Quasi-elastic light scattering. Application to biological problems]

The origin of the light scattered from macromolecular solutions is considered and a discussion is made around the use of laser and light beating spectroscopy to analyze the scattered field spectrum. Several experiments are reviewed where quasi elastic light scattering has been applied.     

A method for measuring sedimentation and diffusion of macromolecules in capillary tubes by total intensity and quasi-elastic light-scattering techniques.

Light scattered from a macromolecular solution in a capillary tube is used to determine both the sedimentation and translational diffusion coefficients. The capillary tube is spun in a preparative centrifuge, removed, and placed in a light-scattering photometer equipped with a scanning mechanism. The intensity distribution of scattered light along the tube represents the concentration profile in the tube and provides the measure of boundary migration. The sedimentation coefficient is determined from this measure and the applied centrifugal field. The diffusion coefficient is obtained from a time-autocorrelation analysis of fluctuations in intensity of light scattered from any fixed point of the profile. These coefficients were obtained for two monodisperse systems, R17 bacteriophage and 28s ribosomal rat liver RNA. The molecular weights obtained from ratios of these coefficients are in good agreement with literature values. In the sedimentation analysis, deviations from linearity between boundary displacement and applied field were found to be less than 1%. This precision confirms that the boundary is stable for the capillary geometry even in the absence of a preformed density gradient. The sedimentation coefficients of identical samples were also measured with the Spinco Model E analytical ultracentrifuge; results of the two methods agree to within 4%. As a consequence of the capillary tube geometry and light-scattering detection, sedimentation coefficients can be obtained from sample volumes of less than 100 μl. This detection techniques is thus far demonstrated to be at least an order of magnitude more sensitive than Schlieren optics, thereby useful when uv absorption is not applicable. For diffusion measurements there are also several inherent advantages. The diffusion coefficient is obtained from the identical sample, and scanning provides the capability to measure D from various parts of the sedimentation profiles and thereby directly explore concentration dependence, homogeneity, and integrity of the sample. The capillary tube with a layer of silicone oil over the sample and centrifugation provides an effective method to cleanse the solution and trap all dust.     

Water and hermal diffusivity in a lipid-water smectic phase.

We report the first application of light scattering to measurement of the hydrodynamic relaxation of inhomogeneities in water concentration within a multilamellar, or smectic A, phospholipid water system (dipalmitoyl) phosphatidyl choline). Although the relaxation process in the multilamellar phase is different from the diffusion process in liquid phases, the relaxation rate can be described in terms of a diffusion coefficient. For diffusion parallel to the lamellae, diffusion coefficients ranging from 8 x 10(-7) to 2 x 10(-5) cm2/s were measured over a range of temperature and water concentrations. We describe a model that expresses the diffusion coefficient in terms of the chemical potential for water inside the multilamellar phase and the effective thickness of a "free water zone." The deduced thickness of this free water zone is in good agreement with estimates from X-ray diffraction results. The activation energy for the diffusion process is also deduced from the data, and is found to decrease monotonically with increasing water concentration. We also found the thermal diffusivity to be about 10(-3) cm2/s with only a weak temperature and water concentration dependence. The experimental technique is a new version of forced Rayleigh scattering. The method uses the phase information of the scattered light to improve the ability to detect weak signals. Experimental details are reported.     

A study of protoplasmic streaming in Nitella by laser Doppler spectroscopy.

Laser light scattered from particles in the streaming protoplasm of a living cell is shifted in frequency by the Doppler effect. The spectrum of the scattered light can be measured and interpreted to infer details of the velocity distribution in the protoplasm. We have developed this approach to study the protoplasmic streaming in the fresh-water alga Nitella. Our results indicate a characteristic flow pattern to which diffusion makes a negligible contribution. No difference in the velocity of particles of different size is indicated. The streaming velocity linearly with temperature with a supraoptimal temperature of 34 degrees C, and the velocity distribution becomes narrower at high temperatures. The protoplasmic streaming can be inhibited by laser light, and this effect has been used to study the photoresponse of the algae. Using beam diameters of about 50 mum, we have shown that the inhibition is very local, becoming minimal at a displacement of about 200 mum in the upstream direction and 400 mum in the downstream direction. Prolonged exposure produces a bleached area free of chloroplasts, which is three orders of magnitude less sensitive to photoinhibition.     

