We propose a model giving the conformation of a star shaped polymer by taking into account the radial variation of the monomer concentration ϕ( r). For an isolated star when increasing r (at the centre of the star r = 0), the variation of ϕ (r) is first given by a constant value (r < f 1/2 l) then has a (r/l)-1 variation (for f1/2 / < r < f1/2 ν-1 /) and finally a (r/l)-4/3 variation (for r > f1/2 ν-1 l); wh ere f is the number of branches, N the number of monomers in a branch and ν and l are the excluded volume and the length associated to a monomer. For all these cases, it is shown that the size of a branch is always larger than that of a linear polymer made of N monomers. Beyond the overlapping concentration the star conformation is obtained from two characteristic lengths essentially : χ(c ) a radius inside which the branches of the other stars do not penetrate, this radius defines a domain where the conformation of a star is similar to that of an isolated one. Beyond χ(c) the interpenetration of branches is characterized by a screening length ξ(c) very similar to that found for semi-dilute solutions of linear polymers. For all these regimes the variation of the size of a star is predicted as a function of N, f, v and c

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BgK is a K؉ channel-blocking toxin from the sea anemone Bunodosoma granulifera. It is a 37-residue protein that adopts a novel fold, as determined by NMR and modeling. An alanine-scanning-based analysis revealed the functional importance of five residues, which include a critical lysine and an aromatic residue separated by 6.6 ؎ 1.0 Å. The same diad is found in the three known homologous toxins from sea anemones. More strikingly, a similar functional diad is present in all K ؉ channel-blocking toxins from scorpions, although these toxins adopt a distinct scaffold. Moreover, the functional diads of potassium channel-blocking toxins from sea anemone and scorpions superimpose in the threedimensional structures. Therefore, toxins that have unrelated structures but similar functions possess conserved key functional residues, organized in an identical topology, suggesting a convergent functional evolution for these small proteins.Functional properties of proteins are frequently associated with a small number of important residues. For example, enzyme activities depend on a few residues that are essential for catalysis. Also, protein-protein recognition processes have been predicted (1) and recently demonstrated (2) to be energetically driven by a small proportion of the residues forming the contacting areas in protein-protein complexes, as identified by x-ray studies (3, 4). Among the proteins whose major functions require protein-protein interactions are animal toxins, which bind to various molecular targets, such as receptors or ion channels, using a small number of binding residues (5-8). As has been shown for enzymes (9), toxins with different architectures are capable of exerting similar functions (10). However, in contrast to enzymes, the molecular basis associated with the conservation of the function in structurally unrelated toxins remains unknown. In this paper, we show that two families of animal toxins with different folding patterns but a comparable capacity to bind to potassium channels include similar functional diads, composed of a critical lysine and an aromatic amino acid separated from each other by 6.6 Ϯ 1.0 Å. MATERIALS AND METHODS Synthesis of Toxin and Mutants-The amino acid sequence of BgK 1 was proposed a few years ago (11). However, chemical synthesis attempts, based on these data, systematically failed. The proposed amino acid sequence was therefore questioned, re-examined, and ultimately corrected.2 The revised amino acid sequence of BgK from Bunodosoma granulifera is: VCRDWFKETACRHAKSLGNCRTSQKYRANCAKTC-ELC. BgK and each alanine-substituted analog were synthesized by solid phase synthesis using an Applied Biosystems model 431A peptide synthesizer, starting from 0.1 mmol of Rink-resin (4-(2Ј,4Ј-dimethoxyphenylhydroxymethylphenoxy resin; 0.48 mmol/g). A 10-fold excess (1 mmol) of Fmoc (N-(9-fluorenyl)methoxycarbonyl)-protected amino acid was used and coupled in N-methylpyrrolidone in the presence of N,NЈ-dicyclohexylcarbodiimide/1-hydroxybenzotriazole. The following side chain protections wer...

Neutron coherent scattering techniques have been used for the determination of the conformation of polymer in bulk and experimental details are given about the application of this method to the study of polymeric systems. Measurements have been made for small and intermediate momentum ranges on a series of eight monodisperse deuterated polystyrenes of molecular weight ranging from 21,000 to 1,100,000. The results lead to the concluson that in amorphous state the conformation of the polymer molecule is indistinguishable from that in solvent and that the Debye scattering function which is valid for unperturbed chains applies for q ~1 as low as 10 Á.

Résumé. 2014 Nous examinons les résultats d'expérience de diffusion de neutrons aux petits angles (10-2 q 2 x 10-1 Å-1) par des solutions de sulfonate de polystyrène dans l'eau pour des concentrations supérieures à 0,5 x 10-2 g. cm-3.Avec de l'eau ultra pure, l'absence de diffusion centrale et la présence d'un maximum pour les plus grandes valeurs de q caractérisent la fonction de diffusion S(q). Les deux effets étudiés, variation de la concentration en polyions et addition de sel, conduisent à des résultats différents : la position du maximum varie avec la concentration de polyions alors qu'avec l'addition de sel, on note l'apparition de diffusion centrale et disparition du maximum.Abstract. 2014 Solutions of polyelectrolyte are studied by small angle neutron scattering for c > 0.5 x 10-2 g. cm -3.The function S(q) is characterized by a small value of S'(q ~ 0) and one broad maximum for higher q's