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Effets du chlorure de guanidinium sur la structure et les propriétés de la caséine- beta en solution et à l'interface avec l'air.

Abstract : The subject of my work comprises three different part: below a summary:

Small angles neutron scattering study:
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1-I examined the small-angle neutron scattering profile of beta-casein in the native state and in highly denaturing conditions.
It shows that the neutron spectra given by unfolded beta-casein is similar to those of excluded volume polymer chains. The
scattering functions of the semi-flexible chains have been determined with precision. Many approximations have been used in
fitting of experimental small-angle neutron scattering data from beta-casein in guanidine hydrochloride.

The small-angle neutron scattering provides a new tool for investigation of the structure of biological particles. (SANS)
data has showed that the denaturation was accompanied by a large decrease of the radius of gyration of beta-casein and its
all completely different from the experimental results of radius of gyration of globular proteins (Phosphoglycerate Kinase).
A complementary approach to interpret the SANS data involves the use of analytical polymer theories to describe the chain
configurational distribution. To our knowledge, very few studies of scattering functions of semi-flexible chains with excluded
volume effects have been done. The only two analytical studies known so far are the one by Sharp and Bloomfield and the other
by Pedersen et. Al.

The models for chains with excluded volume fit our experimental data very well, and various results for the structure of the
chains have been obtained.

2- I still studied the beta-casein micelle in aqueous solution in a wide range of temperatures and denaturant concentrations.
I consider that the micelles consisting of a completely uniform inner core formed from the hydrophobic amino acids blocks
and uniform outer shell composed of the soluble hydrophilic amino acids blocks. Good agreement with the small angle neutron
scattering data on the beta-casein is obtained. The micelles grow in size with increase temperature and the corona are similar
to the outer region of mushroom or a brush consisting of end grafted polymer chains.

Netron reflectivity and tensiometer study:
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Adsorption layers of beta-casein formed from a buffer including various concentrations of guanidine hydrochloride (gdmCl)
have been studied by neutron reflectivity. A transition in the structure and in the properties of the adsorption layer seems
to occur around a gdmCl concentration of 1.5 M. These data are interpreted assuming that the adsorbed protein molecules behave
like multi-block copolymers with alternating hydrophilic and hydrophobic sequences. Below the transition the hydrophilic coils
and the hydrophobic two-dimensional blocks have a larger fractal dimension than beyond the transition where they look more
swollen by the " solvent ". The effect of temperature on these effects indicates that they are not dominated by hydrophobic
interactions. Thus the attractions between amino acids which are ruptured by gdmCl might be hydrogen bounds which are frequently
encountered in the secondary structure of polypeptide chains. These results show that even with the flexible polypeptide chain
of beta-casein, interactions between amino acids contribute significantly to the structure of the adsorption layer formed
from a buffer devoid of denaturing agent.
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Contributor : Adel Aschi <>
Submitted on : Monday, October 13, 2003 - 5:32:27 PM
Last modification on : Monday, February 10, 2020 - 6:12:13 PM
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Adel Aschi. Effets du chlorure de guanidinium sur la structure et les propriétés de la caséine- beta en solution et à l'interface avec l'air.. Analyse de données, Statistiques et Probabilités [physics.data-an]. Institut national agronomique paris-grignon - INA P-G, 2001. Français. ⟨tel-00003548⟩

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