P +Q A −] in Zn-substituted Rhodobacter sphaeroides R26 reaction centers demonstrates a strong dependence on the wavelength and polarisation of the excitation light. This dependence is attributed to photoselection by the plane-polarised laser light utilised in the experiments. The model of spin-correlated radical pairs describes the experimental results well if the effect of photoselection is taken into account.

Photoselection in polarized photolysis experiments on heme proteins

Polarized photolysis experiments have been performed on the carbon monoxide complex of myoglobin to assess the effects of photoselection on the kinetics of ligand rebinding and to investigate the reorientational dynamics of the heme plane. The results are analyzed in terms of the optical theory developed in the preceding paper by Ansari and Szabo. Changes in optical density arising from rotational diffusion of the photoselected population produce large deviations from the true geminate ligand rebinding curves if measurements are made with only a single polarization. The apparent ligand rebinding curves are significantly distorted even at photolysis levels greater than 90%. These deviations are eliminated by obtaining isotropically-averaged optical densities from measurements using both parallel and perpendicular polarizations of the probe pulse. These experiments also yield the optical anisotropy, which gives a novel method for accurately determining the degree of photolysis, as well as important information on the reorientational dynamics of the heme plane. The correlation time for the overall rotational diffusion of the molecule is obtained from the decay of the anisotropy. The anisotropy prior to rotational diffusion is lower than that predicted for a rigidly attached, perfectly circular absorber, corresponding to an apparent order parameter of S = 0.95 +/- 0.02. Polarized absorption data on single crystals suggest that the decreased anisotropy results more from internal motions of the heme plane which take place on time scales shorter than the duration of the laser pulse (10 ns) than from out-of-plane polarized transitions.

Speed of intersubunit communication in proteins.

To determine the speed of communication between protein subunits, time-resolved absorption spectra were measured following partial photodissociation of the carbon monoxide complex of hemoglobin. The experiments were carried out using linearly polarized, 10-ns laser pulses, with the polarization of the excitation pulse both parallel and perpendicular to the polarization of the probe pulse. The substantial contribution to the observed spectra from photoselection effects was eliminated by isotropically averaging the polarized spectra, allowing a detailed comparison of the kinetics as a function of the degree of photolysis. These results show that prior to 1 microsecond both geminate ligand rebinding and conformational relaxation are independent of the number of ligands dissociated from the hemoglobin tetramer, as expected for a two-state allosteric model. After this time the kinetics depend on the ligation state of the tetramer. The conformational relaxation at 10 microseconds can be interpreted in terms of the two-state allosteric model as arising from the R to T quaternary conformational change of both unliganded and singly liganded molecules. These results suggest that communication between subunits requires about 1 microsecond and that the mechanism of the communication which occurs after this time is via the R to T conformational change. The optical anisotropy provides a novel means of accurately determining the extinction coefficients of the transient photoproduct. The decay in the optical anisotropy, moreover, provides an accurate determination of the rotational correlation time of 36 +/- 3 ns.

Dynamics of the quaternary conformational change in trout hemoglobin

The kinetics of conformational changes in trout hemoglobin I have been characterized over the temperature range 2-65 degrees C from time-resolved absorption spectra measured following photodissociation of the carbon monoxide complex. Changes in the spectra of the deoxyheme photoproduct were used to monitor changes in the protein conformation. Although the deoxyheme spectral changes are only about 8% of the total spectral change due to ligand rebinding, a combination of high-precision measurements and singular value decomposition of the data permits a detailed analysis of both their amplitudes and relaxation rates. Systematic variation of the degree of photolysis was used to alter the distribution of liganded tetramers, permitting the assignment of the spectral relaxation at 20 microseconds to the R----T quaternary conformational change of the zero-liganded and singly liganded molecules and spectral relaxations at about 50 ns and 2 microseconds to tertiary conformational changes within the R structure. Analysis of the effect of photoselection by the linearly polarized excitation pulse indicates that a major contribution to the apparent geminate rebinding in the 50-ns relaxation arises from rotational diffusion of molecules containing unphotolyzed heme-CO complexes. The activation enthalpy and activation entropy for the R0----T0 transition are +7.4 kcal/mol and -12 cal mol-1 K-1. Using the equilibrium data, delta H = +29.4 kcal/mol and delta S = +84.4 cal mol-1 K-1 [Barisas, B. G., & Gill, S. J. (1979) Biophys. Chem. 9, 235-244], the activation parameters for the T0----R0 transition are calculated to be delta H = +37 kcal/mol and delta S = +73 cal mol-1 K-1. The similarity of the equilibrium and activation parameters for the T0----R0 transition indicates that the transition state is much more R-like than T-like. This result suggests that in the path from T0 to R0 the subunits have already almost completely rearranged into the R configuration when the transition state is reached, while in the path from R0 to T0 the subunits remain in a configuration close to R in the transition state. The finding of an R-like transition state explains why the binding of ligands causes much smaller changes in the R----T rates than in the T----R rates.

