Macromolecular Interactions Characterization by Analytical Ultracentrifugation

polydisperse polymers of amino acid residues - the popular concept amongst colloid chemists at that time. This revolutionary finding was the seminal contribution that generated the present-day fields of protein chemistry and molecular biology. Indeed, for 40 years, the analytical ultracentrifuge was the primary source of information on the molecular mass and heterogeneity of proteins, as well as on the polydis-persity of polymer preparations. At that stage, preparative ultracentrifugation, gel permeation (size exclusion) chromatography and gel electrophoresis were developed to take advantage of macromolecular separation on the basis of size and shape (as well as charge in some forms of gel electrophoresis). Meanwhile, the sedimentation equilibrium variant of analytical ultracentrifugation retained supremacy for many years as the benchmark standard of molecular mass measurement for a homogeneous protein - a role now being taken over by mass spectrometry. Furthermore, inroads into its status as the method of choice for determining molecular size distributions for polydisperse polymer preparations have now been made by light scattering as the means of analysing the column eluate in gel permeation chromatography. Nevertheless, such is the strength of analytical ultracentrifugation that it has survived those losses of traditional roles to become an invaluable tool for the characterization of reversible macro-molecular interactions - a task which is beyond most of the techniques that have usurped its traditional roles.

Many proteins comprise a mixture of monomeric and polymeric states that coexist in rapid association equilibrium, whereupon the relative proportions of the two macromolecular states vary with total solute concentration in accordance with Le Cha-telier's principle: the polymeric state is favoured by an increase in solute concentration whereas monomer is favoured by dilution. Analytical ultra-centrifugation has great potential for characterizing the self-association equilibrium by virtue of these concentration-dependent changes in the average macromolecular state of the solute, and also for the characterization of rapid equilibria involving dissimilar reactants, for which similar considerations apply. The main emphasis in current analytical ultracentrifugation is thus the study of noncovalent macromolecular association equilibria: proteinprotein interactions such as those involved in enzyme self-association or the binding of an antibody to its eliciting protein antigen; and protein-nucleic acid interactions such as those associated with regulation of the transcription of genetic information.

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Solar Panel Basics

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