Contribution à l'étude de la structure de la chromatine: I - Etude physico-chimique de la stabilité de la chromatine

Physicochemical studies of calf thymus chromatin were performed on micromicellar suspensions by thermal denaturation. These diluted suspensions were obtained, by a controlled shearing method, from a compact gel chromatin. Sedimentation and free-flow electrophoresis determined the size distribution of these particles. The most important result is a new transition on the melting profiles corresponding to a sudden increase of solution turbidity. This chromatin solution transition occurs at a higher temperature than usual DNA transition. The degree of « turbidity transitiondiminishes with micelle size but disappears when they are very mildly degraded by DNAases and when F₁ histone fraction is removed.This transition is not only size dependent but also depends on the micellar structure. This phenomenon is interpreted as an excluded volume effect by contact between compact and native regions of nucleoprotein micelles and denatured coils of DNA. Our study tried to show that the degree of turbidity transition can be a criterion of chromatin native structure.     

Etude,par spectroscopie infra-rouge,de la conformation de quelques composés peptidiques modèles

Some experimental data are given on the infrared spectra between 3300 and 3500 cm^?1 of dilute solutions in carbon tetrachloride of three types of model compounds: CH_3?CONH-CH(R₁)-CONH(R₂), (I); CH₃-CON(CH₃)-CH(R₁)-CONH(R₂), (II) and CH₃-CONH-CH(R₁)-CON(R₂)₂, (III). In studying the N-H stretching bands, it was found that there are two types of intramolecular hydrogen bonds in these molecules; these result in two different cyclized conformations, C₅ and C₇, which contain respectively, five and seven atoms in the ring. By using model substances I, II, and III, in which the nitrogen atoms are unequally substituted, it is possible to identify the N-H stretching bands which are to be ascribed to the N-H oscillators included in the two different chelated conformations. It is found also that the stretching frequency of a free N-H oscillator depends upon the substituent on the nitrogen atom. Thus, it is possible to observe, with some of the model compounds I, four different absorption bands located at 3340, 3420, 3440, and 3460 cm^?1. The first two are ascribed to the N-H oscillators included in the H? bonds which lock the C₇ and C₅ conformations; the last two correspond to free N-H which differ with the substituent on the nitrogen atom.     

Etude de quelques propriétés de la phosphofructokinase érythrocytaire de l'homme normal

Human erythrocyte phosphofructokinase was purified 150 fold by DEAE cellulose adsorption and ammonium sulfate precipitation.At pH 7,5 the enzyme exhibits allosteric kinetics with respect to ATP, fructose 6 phosphate, and Mg²⁺.ATP at high concentration acted as an inhibitor and ADP, 5′AMP, 3′,5′, AMP, acted as activators. Both effectors seemed to decrease the homotropic interactions beetween the fructose 6 phosphate molecules.The activators increased the affinity of phosphofructokinase for the substrate (F6P), the inhibitor decreased it.These ligands had no effect on the maximum velocity of the reaction except in the case of ADP.Interactions between the substrates and the effector ligands on the enzyme were considered in terms of the Monod - Changeux - Wyman model for allosteric proteins.With GTP and ITP, no inhibition was observed. At saturing concentration of GTP, ATP still inhibited phosphofructokinase.Both 3′5′ AMP and fructose 6 phosphate increased the concentration of ATP required to produce an inhibition of 50 %.Citrate, like ATP, inhibited phosphofructokinase by binding most likely at the same allosteric site. Erythrocyte phosphofructokinase is inhibited by 2–3 DPG.The study of the relation log V max = f (pH) suggested, that the active center contains at least one imidazole and one sulfhydryl group.     

Etude ultrastructurale comparée du processus de dégradation de l'hémoglobine par P. berghei (Vincke et Lips, 1948) en fonction de l'état de maturité de la cellule hǒte1

By serial sectioning and 3D reconstruction we have been able to demonstrate that the type of system for hemoglobin digestion in two strains of Plasmodium berghei, N and RC, is dependent on the maturity of the host cell. In parasites growing in erythrocytes, both systems for the endocytosis of hemoglobin—micropinocytosis and the cytostomal system (i.e. a cytostome budding a cytostomal tube that releases food vacuoles)—are fully functional and produce a great quantity of residual pigment. Parasites growing in reticulocytes have a disrupted cytostomal system; no tube is formed and only food vacuoles are visible in their cytoplasm. Residual pigment is smaller in size and in quantity. The reduced quantity of pigment in reticulocytes is explained by our observation of the exocytosis of pigment. We propose a hypothesis that relates the process of degradation of hemoglobin to the maturity of the host cell and a possible mechanism of protection against chloroquine, a drug known for its affinity for malarial pigment.     

