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Acetyl-APhe-L-Ala- APhe-l)-Ala MethylEster
No full textThe peptide chain of acetylphenylalanine-L-alaninephenylalanine-~
alanine methyl ester, C27H30N406,
adopts a 3~0-helical conformation having tight-handed
screw sense. The 310-helix is stabilized by intramolecular
hydrogen bonds, between CO of the acetyl group and
NH of APhe 3, and between CO of APhe I and NH of
D-Ala 4. The hydrogen bonds form two consecutive tenmembered
rings whose (% z/.,) torsion angles are quite
close to the standard values for type-Ill/3-turns. In the
crystal, the molecules are linked head-to-tail by intermolecular
hydrogen bonds to form continuous helical
columns. These are aligned along axes parallel to the
c axis, with neighbouring columns running in opposite
directions. There are no lateral hydrogen bonds between
helical columns, but only hydrophobic interactions provided
by the interdigitation of apolar side chains of the
dehydro-phenylalanine residues, as well as of the Cterminal
methyl ester groups
Optically active polymers bearing side-chain photochromic moieties: synthesis and chiroptical properties of methacrylic homopolymers with pendant trans-azobenzene chromophores bound through L-leucine, L-valine and L-proline amino acid spacers
No full textFull Paper: Novel optically active monomers, based on different L-amino acid residues such as trans-(S)-4-(2-methacryloylamino-3-methylbutanoylamino)azobenzene, trans-(S)-4-(2-methacryloylamino-4-methylpentanoylamino)azobenzene and trans-(S)-4-(N-methacryloyl-2-pyrrolidinoylamino)azobenzene, have been prepared and homopolymerized by free radical initiation. Circular dichroism spectra of the resulting polymers, as compared with those of the corresponding low molecular weight analogues, purposely synthesized, allow one to suggest that the macromolecules assume in solution achiral or chiral conformations with a prevailing screw sense, depending on the bulkiness and rigidity of the L-amino acid residue present in the side chains. The role of intra- and/or intermolecular hydrogen bonding between side-chain amido groups along the backbone and the structural requirements of the chiral groups in determining the macromolecular arrangement is also discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
3(10)-Helices, helix screw sense and screw sense reversal in the dehydro-peptide Boc-Val-delta Phe-Gly-delta Phe-Val-OMe
No full textTitle pentapeptide Boc-Val-ΔPhe-Gly-ΔPhe-Val-OMe [Boc = Me3CO2C; ΔPhe = (Z)-dehydrophenylalanine] has been synthesized and its structure investigated. In the cryst. state, the mol. adopts a right-handed 310-helical conformation stabilized by two intramol. hydrogen bonds between CO of Val1 and NH of ΔPhe4, and between CO of ΔPhe2 and NH of Val5, resp. NMR measurements are consistent with the presence of 310-helical structures also in acetonitrile and DMSO soln.: the distances between backbone protons estd. from NOE connectivities are in overall agreement with those obsd. in the solid state; the chem. shifts of the amide protons show the smaller temp. coeffs. for the NHs that in solid state are involved in intramol. hydrogen bonds. The CD spectra in acetonitrile, chloroform, methanol and DMSO display exciton couplets of bands corresponding to the ΔPhe electronic transition at 280 nm; the sign of the bands is consistent with the presence of helical structures having a prevalent left-handed screw sense. Addn. of 1,1,1,3,3,3-hexafluoro-propan-2-ol gives rise to the gradual appearance of a couplet of opposite sign, suggesting the helix reversal from left-handed sense to right-handed sense. The conformational behavior is discussed on the basis of the specific sequence of the peptide
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Photoresponsive Polypeptides: Photochromism and Conformation of Poly(l-Glutamic Acid), Containing Spiropyran Units
No full textSpirobenzopyran units were bound to the side chains of poly (L-glutamic acid) and partially methylated poly(L-glutamate)s. The modified polymers were found to exhibit ''reverse photochromism'' in hexafluoro-2-propanol (HFP), so the samples kept in the dark were characterized by an intense absorption band in the visible range of the spectrum, which was completely erased upon exposure to sunlight or irradiation at 500-550 nm.
The CD spectra showed that the macromolecules adopted a random coil conformation in the dark, whereas the bleached solutions after exposure to light displayed the typical CD pattern of the alpha-helix. The back reaction in the dark was accompanied by the progressive decrease of the helix content and recovery of the original disordered conformation. The photoinduced conformational changes resulted in large and reversible viscosity variations.
When spiropyran side chains were converted to ''spiropyran salts'' of trifluoroacetic acid, the system was still photochromic, but the macromolecules were disordered both in the dark and light conditions. However, when appropriate amounts of methanol were added as a cosolvent to the HFP solutions, the system responded to light, giving reversible variations of the alpha-helix content. Irradiation at appropriate solvent compositions allowed modulation of the extent of the photoresponse
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Solid state and solution structure of Boc-L-Ala-Δphe-Δphe-NHMe: a dehydropeptide showing propensity for 310-helices of both screw senses
No full textThe crystal and molecular structure of the peptide Boc-l-Ala-Δphe-Δphe-NHMe, containing two consecutive dehydro-phenylalanine (Δphe) residues, has been solved by x-ray diffraction. Two independent molecules, X and Y, are present in the crystallographic unit. Their conformation corresponds approximately to an incipient 310-helix stabilized by two intramolecular hydrogen bonds. The (φ, ψ) torsion angles, however, have negative and positive signs in the two molecules X and Y, respectively. Therefore, in spite of the presence of an amino acid residue of the l configuration, the two helical molecules have opposite screw senses, even though the right-handed helix is less distorted than the left-handed one in correspondence of the l-Ala residue.
The CD spectra in various solvents exhibit exciton bands originating from dipole–dipole interaction between the Δphe side chains. Addition of DMSO to the chloroform solution produces, as a first step, a strong increasing of the CD bands, which are then progressively canceled by increasing DMSO concentration. The nmr data parallel the behavior observed in the CD spectra. In CDCl3 solution, the temperature coefficients of the NH resonances are consistent with the involvement of the last two amide protons of the sequence in intramolecular hydrogen bonds, but only negligibly small nuclear Overhauser effects (NOE) are observed. Addition of 5% DMSO-d6 allows the observation of diagnostic NOEs. CD and nmr data indicate that the solid state structure is retained in solution, and are consistent with the presence of right-handed and left-handed conformers, with a prevalence of the more stable right-handed one
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