Showing papers by "Carlos Bustamante published in 1988"

Design and application of a computer‐controlled confocal scanning differential polarization microscope

Abstract: Based on the confocal scanning and differential polarization imaging technique, a computer‐controlled confocal scanning differential polarization microscope has been built. This system possesses extensive image processing capability that provides digitized regular and/or differential polarization images. This microscope combines the advantages of the confocal design and the extended electronic sensitivity of polarization modulated instrumentation to achieve the measurement of an anisotropy ratio as small as 10−5. In this paper, the basic theory and the instrument are described. In addition, some samples are used to test the actual performance of the system, and it is found that the system provides high resolution (0.3 μm at wavelength 546 nm), reduced depth of field, and high signal‐to‐noise ratio (83 dB at 10‐ms integration time) images. Finally, some biological applications are shown.

Helically organized macroaggregates of pigment-protein complexes in chloroplasts: evidence from circular intensity differential scattering

Abstract: Angle dependence of circular intensity differential scattering (CIDS) and of nonpolarized scattering was determined in isolated spinach chloroplasts at 514.5 nm. CIDS between 0 degrees and 170 degrees was independent of the nonpolarized scattering and showed intense lobes of alternating signs, exhibiting the negative and positive maxima around 15 degrees and 70 degrees, respectively. These results provide experimental evidence for the existence of large helically organized macroaggregates of pigment-protein complexes in thylakoid membranes. Modeling of the CIDS data by a simple helical array of uniaxial polarizable groups suggests that the chiral structure is left-handed with pitch and radius of the order of 385 nm.

Statistical effects in the absorption and optical activity of particulate suspensions.

Abstract: The phenomenon of Duysens flattening of the absorption spectra resulting from the inhomogeneous distribution of the chromophores in the solution is analyzed. These inhomogeneities are treated as localized statistical fluctuations in the concentration of the absorbing species, by using the Gaussian distribution. A law of absorbance is obtained, and the effect of light scattering on the flattening is also characterized. The flattening in the circular dichroism spectra of particulate suspensions is then analyzed. It is shown that the degree of flattening of the circular dichroism of a suspension is, in general, different from the corresponding flattening of its absorption spectrum. A quantitative relationship between the two effects is established.

A study of some lyotropic cholesteric mesophases by circular and linear dichroism and by circular intensity differential scattering

Abstract: The handedness of lyotropic cholesterics formed by decylammonium chloride, ammonium chloride, water and chiral dopants was determined by circular and linear dichroism and, independently, by circular intensity differential scattering. From the study of the twisting power of a few dopants and from the magnetic orientation of cholesterics with long and short pitches, it is concluded that distortion of the micelles is likely to be determinant for the formation of the cholesteric phase.

Why does sinusoidally modulated polarization introduce a systematic error in linear dichroism measurements? analytical and instrumental solutions

Abstract: Photoelastic modulation spectroscopy introduces a systematic error in linear diehroism measurements. It is shown that this instrumental artefact is due to an imbalance in the number of photons of the two perpendicularly polarized light components used for linear diehroism measurements. The numerator, (I⊥ - I∥), and the denominator, (I⊥ + I∥), of the dichroic ratio are measured by the lock-in amplifier and the DC integrator respectively. These uneven populations of the orthogonal photons affect only the denominator measurements. The “mechanical approach” to the physics of the modulation technique used here shows very clearly how the diehroism of the sample affects the average intensity (I⊥ + I∥) measurement, thereby introducing a systematic error. This insight also suggests several analytical and instrumental solutions.

Differential Polarization Imaging: Theory and Applications

Abstract: A theory of differential polarization imaging is presented using Mueller calculus It is shown that, for any arbitrary object, 16 images (in general different) can be obtained by combining different incident polarizations of light and measuring the specific polarization components transmitted or scattered by the object Mathematical expressions of these images for an object of arbitrary geometry are derived using classical vector diffraction theory This theory is then applied to study the symmetry behavior of the Mueller matrix elements upon infinitesimal rotations of the optical components about the optical axis of the imaging system and to obtain the the phenomenological equations of the sixteen bright-field Mueller elements in terms of the optical coefficients The criterion of spatial resolution between adjacent domains of different optical anisotropy is discussed Also, the feasibility of optical sectioning in differential polarization imaging will be discussed Finally, a test of the predictions of the differential polarization imaging theory will be presented, through the characterization of the patterns of polymerization of hemoglobin in red blood cells from patients with sickle cell anemia A differential polarization microscope designed and built in our laboratory was used to carry out this study On the basis of the differential polarization images obtained, models of the patterns of polymerization of the hemoglobin S inside the sickle cells are proposed and their M 12 and regular images are calculated by means of the theory (Calculated images are displayed in contour plots) Good agreement between those models and the experimental systems is found, as well as with the results previously reported

Theory of Absorption and Circular Dichroism of Large Inhomogeneous Molecular Aggregates

Abstract: A general method to describe the spectroscopy of large, internally inhomogeneous particles is presented. The theory utilizes an approach similar to the one used by De Voe in the treatment of the optical properties of polymers. It is found that if the particle is dense the intermediate and radiation coupling mechanisms must be included in addition to the dipole-dipole coupling. Through these coupling mechanisms it is found that the excitation generated at each group in the chromophore can delocalize over regions comparable to the size of the wavelength of light. The spatially averaged equations of the absorbance for a collection of large inhomogeneous arbitrarily shaped aggregates will be presented. This theory is then applied to the polymer and salt induced Psi-type circular dichroism observed in DNA aggregates. Using the formalism developed, it is shown that the anomalously large signals observed in the circular dichroism of certain molecular aggregates result from: (a) the presence of a long-range chiral structure in the aggregate; (b) delocalization throughout the entire particle of the light-induced excitations in the chromophores. This is the first successful attempt to explain the physical origin of the psi-type CD effect. Useful information regarding the chiral structure of the aggregates can be inferred from the theory.