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Cotton Effects (cotton + effects)
Selected AbstractsSynthesis and study of a gramicidin B mutant possessing a ditryptophan crosslinkJOURNAL OF PEPTIDE SCIENCE, Issue 9 2002Alice L. Presley Bodnar Abstract Recent studies of peptide dimers linked by Trp-Trp (ditryptophan) crosslinks suggest that the crosslinks can reinforce antiparallel ,-structure. Depending on environment, gramicidins A, B and C form either helical ion channels with parallel ,-structure or non-functional pores with antiparallel ,-structure. In the channel conformation of the gramicidins Trp9 and Trp15 are close in space, but in the pore conformation Trp9 and Trp15 are far apart. We hypothesized that a ditryptophan crosslink between Trp9 and Trp15 could pre-organize gramicidin in an active conformation. To test the potential for pre-organization, an intramolecular ditryptophan crosslink was formed between Trp9 and Trp15 in a W13F mutant of gramicidin B. Photooxidative conditions were shown to generate ditryptophan crosslinks in low yields. While not preparatively useful, photooxidative tryptophan crosslinking may have implications for protein aging processes like cataract formation. The ditryptophan crosslink in the gramicidin B mutant substantially lowered the antibiotic activity of the gramicidin B mutant, unlike the ditryptophan crosslink in the antibiotic X-indolicidin. The biaryl chromophore generated diagnostic Cotton effects in the CD spectrum that revealed the absolute stereochemistry of the biaryl chromophore, but the biaryl chromophore obscured diagnostic features below 220 nm. However, changes in peptide conformation were reflected in changes in the biaryl region of the CD spectrum above 240 nm. Copyright © 2002 European Peptide Society and John Wiley & Sons, Ltd. [source] Synthesis and Polymerization of Optically Active N -Propargylphosphonamidates: A Novel Helical Polymer Carrying a P-Chiral CenterMACROMOLECULAR RAPID COMMUNICATIONS, Issue 17 2006Dongmei Yue Abstract Summary: A diastereomeric pair of novel N -propargylphosphonamidates, HCCCH2NHP(O)(CH3)O- L -menthyl was synthesized by the successive condensations of methylphosphonic dichloride with L -menthol and propargylamine. The (R)-P-isomer (1a) was isolated, and the absolute configuration was determined by XRD. Polymerization of 1a, and a mixture of 1a and (S)-P-isomer (1b) was carried out with a zwitterionic Rh complex as a catalyst. cis -Stereoregular polymers with number-average molecular weights of 5,600,9,800 were obtained in good yields. Poly(1a) and poly(1a29 - co - 1b71) exhibited large specific rotations (+408 and ,146°), and intense Cotton effects ([,],=,+2.25 and ,0.9,×,104 deg,·,cm2,·,dmol,1) based on the conjugated polyacetylene backbone around 325 nm in CHCl3, indicating that these polymers have helical structures, whose predominant helical senses are opposite. Polymerization of N -propargylphosphonamidate. [source] Synthesis, chiroptical properties, and their theoretical simulation of some highly rotating benzotricamphor derivatives,,CHIRALITY, Issue 1E 2009Giuseppe Mazzeo Abstract The large molecules 1,3 (69, 90, and 102 atoms, respectively), prepared by cyclotrimerization of enantiomerically pure derivatives of (,)-bornyl acetate, show intense ECD spectra, high optical rotation (OR) values (200,1300, in absolute value) dominated in sign and order of magnitude by the lowest-energy Cotton effects, that is, they are the ideal candidates to test the reliability of our "approximate" (TDDFT/B3LYP/6-31G* or smaller basis set) approach to the calculation of chiroptical properties. As a matter of fact, a correct simulation of the OR values and ECD spectra of 1 and 2 can be obtained even using STO-3G basis set and semiempirical or molecular mechanics input geometries: for 1, at the TDDFT/B3LYP/STO-3G level, the OR values are of the order of 500,550, versus an experimental value ranging between 660 and 690, depending on the solvent. On the contrary, the case of 3 (exp. OR between ,1330 and ,1500) is really