Etude par spectroscopie infrarouge de la transformation hélice–chaîne statistique des polypeptides. II. Etude de l'interaction de quelques polypeptides à chaînes latérales polaires avec l'acide trifluoracétique

The various polymer–acid solvation possibilities occuring in the helix–coil transition process of polypeptides with polar side chains were systematically analyzed by infrared spectroscopy. The following samples have been considered: poly-γ-benzyl-L -glutamate (PBLG), alternating poly-γ-benzyl-D ,L -glutamate (PBD-LG), and poly-β-benzyl-L -as-partate (PBLA). The behavior of the amide A, I, II, and νC?O ester absorptions of each polymer dissolved in trifluoroacetic acid–chloroform mixtures was studied in depth. The classical assumptions concerning the interaction between a polypeptide and a proton donor solvent are discussed. This interaction was previously proposed in a theoretical model of helix–coil transition. For PBLG, the spectral characteristics of the cooperative transition are evidenced by the amide bands. These bands also show main chain–acid hydrogen bonding (I) Quantitative analysis of phenomenon (I) was performed in order to localize the “binding sites” of the polymer. In agreement with the theory, only the complexation of peptide units belonging to random coil and terminal helical regions were observed. However, in contrast to the theory in which the association constants KCO and KNH of these residues are generally kept equal, the present results have shown that the main binding site is the carbonyl group (KNH ? 0 or « KCO ). The behavior of the polar side chains of these polypeptides were analyzed during the transition. Similarly to the peptide backbone, they bind the acid by hydrogen bonding (II) Furthermore, this association is more important when the side chains are localized in the coiled regions than in the helical ones. This result suggests, by analogy with the main chain behavior, that the helix–coil transition theory should take into account two more association constants for polar side chains, namely k₁ for the helical regions and k₂ > k₁ for the coiled ones.     

Intensification de la diapause embryonnaire d‘Archanara sparganii par la température: Incidences écologiques

Résumé L'incubation des oeufs d'Archanara sparganii, Esper (Lepidoptera: Noctuidae) s'étend sur 8 à 9 mois dans les conditions naturelles. Elle est temporairement interrompue par une diapause qui intervient à un stade précoce de l'embryogénèse et se prolonge de l'été à la fin de l'hiver. Au laboratoire, l'incubation est néanmoins possible à température constante entre 8 et 21 °C. Une étude expérimentale a montré que l'intensité de la diapause s'accroît lorsque la température en début d'incubation est portée pendant 15 jours de 11 °C à 15 °C et 21 °C. Aucun des modèles expérimentaux faisant intervenir la température n'a permis d'éviter totalement la diapause. La durée de celle-ci a néanmoins été réduite des trois quarts dans certaines circonstances, rendant possible l'éclosion des larves avant l'hiver. Ces résultats servent à interpréter les altérations du cycle biologique du nord au sud de la distribution d'A. sparganii. La perte rapide de réceptivité de Typha spp. à l'égard des larves jeunes justifie par ailleurs une entrée en diapause précoce de l'espèce sous climat méditerranéen.     

Etude électroantennographique de la réception de stimulus odorants d'origine végétale chez Acrolepiopsis assectella

Résumé Les réponses E.A.G. d'Acrolepiopsis assectella Zell., Lépidoptère Hyponomeutoïdea spécialiste des Allium, montrent une sensibilité à l'odeur de la plante-hôte larvaire A. porrum saine ou attaquée, et à ses composés volatils soufrés spécifiques, toutes odeurs actives dans l'attraction des deux sexes en olfactomètre. Le Ti-P₂, le plus abondant et le plus spécifique du végétal est toujours le plus efficace. Ceci confirme le rôle prépondérant des substances allélochimiques soufrées dans la recherche de la plante-hôte. Une réactivité antennaire plus ou moins importante est également enregistrée avec l'odeur de certaines plantes non-hôtes (chou et tomate). L'antenne du lépidoptère est également très sensible à certains produits volatils du complexe odorant vert des végétaux qui sont identifiés dans l'odeur du poireau, le cis-hexen-3-ol-1 le plus actif est aussi le seul à être attractif. Le rôle biologique exact des composés verts et l'interférence de l'odeur complète de la plantehôte avec celle de plantes non-hôtes, telle que la tomate, efficace sur l'antenne et sur le comportement sont discutés.     