Polarized light in optics and spectroscopy

The Nature of Light. Polarization Properties of Light. Devices for Control of Polarized Light. Introduction to the Jones Calculus, Mueller Calculus and Poincare Sphere. The Jones Calculus, Mueller Calculus and Poincare Sphere Revisited. Spectroscopy with Polarized Light. Orientation and Photoselection Effects. Polarized Light in Condensed Phases. Appendixes. Index.

The excited-state absorption of Ru(bpy) 2+3 reexamined

The excited-state absorption of Ru (bpy) 3 2+ ( RBY ) in the range 240 to 2200 nm is reported, extending the range and improving the resolution of previous measurements. The excitation power dependence of the excited-state populations are quantified and absorption measurements made in H 2O and D 2O solutions at 300 K and in poly (vinyl-alcohol) (PVA) films at 20 K. No evidence is found for an intervalence charge-transfer type transition in the near IR, but the Ψ ★→ χ ★ ligand excitation transition is clearly seen near 11000 and 9000 cm −1 in solution and in PVA respectively. The visible and UV excited-state spectra are similar in the two environments and differences are attributed to the effects of photoselection in PVA. The Ψ ★→ χ ★ transition shows a marked shift in passing from a rigid to a fluid medium, indicating the possibility of a change in ligand geometry.

The study of the state [P870 + B800 −] in bacterial reaction centers by selective picosecond and low-temperature spectroscopies

The selective picosecond excitation of Rhodopseudomonas sphaeroides (R-26) reaction centers (RCs) at 870 nm induces the formation of the transient state within <1 ps followed by the conversion into the state P F (P ± Bph ±− during 7 ± 2 ps at both 293 K and 110 K. The transient state including the intense bleaching at 800 nm has been shown not to be due: (a) to photon excitation at 870 nm; (b) the excitation of P +; (c) photoselection effects. The transient state is interpreted as the state 1[P +B −] in agreement with earlier works. The primary formation of the state 1P +B −] and the big effective singlet-triplet splitting in this state correspond to the spectral splitting of the P band at 900 nm in R-26 RCs and at 1000 nm in Rhodopseudomonas viridis RCs found at 4.2 K and attributed to the optical transition to both 1P and 1[P +B −] states.

Effects of photoselection on the kinetics of colour decay in a photochromic spiropyran/poly(alkyl methacrylate) matrix

AbstractPhotochromic measurements are reported on the decay of the coloured form of 1′,3′,3′‐trimethyl‐6‐nitro[2H‐(1)‐benzospiropyran‐2,2′‐indoline] dispersed in poly(methyl methacrylate) and poly(ethyl methacrylate) matrices. Polarized light was used to photoselect molecules in specific orientations, and the data are analysed in terms of a free volume distribution controlling the kinetic matrix effect. A time correlation function is obtained which characterizes both the rotation and the isomerization of the molecule.

The polarization of pure electric quadrupole transitions in the electronic spectra of molecules

The polarization properties of electric quadrupole contributions to molecular electronic transitions are examined theoretically. Possible manifestations of these electric quadrupole contributions in various types of polarized absorption and emission spectra are considered in detail. The types of polarized spectra considered are circular dichroism (CD), linear dichroism (LD), circularly polarized luminescence (CPL), linearly polarized luminescence (LPL), magnetic circular dichroism (MCD), magnetic linear dichroism (MLD), magnetic circularly polarized luminescence (MCPL), and magnetic linearly polarized luminescence (MLPL). In emission the possible effects of photoselection and reorientational relaxation are investigated. Calculations are presented for the A2g↔T1g transitions of octahedral (Oh) systems and for the Σ+g↔Πu, Πg, and Δg transitions of homonuclear diatomic molecules (D∞h) under a variety of conditions. The identification and characterization of pure electric quadrupole (electronic) transitions using polarized absorption/emission spectroscopy are discussed.