Protein self-association induced by macromolecular crowding: a quantitative analysis by magnetic relaxation dispersion

In the presence of high concentrations of inert macromolecules, the self-association of proteins is strongly enhanced through an entropic, excluded-volume effect variously called macromolecular crowding or depletion attraction. Despite the predicted large magnitude of this universal effect and its far-reaching biological implications, few experimental studies of macromolecular crowding have been reported. Here, we introduce a powerful new technique, fast field-cycling magnetic relaxation dispersion, for investigating crowding effects on protein self-association equilibria. By recording the solvent proton spin relaxation rate over a wide range of magnetic field strengths, we determine the populations of coexisting monomers and decamers of bovine pancreatic trypsin inhibitor in the presence of dextran up to a macromolecular volume fraction of 27%. Already at a dextran volume fraction of 14%, we find a 30-fold increase of the decamer population and 510(5)-fold increase of the association constant. The analysis of these results, in terms of a statistical-mechanical model that incorporates polymer flexibility as well as the excluded volume of the protein, shows that the dramatic enhancement of bovine pancreatic trypsin inhibitor self-association can be quantitatively rationalized in terms of hard repulsive interactions.     

Multichannel photometer-nephelometer.

We describe an instrument for monitoring either transmitted or scattered light intensity, or both, simultaneously on up to eight channels. The use of a laser light source (at 632.8-nm wavelength) provides high accuracy and dynamic range: optical density can be measured from 0.0004 up to 6, and a scattered light fraction down to 10(-6) can be resolved. Built-in thermostat and magnetic stirrers allow precise monitoring of aqueous microbial growth over a practical range of 4 orders of magnitude of cell concentration.     

Localized dynamic light scattering: a new approach to dynamic measurements in optical microscopy.

We present a new approach to probing single-particle dynamics that uses dynamic light scattering from a localized region. By scattering a focused laser beam from a micron-size particle, we measure its spatial fluctuations via the temporal autocorrelation of the scattered intensity. We demonstrate the applicability of this approach by measuring the three-dimensional force constants of a single bead and a pair of beads trapped by laser tweezers. The scattering equations that relate the scattered intensity autocorrelation to the particle position correlation function are derived. This technique has potential applications for measurement of biomolecular force constants and probing viscoelastic properties of complex media.     

The Role of Coupled Nanoplasmon Excitation in Growth Mechanism of Laser-Induced Self-Organized Nanostructures in AgCl-Ag Waveguide Thin Films

Formation mechanism of laser-induced spontaneous periodic nanostructures in thin light-sensitive AgCl waveguide films, doped by silver nanoparticles, is studied. It is found that the initial size, geometry, and surface coverage parameters of Ag nanoparticles instate preconditions for the nanostructure formation. These parameters play essential roles in coupled nanoplasmon excitation in silver nanoclusters, which in turn influences scattering of the incident light from the silver clusters. Some parts of the scattered light propagate as waveguide modes in the film and interference with the incident light. Afterward, migration of the Ag nanoparticles into the minima of the interference pattern forms the nanostructure. Simultaneously, excitation of coupled nanoplasmons in the neighboring clusters enhances the scattered light intensity. It is observed that longer exposure results in destruction of the formed nanostructures, because of creation of electrical joint between agglomerated clusters in the interference pattern’s minima, which leads to weakening of the TE mode excitation and, consequently, domination of the Gaussian profile of the incident light. This leads to the deceleration of the self-organized nanostructure development, growth rate, and quality. We have found that competition between the Gaussian profile of the incident laser field and the interference field causes a finally saturating oscillatory behavior of the self-organizing process.     

Rayleigh linewidths of laser light inelastically scattered from neural membranes

Laser light is scattered off the surface of nerve from the walking legs ofHomarus Americanus and the spectral power density of the Rayleigh line is obtained. The data is interpreted in terms of a relaxation time of a scattering unit. The experimental results are consistent with a scattering unit having a molecular weight of the order of 30,000 in a medium of several poise viscosity. The relaxation time for scattering is calculated to be 16±2μsec in the resting state and 10±2μsec in the depolarized state.     

Quasi-elastic laser light scattering from solutions and gels of hemoglobin S.

