Showing papers by "Carlos Bustamante published in 1980"

The Optical Activity of Nucleic Acids and their Aggregates

Abstract: This review includes experimental and theoretical work on absorption and scattering of light by chiral macromolecules. All molecules absorb light, but only chiral (handed) molecules show a preferential absorption for right or left circularly polarized light. This phenomenon of circular dichroism (CD) has been very useful in characterizing any chiral aggre­ gate of chromophores-including proteins, nucleic acids, and their complexes. We discuss theoretical methods that relate the absorption and circular dichroism of a polymer or aggregate to the optical proper­ ties of its constituent parts. The experimental data reviewed is limited essentially to nucleic acids, for lack of space. Two new experimental methods that are particularly useful for mac­ romolecules or for systems that scatter a significant fraction of the incident light are fluorescence-detected circular dichroism and circular intensity differential scattered light. Fluorescence-detected circular di­ chroism (FDCD), as the name implies, used the intensity of the fluores­ cence emitted to monitor the intensity of the light absorbed (167). This method provides two main advantages: (a) The spectrum of a complex system containing many chromophores, but only a few fluorophores is greatly simplified, and (b) scattering artifacts, which plague circular dichroism studies using transmitted light measurements, are partly avoided by use of fluorescence detection. The theory (159, 160) and

Circular intensity differential scattering of light by helical structures. I. Theory

Abstract: General expressions for the fields and intensities of scattered electromagnetic radiation by helical structures whose optical properties are described by a uniaxial polarizability along the tangent to the helix are obtained Analytical expressions for the circular intensity differential of scattering (CIDS) as a function of the helix parameters and the wavelength of light are derived It is shown that a sufficient condition for the existence of differential scattering for right and left circularly polarized light is the existence of an asymmetric polarizability The choice of a uniaxial polarizability is found to give rise to form CIDS For incident plane‐polarized light, the scattered fields are found to be generally elliptically polarized The Stokes parameters describing the state of polarization of the scattered radiation are derived These results are expected to be relevant both as a new method to characterize chiral regions in macromolecular structures as well as to describe the light scattering pr

Circular intensity differential scattering of light by helical structures. II. Applications

Abstract: Analysis and numerical calculations were made of the total scattering and differential scattering of circularly polarized light by helices as models of chiral structures. The differential scattering patterns are much more sensitive than the total scattering to helical parameters. For large helices the angular dependence of these patterns show lobes which alternate in sign. The number of lobes and the positions of the zeros directly measure the ratios of radius and pitch to wavelength. The signs depend on the sense of the helix. The results are compared with measured circular intensity differential scattering of membranes from the bacterium Spirillum serpens. Good qualitative agreement is obtained.

Circular intensity differential scattering of chiral molecules

Abstract: In this thesis a theory of the Circular Intensity Differential Scattering (CIDS) of chiral molecules as modelled by a helix oriented with respect to the direction of incidence of light is presented It is shown that a necessary condition for the existence of CIDS is the presence of an asymmetric polarizability in the scatterer The polarizability of the scatterer is assumed generally complex, so that both refractive and absorptive phenomena are taken into account