Conformational isomerism

About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.

Topology switching in [32]heptaphyrins controlled by solvent, protonation, and meso substituents.

Abstract: The switching of topology between "figure-eight", Mobius, and untwisted conformations in [32]heptaphyrins(1.1.1.1.1.1.1) has been investigated by using density functional theory calculations. Such a change is achieved by variation of one internal dihedral angle and, if properly controlled, can provide access to molecular switches with unique optical and magnetic properties. In this work, we have explored different conformational control methods, such as solvent, protonation and meso substituents. Despite its antiaromatic character, most of the [32]heptaphyrins (R=H, CH(3), CF(3), Ph, C(6)F(5)) adopt a figure-eight conformation in the neutral state, owing to their more-effective hydrogen-bonding interactions. The aromatic Mobius topology is only preferred with dichlorophenyl groups, which minimize the steric hindrance that arises from the bulky chlorine atoms. The conformational equilibrium is sensitive to the solvent, so polar solvents, such as DMSO, further stabilize the Mobius conformation. Protonation induces a conformational change into the Mobius topology, irrespective of the meso-aryl groups. In the triprotonated species, the conformational switch is blocked and a non-twisted conformer becomes much more stable than the figure-eight conformation. We have shown that the relative energies of the protonated [32]heptaphyrins are dominated by aromaticity. Importantly, this topology switching induces a dramatic change in the magnetic properties and reactivity of the macrocycles, as revealed by several energetic, magnetic, structural, and reactivity indices of aromaticity.

Synthesis, Structural Characterization, and Alkali-Metal Complexation of the Six Possible (1,3)- and (1,2)-Bridged p-tert-Butylcalix[4]crown-5 Conformers Bearing α-Picolyl Pendant Groups

Abstract: Complementary synthetic strategies leading to the six possible regioisomers and conformational isomers of bis[(α-picolyl)oxy]-p-tert-butylcalix[4]crown-5 (1−6) are reported. The overall conformation of 1−6 was deduced by NMR spectroscopy and further proven by single-crystal X-ray analysis for (1,3)-bridged cone and (1,2)-bridged 1,2-alternate conformers 1 and 6. Compounds 1−6 form 1:1 complexes with alkali-metal cations. The complexation sites and solution structures for Na+ and K+ complexes have been determined by 1H NMR titration experiments. The binding affinities of ligands 1−6 for alkali-metal ions have been assessed by phase transfer and stability constant measurements using spectrophotometric, potentiometric, and calorimetric techniques. All 1,3-bridged calixcrowns 1−3 show a strong affinity for K+ (partial cone 2 and 1,3-alternate 3 also for Rb+), conformer 3 being slightly more selective than the naturally occurring ionophore valinomycin. Conversely, 1,2-bridged calixcrowns 4−6 are less efficient...

Peptide models. IX. A complete conformational set of For-Ala-Ala-NH2 from ab inito computations

Abstract: This paper reports the complete set of minimum energy (i.e. stable) conformations of For-Ala-Ala-NH2 (containing three peptide bonds), computed by ab initio geometry optimization. A more confident structure and relative energy comparison of the 49 different relaxed geometries was possible using the HF/3-21G method as compared to empirical (force field) methods. The analysis of the different conformers resulted in a useful structure database, incorporating 30 different β-turns, which will have some relevance to the description of peptide folding. Spectroscopic (e.g. NMR) consequences of the reported ab initio calculations are also discussed, and a 3D-structure assignment method for proteins is provided.

Differential flexibilities in three branches of an N-linked triantennary glycopeptide.

Abstract: The solution conformation behavior of complex oligosaccharides was studied by resonance energy transfer, as measured by the time-resolved fluorescence method, to determine the conformational heterogeneity of a triantennary glycopeptide at various temperatures. Groups that acted as a fluorescence donor (naphthyl-2-acetyl, Nap) or acceptor (dansylethylenediamine, Dan) were selectively attached to the N terminus of the peptide and a Gal residue [either 6' (shown below), 6, or 8] of the oligosaccharide, respectively. [formula: see text] Time-resolved fluorescence energy-transfer measurements revealed two populations of conformers when Dan was attached to either Gal-6' or Gal-6. One conformer contained the antenna folded back toward the core region, and a second was in an extended conformation. The two conformations differed in donor-acceptor distance by about 10 A. Systematically increasing the temperature from 0 degrees C to 40 degrees C increased the ratio of extended to folded forms 2-fold for the Gal-6 isomer and 4-fold for the Gal-6' isomer, whereas the Gal-8 isomer showed only a single distance population throughout this temperature range. From these data, delta H and delta S for the reversible conformational change were calculated to be 3.1 kcal/mol and 10.8 cal/(mol.K) for the Gal-6 isomer and 7.1 kcal/mol and 25.8 cal/(mol.K) for the Gal-6' isomer. In addition to the structural microheterogeneity commonly associated with glycoproteins, the differential flexibilities of the different branches in the oligosaccharides contribute conformational heterogeneity and should be considered in conformational analysis. The data are discussed in terms of the most probable linkages that contribute to the observed flexibility of the individual triantennary branches, and the biological significance of flexible linkages in complex carbohydrates is considered.