Quasi-elastic light scattering has been used to examine solutions and gels of deoxyhemoglobin S. The autocorrelation function is found to decay with a characteristic exponential relaxation which can be ascribed to the diffusion of monomer (64,000 molecular weight) hemoglobin S molecules. In the absence of polymers, the relaxation time is in good agreement with previous measurements of the diffusion coefficient for solutions of normal human hemoglobin. In the presence of the polymer phase, a large (greater than 200-fold) increase in the scattered intensity is observed but no contribution to the decay of the autocorrelation function from the motion of the aligned polymer phase can be detected. Heterodyning between the time-independent scattering amplitude from the polymers and the time-dependent scattering of the diffusing monomers results in a twofold increase in the relaxation time arising from monomer diffusion.     

Free energy changes associated with amino acid substitution in proteins

The estimation of free energy differences from computer simulation of macromolecular systems is important for rational strategies for drug design and for protein engineering. As an example of one mutation, we have studied the free energy change resulting from the conversion of a polar group (OH) to an apolar group (CH3) in aqueous solution. We have estimated the effect of various local environments on the magnitude of the free energy difference and find that significant environmental effects are found. We have also studied the reliability of the results in detail.     

A phase-shift fluorometer using a laser and a transverse electrooptic modulator for subnanosecond lifetime measurements.

We described a simple phase-shift fluorometer using continuous laser excitation. The laser enables the use of a transverse mode electrooptic modulator with a half-wave retardation voltage of about 200 V (in contrast to many kilovolts of longitudinal modulators) at frequencies up to 100 MHz. The modulated fluorescence signal is detected, after passing through a double monochromator, by a photomultiplier tube feeding a radio frequency (RF) tuned amplifier. THE RF phase is then determined by phase-sensitive detection using a double balanced mixer with the reference obtained from a PIN photodiode-turned amplifier combination which detects light split off from the main exciting beam. The laser and double monochromator allow the observation of modulated Raman solvent and Rayleigh scatterin, which are convenient for determining the zero reference phase.     

共振瑞利散射测试血清蛋白酸碱度影响的探讨

提出了一种基于共振瑞利散射（RRS）原理测量人体血清蛋白的新方法。在缓冲溶液的作用下,把配制好的人体血清蛋白稀释液按比例与四羧基酞菁锌混合,经过化学作用后在波长为400 nm左右蓝色波段强光照射下,散射出480 nm左右的共振瑞利散射光强信号。考察在不同pH对共振瑞利散射光强信号与混合物中的血清蛋白反应线性关系的影响。结果表明,pH在6.0～8.0范围内混合溶液共振瑞利散射光强信号与血清蛋白的线性关系良好。     

Probing the electronic structure of transition metal ion centres in proteins by coherent Raman-detected electron paramagnetic resonance spectroscopy

The simultaneous excitation of a paramagnetic sample with optical (laser) and microwave radiation can cause an amplitude or phase modulation of the transmitted light at the microwave frequency. The detection of this modulation indicates the presence of coupled optical and electron paramagnetic resonance (EPR) transitions in the sample. Here we report the first application of this technique to a biomolecule: the blue copper centre of Pseudomonas aeruginosa azurin. Using optical excitation at 686 nm, in the thiol to copper(II) charge transfer band, we measure a coherent Raman-detected EPR spectrum of a frozen aqueous solution. Its lineshape is characteristic of the magnetic circular dichroism along each principal g-value axis. This information allows electronic and structural models of transition metal ion centres in proteins to be tested.     

Phase transition of dimyristoylphosphatidylglycerol bilayers induced by electric field pulses

The phase transition of dimyristoylphosphatidylglycerol (DMPG) bilayers has been studied by measurements of light scattering under high electric field pulses. Midpoints of phase transitions have been identified by a clear discontinuity of field induced relaxation amplitudes. We show that the phase transition of DMPG suspensions in monovalent salt is virtually independent of the electric field strength up to approx. 35 kV/cm. A shift of the lipid phase by electric field pulses has been observed, however, for DMPG suspensions in the presence of Ca2+ ions. DMPG suspensions exhibit a jump of the phase transition temperature from 17 degrees C at Ca/DMPG molar ratios r less than 1/7 to 32 degrees C at r greater than 1/7. Field pulses of 60 to 100 microseconds applied to DMPG suspensions with Ca2+ at r greater than 1/7 induce discontinuities of relaxation amplitudes in the temperature range 15 to 22 degrees C in addition to the 'standard' one at 32 degrees C, when the electric field strength is above 15 kV/cm. These results indicate that electric field pulses induce a transition from the phase formed at 'high' Ca(2+)- to the one formed at 'low' Ca(2+)-ion concentrations. Our results are consistent with a dissociation field effect on Ca(2+)-lipid complexes which drives the phase transition.     

