PubMed Health⌕ Search

Biomedical subjects

B Busson

Publications and source records attributed to B Busson.

7 recordsLinked to original sources
PubMed2000

Quasi-phase-matching in chiral materials

The second-order nonlinear optical coefficients associated with chirality differ in sign for the two mirror-image forms (enantiomers) of a chiral material. Structures comprised of alternating stacks of the enantiomers can therefore be used for quasi-phase-matched frequency conversion, as we demonstrate here by second-harmonic generation from Langmuir-Blodgett films of a helicenebisquinone. Such structures could lead to new types of frequency converters in which both the second-order nonlinear response and quasi-phase-matching arise from the chirality of a material rather than its polar order.

Journal Article↗
PubMed2000

Distribution and interference functions for two-dimensional hexagonal paracrystals.

The notion of a paracrystal is particularly well adapted to the calculation of the scattering interference function of distorted crystallographic lattices in which the long-range order does not exist. However, classical paracrystal modelling cannot be used directly for hexagonal lattices because it does not respect the hexagonal symmetry. Here an analytical determination of the distribution and interference functions for two-dimensional hexagonal paracrystals is presented.

Journal Article↗
PubMed2000

Exploring a biological tissue from atomic to macroscopic scale using synchrotron radiation: example of hair.

A combined approach, using synchrotron radiation-based diffraction and infrared microspectrometry, has been used to study the structure and molecular composition of hair samples. These methods allowed us to get an insight at different structural scales into the composition and structure of hair. Firstly, information about the configuration of amino-acid residues was obtained at atomic scale, secondly, a model was presented for the geometry and the packing of the microfibrils at medium scale and finally different structural zones were evidenced by microdiffraction at macroscopic scale. We also showed that the two main components of hair--proteins and lipids--are not evenly distributed within the fiber. In addition, these two components exhibit different structure, depending upon their location. Moreover, diffraction and microdiffraction data indicate that the cuticle zone is mainly composed of lipid granules, whereas the cortex and the medulla zones are composed primarily of alpha-keratin. Infrared microspectroscopy, using an enhanced lateral resolution thanks to synchrotron radiation, indicates, on one hand, that the protein structure between the cuticle and cortex are different, and on the other hand, that the concentration of lipids, inside the medulla, is much higher than everywhere else. This work emphasizes the complementarity between both techniques, and highlights the potentialities they can offer in the case of various other studies in biology.

Hair↗
PubMed1999

Side-chains configurations in coiled coils revealed by the 5.15-A meridional reflection on hard alpha-keratin X-ray diffraction patterns.

The origin of the 5.15-A meridional reflection on hard alpha-keratin X-ray diffraction patterns is discussed in terms of side-chains conformations. We show it to reveal specific configurations of the side chains which are common to all two-stranded alpha-helical coiled coils. Combining literature data on crystallised coiled coil pieces and molecular dynamics results with our X-ray diffraction pattern simulations, we propose rules for the attribution of chi1 torsion angles for coiled coils involved in fibres whose structure cannot be resolved at atomic resolution: in a (a b c d e f g) heptad repeat, a and d residues, respectively, adopt mean t and g+ configurations, whereas statistical rules are given for the other residues.

Bacterial Proteins↗
PubMed1999

Modeling alpha-helical coiled coils: analytic relations between parameters.

This paper deals with the alpha-helical coiled coil secondary structure of proteins, which is found not only in many fibrous proteins but also in globular proteins. The standard model used nowadays to describe a coiled coil structure is derived from the mathematical description established more than 40 years ago by F. H. C. Crick (1953, Acta Crystallogr. 6, 685-689) from geometrical arguments. In this paper, we apply stereochemical constraints to the protein chains to refine this model. We present a model based on Crick's calculations with less restrictive hypotheses than the standard model and only requiring a set of initial parameters that can be experimentally measured. In addition, the metrics equation method developed here ensures a minimization of the distortions occurring during the coiling process relating the original straight alpha-helix and the coiled coil minor helix. It leads to a modification of the widely used relation between the numbers of residues per turn in the minor and alpha-helices, mathematically demonstrating a previously semiempirical result. This method can be extended to a wide range of coiled structures.

Models, Molecular↗
PubMed1998

Organization of microfibrils in keratin fibers studied by X-ray scattering modelling using the paracrystal concept.

Low-angle X-ray scattering patterns of hard alpha-keratin fibers have been studied for more than 50 years but a completely convincing modelling has never been presented. The models which have been proposed so far are specific to the sample and cannot be adapted to others, mainly because they do not use a parametric analytical expression of the distribution function describing the relative positions of the microfibrils. Our new approach is based on a paracrystal distribution function. In addition, a huge background originating from a non-ordered matrix is taken into account. Various hard alpha-keratins from different origins have been studied using our approach. From the rather good modellings obtained, it appears that the diameter of the microfibril is not origin dependent (7.4 nm) whereas the distances between microfibrils and their electron density profiles are. Hair microfibrils can be reasonably approximated by a solid cylinder but a core and an outer ring are necessary for porcupine. Our method is of course not limited to keratin microfibrils; it can be used for modelling equatorial X-ray scattering profiles of all types of hexagonal fibrillar assemblies, which are in fact widely found in biological tissues.

Animals↗