M.C.D./M.C.P.L. saturation theory with application to molecules inD∞hand its subgroups

The M.C.D. of a system with a paramagnetic ground state will deviate from simple H/T dependence at sufficiently high field and/or low temperature, and this deviation is referred to as a saturation effect. A detailed theoretical discussion of this phenomenon is presented, and specific application is made to molecules in point group D ∞h and in its subgroups down through C 3. A single general formula is found to apply in all cases to an allowed electronic transition between two ‘isolated’ electronic states. The properties and applications of this formula are described in some detail. The formula is then generalized to apply to saturation behaviour for the corresponding emission phenomenon, magnetic circularly polarized luminescence (M.C.P.L.), where the additional effect of photoselection must be considered. A saturation analysis in M.C.D. is also presented for one special case of accidental degeneracy, that for a 2P3/2 ‘atomic’ ground state in a linear molecule. Various experimental considerations in saturation work are also discussed, and a method is presented for correcting for the effect of magnet reluctance when a large superconducting solenoid is used. Corrections may easily be made for the presence of B terms.

Magnetically Induced Circular Polarization of Emission: Application to Inorganic Complexes

A sensitive spectrometer for the measurement of circular polarization of emission, utilizing a photoelastic modulator, is presented. This spectrometer, when augmented by a superconducting magnet and dewar assembly, provides an excellent system for studying magnetically induced circular polarization of emission (MICE). In order to exemplify both the system and the usefulness of MICE, a partial analysis of the MICE observed from Ru-(4,7-dipheny1-1,10-phenanthroline)3C12 dispersed in a polymethylmethacrylate (PMM) matrix in the 2 to 100 K temperature range is reported. Also presented is a discussion of the effects of photoselection on the observed MICE for these D3 symmetry complexes. It is found that MICE is more sensitive to the detailed nature of the levels involved in emission than a combination of zero field techniques. MICE should prove to be an exceptionally useful tool for probing excited states of inorganic complexes.

The effects of photoselection and orientational relaxation on the magnetically induced circular polarization of emission of molecular systems of Cn (n.gtoreq. 3) symmetry

The effects of photoselection and orientational relaxation on the magnetically induced circular polarization of emission of molecular systems of Cn (n.gtoreq. 3) symmetry

Effect of photoselection on fluorescence-detected circular dichroism

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEffect of photoselection on fluorescence-detected circular dichroismB. Ehrenberg and I. Z. SteinbergCite this: J. Am. Chem. Soc. 1976, 98, 5, 1293–1295Publication Date (Print):March 1, 1976Publication History Published online1 May 2002Published inissue 1 March 1976https://doi.org/10.1021/ja00421a057RIGHTS & PERMISSIONSArticle Views117Altmetric-Citations30LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (439 KB) Get e-Alerts Get e-Alerts

A theoretical evaluation of the effect of photoselection on the measurement of the circular polarization of luminescence

The orientation of molecules excited by a beam of light is not isotropic as a rule. If rotatory Brownian motion is not fast during the lifetime of the excited state the polarization properties of the light emitted from the excited molecules will depend as a rule on the mode of photoselection of the molecules by the excitation beam. A general theoretical method is developed for proper weighting in the process of summing of the contributions of the various photoselected molecules to the emitted light when Brownian rotational motion is frozen. A formula is obtained for the calculation of the probabilities of emission of light of a specified polarization from the photoselected system. To use this formula average values of multiples of elements of the Euler matrix are to be evaluated first; the polarization properties of the emitted light are then obtained by insertion of the components of the electric and magnetic transition dipole moments characterizing the absorption and emission of light by the molecule under discussion. By use of this formula, the Perrin-Jablonski formula for the linear polarization of emitted light is derived in a simple fashion. It is shown that for asymmetric molecules the degree of circular polarization of the emitted light is affected by photoselection if the electric transition moments responsible for absorption and emission are not parallel. It is of much interest that under such circumstances the angle between the emitting electric and magnetic dipole moments can in principle be obtained.

Effects of photoselection on decomposition kinetics in frozen solutions using unpolarized light

Effects of photoselection on decomposition kinetics in frozen solutions using unpolarized light