Quasi-elastic light scattering from migrating chemotactic bands of Escherichia coli.

We report the observation of migrating chemotactic bands of Escherichia coli in a buffer solution. The temporal development of the bacterial density profile is observed by the scattered light intensity as the band migrates through a stationary laser beam. We have made a preliminary analysis of the observed band profile with help of the Keller-Segel theory. The model accounts for only some aspects of the observed time evolution of the density profile. The microscopic motility characteristics of the E. coli in the band are simultaneously studied by photon correlation. The measured correlation functions are analyzed to obtain the spatial dependence of the half-width within the band. A simple analytical model is proposed to account for the contribution of the twiddle motion to the correlation function. By analyzing the correlation function as a superposition of straight-line and twiddle motions, we obtain a satisfactory agreement between the theory and the measured angular dependence of the line shape. As a consequence we are able to extract a parameter beta, which measures the average fraction of twiddling bacteria in the center of the band at a given time.     

Unravelling coherent dynamics and energy dissipation in photosynthetic complexes by 2D spectroscopy

Spectroscopic studies of light harvesting and the subsequent energy conversion in photosynthesis can track quantum dynamics happening on the microscopic level. The Fenna-Matthews-Olson complex of the photosynthetic green sulfur bacteria Chlorobium tepidum is a prototype efficient light-harvesting antenna: it stores the captured photon energy in the form of excitons (collective excitations), which are subsequently converted to chemical energy with almost 100% efficiency. These excitons show an elaborate relaxation pattern involving coherent and incoherent pathways. We make use of the complex chirality and fundamental symmetries of multidimensional optical signals to design new sequences of ultrashort laser pulses that can distinguish between coherent quantum oscillations and incoherent energy dissipation during the exciton relaxation. The cooperative dynamical features, which reflect the coherent nature of excitations, are amplified. The extent of quantum oscillations and their timescales in photosynthesis can be readily extracted from the designed signals, showing that cooperativity is maintained during energy transport in the Fenna-Matthews-Olson complex. The proposed pulse sequences may also be applied to reveal information on the robustness of quantum states in the presence of fluctuating environments in other nanoscopic complexes and devices.     

Structural dynamics of frog muscle during isometric contraction

Intensity fluctuation autocorrelation functions of laser light scattered by actively contracting muscle were measured at points in the scattered field. They were reproducible and showed characteristics which depended on the physiological state of the muscle and the parameters of the scattering geometry. The autocorrelation functions had large amplitudes and decay rates that varied significantly with the phase of the contraction-relaxation cycle. The dependence of the autocorrelation function on scattering geometry indicated many elements with diameters on the order of 0.5 mum (presumed to be myofibrillar sarcomeres or their A bands or I bands) undergo independent random changes in their axial positions and their internal distribution of optical polarizability during the plateau of an isometric tetanus. The experimental results are interpreted in terms of a model in which most of the scattering elements in isometrically contracting muscle have random fluctuating axial velocities of average magnitude 20 nm/ms that persist for a few milliseconds at least. In addition to these axial motions there are local fluctuations in polarizability. Similar intensity fluctuation autocorrelation functions were observed throughout the active state on two muscle preparations, whole sartorius muscle and small bundles of single fibers (three to eight) of semitendinosus muscle. These results imply that the tension developed during an isometric tetanus contains a fluctuating component as well as a constant component.     

Viscoelasticity of living materials: mechanics and chemistry of muscle as an active macromolecular system

At the molecular and cellular level, mechanics and chemistry are two aspects of the same macromolecular system. We present a bottom-up approach to such systems based on Kramers' diffusion theory of chemical reactions, the theory of polymer dynamics, and the recently developed models for molecular motors. Using muscle as an example, we develop a viscoelastic theory of muscle in terms of an simple equation for single motor protein movement. Both A.V. Hill's contractile component and A.F. Huxley's equation of sliding-filament motion are shown to be special cases of the general viscoelastic theory of the active material. Some disparity between the mechanical and the chemical views of cross-bridges and motor proteins are noted, and a duality between force and energy in discrete states and transitions of macromolecular systems is discussed.