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Internal Position (internal + position)
Selected AbstractsEarly Mesozoic evolution of alivincular bivalve ligaments and its implications for the timing of the ,Mesozoic marine revolution'LETHAIA, Issue 2 2004MICHAEL HAUTMANN The early Mesozoic radiation of the Pteriomorphia was accompanied and furthered by the development of several new types of alivincular ligaments. These new types evolved as modifications of the primitive alivincular-areate (new term) ligament, which is characterized by an ontogenetic shift of both the central resilium and the straight lateral ligament in the direction of main shell growth. Arching of the attachment surface of the ligament led to the alivincular-arcuate (new term) ligament type, which has been realized by the Ostreidae only. By contrast, a replacement of the lateral ligament by hinge teeth, limiting the (primary) ligament to a central groove (alivincular-fossate, new term), has evolved independently in three families (Dimyidae, Plicatulidae and Spondylidae). Functionally, both kinds of modification effectively impede shearing of the valves and are interpreted as an antipredatory adaptation advantageous in the cemented habit of these families. The alivincular-alate (new term) ligament of the Entoliidae and Pectinidae differs from the other types of alivincular ligaments by different growth directions of resilium and lateral ligament, which result in an internal position of the resilium suitable for fast and powerful opening of the valves. This arrangement is an important prerequisite for effective swimming, which, in its turn, is a behaviour chiefly used to escape from predator attacks. The simultaneous early Mesozoic appearance of different antipredatory adaptations within independent clades hints at increased predator pressure as a stimulant and may therefore point to a contemporaneous proliferation of durophagous predators. Hence, an important aspect of the ,Mesozoic marine revolution' might have started earlier than previously thought. [source] Emerging views on tmRNA-mediated protein tagging and ribosome rescueMOLECULAR MICROBIOLOGY, Issue 4 2001Reynald Gillet Transfer- messenger RNA (tmRNA), also known as SsrA or 10Sa RNA, is a bacterial ribonucleic acid that recycles 70S ribosomes stalled on problematic messenger RNAs (mRNAs) and also contributes to the degradation of incompletely synthesized peptides. tmRNA acts initially as transfer RNA (tRNA), being aminoacylated at its 3,-end by alanyl-tRNA synthetase, to add alanine to the stalled polypeptide chain. Resumption of translation ensues not on the mRNA on which the ribosomes were stalled but at an internal position in tmRNA. Termination soon occurs, tmRNA recruiting the appropriate termination factors allowing the release of the tagged protein that is subsequently recognized and degraded by specific cytoplasmic and periplasmic proteases, and permits ribosome recycling. Recent data suggest that tmRNA tags bacterial proteins in three other instances; when ribosomes stall at internal sites; during ,readthrough' of canonical termination codons; and when ribosomes are at the termination codon of intact messages. The importance of bacterial tmRNAs for survival, growth under stress, and pathogenesis is also discussed. Recent in vivo and in vitro studies have identified novel ligands of tmRNA. Based on the available experimental evidences, an updated model of tmRNA mediated protein tagging and ribosome rescue in bacteria is presented. [source] Sequence dependence of ,-hairpin structure: Comparison of a salt bridge and an aromatic interactionPROTEIN SCIENCE, Issue 12 2003Sarah E. Kiehna Abstract A comparison of the contributions and position dependence of cross-strand electrostatic and aromatic side-chain interactions to ,-sheet stability has been performed by using nuclear magnetic resonance in a well-folded ,-hairpin peptide of the general sequence XRTVXVdPGOXITQX. Phe,Phe and Glu,Lys pairs were varied at the internal and terminal non,hydrogen-bonded position, and the resulting stability was measured by the effects on ,-hydrogen and aromatic hydrogen chemical shifts. It was determined that the introduction of a Phe,Phe pair resulted in a more folded peptide, regardless of position, and a more tightly folded core. Substitution of the Glu,Lys pair at the internal position results in a less folded peptide and increased fraying at the terminal residues. Upfield shifting of the aromatic hydrogens provided evidence for an edge-face aromatic interaction, regardless of position of the Phe,Phe pair. In peptides with two Phe,Phe pairs, substitution with Glu,Lys at either position resulted in a weakening of the aromatic interaction and a subsequent decrease in peptide stability. Thermal denaturation of the peptides containing Phe,Phe indicates that the aromatic interaction is enthalpically favored, whereas the folding of hairpins with cross-strand Glu,Lys pairs was less enthalpically favorable but entropically more favorable. [source] The [2+2]-Cycloaddition Reaction of Bisallenes: A Dramatic Substituent Effect,CHINESE JOURNAL OF CHEMISTRY, Issue 9 2010Ping Lu Abstract The bisallenes 1 possessing a phenylsulfonyl or n -butyl substituent at the internal position of allene moieties could be smoothly transformed to the bicyclo[5.2.0] or bicyclo[6.2.0] derivatives 2 in moderate to excellent yields with good regioselectivity upon refluxing in toluene. This [2+2]-cycloaddition reaction shows an interesting substituent effect as unveiled by comparing with the results of unsubstituted substrates. However, further attention should be paid to the control of the regioselectivity of the [2+2]-cycloaddition involving different CC bonds. [source] Reaction of 1,1,-Divinyl Ferrocene with One-Electron Oxidants: Entry into Functionalised [4]Ferrocenophanes and Observation of an Isotope-Dependent Chemoselectivity EffectCHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2010Renate Abstract 1,1,-Divinyl ferrocene (2) reacts with K3[Fe(CN)6] under basic biphasic conditions to give a [4]ferrocenophane (4) in good yield. Incorporating deuterium labels into the internal positions of the vinyl groups of 2 affects the chemoselectivity of the reaction; thus under identical reaction conditions, [D2]- 2 reacts to provide a diol-functionalised [4]ferrocenophane, [D2]- D/L - 6 in addition to the expected keto-alcohol, [D1]- 4. Variants on this one-electron oxidative cyclisation methodology can be used to give other [4]ferrocenophanes; thus, the reaction of 2 with CuCl2 in MeOH or iPrOH leads to dialkoxy [4]ferrocenophanes 19 and 20, respectively, whereas the reaction of 2 with benzyl carbamate in the presence of tBuOCl gives a bis(carbamate)[4]ferrocenophane, 21. Mechanisms to account for the formation of the products, the stereoselectivity, and the unusual isotope-dependent chemoselectivity in the reaction of 2 and [D2]- 2 with K3[Fe(CN)6] are proposed. [source] Solid-Phase Synthesis of DOTA,PeptidesCHEMISTRY - A EUROPEAN JOURNAL, Issue 5 2004Luis M. De León-Rodriguez Dr. Abstract A general synthetic route to two DOTA-linked N -Fmoc amino acids (DOTA-F and DOTA-K) is described that allows insertion of DOTA at any endo -position within a peptide sequence. Three model pentapeptides were prepared to test the general utility of these derivatives in solid-phase peptide synthesis. Both DOTA derivatives reacted smoothly by means of standard HBTU activation chemistry to the point of insertion of the DOTA amino acid, but extension of the peptide chain beyond the DOTA-amino acid insertion required the use of pre-activated C -pentafluorophenyl ester N - , -Fmoc amino acids. Three Gal-80 binding peptides (12-mers) were then prepared by using this methodology with DOTA positioned either at the N terminus or at one of two different internal positions;the binding of the resulting GdDOTA-12-mers to Gal-80 were compared. The methodology described here allows versatile, controlled introduction of DOTA into any location within a peptide sequence. This provides a potential method for the screening of libraries of DOTA-linked peptides for optimal targeting properties. [source] | ||||||||||