Contribution Biochimique à la Systématique de l''Ordre des centrospermales

A chemosystematic survey of flavonoids have been made in 55 centrospermous species. On the basis of both the experimental results and published data, the evolutionary significance of distribution patterns of proanthocyanidins, anthocyanidins, C-glycoflavones, flavonols and flavones is discussed. The Nyctaginaceae and Didiereaceae appear to be particularly primitive families, while Caryophyllaceae, Aizoaceae, Cactaceae, Amaranthaceae and Chenopodiaceae are the most advanced of the order.     

Variations spectroscopiques induites par illumination et par action chimique observées à—196°C dans la chlorophylle a de Porphyridium

Spectroscopic Changes in the Chlorophyll a of Porphyridium Induced by Illumination and Chemical Action and Observed at ?196°C. Photo-oxidation of P700 by 708 nm light can take place under weak intensity (10^?6 W × cm^?2) when the medium is frozen. Spectral characteristics of “700 nm” and “690 nm” variations are accurately measured. The amplitude of the photoinduced changes of absorption are similar to those induced by chemical action. In the case of Porphyridium, an apparent increase of the extinction power of P700 at ?196°C is observed. This fact seems to be due to a diminution of the bandwith of the neighbouring pigments. Irradiation with red light (685 nm), of a relatively high intensity (10^?2 W × cm^?2), in the presence of oxygen at ?196°C, induces a slight shift (0.5 nm) of the red absorption band maximum towards longer wavelengths. This change is similar to the one promoted by ferricyanide in the dark. The origin and the functional significance of the phenomenon is discussed.     

Etude par Spectroscopie Infrarouge de la Transformation Hélice—Chaine Statistique des Polypeptides. I. Etude Comparée de l'Interaction de la Poly-L-alanine et d'un Amide Modèle avec l'Acide Trifluoroacétique

Infrared spectra of poly-L -alanine in trifluoroacetic acid-chloroform mixtures have been investigated and compared with those of a model amide (N-methylacetamide). The purpose of this work is to determine the nature of peptide-acid specific interactions responsible for the helix-random coil transition of polymer chains. Analysis is made in using amide (A, I, II, III) and acid (νC?O, νOH) vibrations which are specially sensitive to molecular interactions. We examined a model compound to determine the spectral characteristics of the different complexes or species formed between amide and acid. At a low acid concentration, hydrogen-bonded complexes: ? (NH) C?O…?HOOCCF₃ (1) are evidenced but no association between amide NH and acid CO groups (complexes A) is observed. For higher acid concentrations complexes (I) are progressively changed into ions pairs and free ions, which result from amide protonation by acid, according to the exothermic equilibrium (I)?? (NH)COH⁺, ^?OOCCF₃(II). Amidium and carboxylate bands are localized between 1680–1705 cm^?1 and 1620–1625 cm^?1, respectively. If the cation band is always clearly seen, the anion band is only observed for the most acidic solutions. For the polymer, a gradual complexation of type (I) is observed for all acid concentrations. From our results, the assumption of an (A) type interaction seems very unlikely but cannot be excluded. Moreover, proton transfer—similar to that observed with a model amide—is never evidenced since, in particular, the amidium band characteristic of protonation is never seen. In contrast to previous investigations, we conclude that the helix-random coil transition of polypeptides is not due to the protonation of the peptide functions. This transition does suggest a strong interaction by hydrogen bonds between polymer and acid molecules.     

Biosynthèse de cellulose bacterienne à partir de glucose sélectivement deuterié en position 6: Étude par RMN

D-glucose specifically deuterated at C-6 was prepared and used for the biosynthesis of bacterial cellulose with Acetobacter xylinum. The material obtained was converted into glucitol hexaacetate and analyzed by 250 MHz n.m.r. and mass spectrometry. These spectra indicated that about 70% of the starting D-glucose was incorporated without modification of deuteriation at the C-6 position. However an explanation is required of the finding that deuterium was also incorporated at the C-2 and C-1 positions of the cellulose, arising respectively from the 6 pro-R and 6 pro-S positions of the initial D-glucose.