complex (for instance, the OR values range between ,3216 and ,729 (TDDFT/B3LYP/6-31G* calculations) or ,1824 and ,444 (TDDFT/B3LYP/STO-3G calculations)), making the comparison between calculated and experimental values more difficult. The behavior of 3 is due to its molecular flexibility, whereas 1 is a really rigid molecules and 2 behaves (vide infra) as it were a rigid system. These observations strongly indicate that the conformational freedom constitutes one of the major difficulties for a correct but simple simulation of the chiroptical properties. Chirality 21:E86,E97, 2009. © 2009 Wiley-Liss, Inc. [source] Self-assembly of multinuclear complexes with enantiomerically pure chiral binaphthoxy imine ligands: Effect of the alkyl spacer connecting two binaphthyl units on the metal bindingCHIRALITY, Issue 9 2006Takeshi Maeda Abstract Metal complexing behavior of enantiomerically pure ,,,-diiminoalkanes possessing the two terminal binaphthyl units (L1 and L2) was studied. The ligands L1 and L2 were prepared by the reaction of optically pure 2,-butoxy-3-formyl-2-hydroxy-1,1,-binaphthyl with propane and pentane diamines. Reactions of L1 and L2 with equimolar amount of Cu(OAc)2 afforded quantitatively multinuclear complexes 1. The structure of 1 was confirmed by MALDI,TOF MS spectroscopy, X-ray single-crystal-structure analysis, and UV/vis and CD spectroscopic analyses. The reaction of L1 having a 1,3-propanediyl spacer resulted in the formation of a self-assembled product, which was assigned as enantiopure trinuclear circular helicate 1a, while the ligand having a 1,5-pentanediyl spacer L2 gave a different self-assembled product, dinuclear side-by-side complex 1b. The circular dichroism (CD) spectrum of 1a in solution showed intense Cotton effects in both the ,,,* transition of the naphthalene units and the LMCT region of the N,O -chelate moieties. The CD spectrum of 1b was completely different from that of 1a; in particular the Cotton effects in the LMCT region were very weak, contrary to those of 1a. These results suggest that 1a retains some chirality induced on the N,O -chelating moieties even in solution, while the induced chirality on the N,O -chelating moieties in 1b is not very significant, being consistent with the consequences of the X-ray single-crystal-structure studies. Chirality, 2006. © 2006 Wiley-Liss, Inc. [source] Effect of intramolecular interactions on circular dichroism of ortho-substituted 1-phenethylamines,CHIRALITY, Issue 8 2004V.M. Demyanovich Abstract The synthesis of ortho-substituted (S)-1-phenethylamines via ortho-lithiation of its N,N-dimethyl derivative followed by reactions with different reagents is described. Circular dichroism spectra of synthesized compounds were measured. It is shown that the observed short-wave Cotton effects greatly depend on the existence of cyclic structure due to possible interactions such as hydrogen bond or electrostatic interaction. Chirality 16:486,492, 2004. © 2004 Wiley-Liss, Inc. [source] Chirality inversion in the bilirubin molecular excitonCHIRALITY, Issue 5 2001Stefan E. Boiadjiev Abstract The bichromophoric pigment bilirubin acts as a molecular exciton in its UV-visible and circular dichroism (CD) spectroscopy. In both polar and nonpolar solvents, an optically active analog, (,R,,,R)-dimethylmesobilirubin-XIII, (1), exhibits intense bisignate CD Cotton effects in the region of its long wavelength UV-vis absorption near 400 nm: ,, + 337, ,, , 186 (CHCl3), and ,, + 285, ,, , 177 (CH3OH). However, introduction of an amine into a CHCl3 solution of 1 causes the Cotton effect signs to become inverted, e.g., after addition of NH3, ,, , 345, ,, + 243, and after addition of ethylene diamine, ,, , 420, ,, + 299. The sign inversions imply inversion of molecular chirality of the bilirubin and the phenomenon appears to be general for amines, including ,,,-diamines. 1,8-Diaminooctane was found to be more effective than longer or shorter chain analogs in producing CD sign inversion. Chirality 13:251,257, 2001. © 2001 Wiley-Liss, Inc. [source] | ||||||||||