Conformational isomerism

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

Investigation of intramolecular interactions in n-alkanes. Cooperative energy increments associated with GG and GTG' [G = gauche, T = trans] sequences

Abstract: Energies of rotational isomers of n-alkanes are largely determined by the number of individual gauche bonds (G). However, according to molecular mechanics calculations (MM2), within a group of rotamers with equal number of G bonds, there are characteristic energy variations due to cooperative effects involving sequences of several bonds. For example, the energy is increased by inserting a trans bond (T) between two consecutive G bonds of the same sign (e.g., TGGG

UV irradiation of nucleic acids: formation, purification and solution conformational analysis of the ‘6-4 lesion’ of dTpdT

Abstract: Irradiation of dTpdT with 300 kJ/m2 of 254 nm produces numerous photo-products, one of which labeled dT6pd4T[1] was purified by HPLC. dT6pd4T has a UV spectrum (H20, pH 7) with lambda max = 326 nm and lambda min = 265 nm, and a P-31 NMR resonance at -3.46 ppm (normal dTpdT occurs at -4.01 ppm; TMP, 30 degrees C). 2-D COSY NMR spectra facilitated proton resonance assignments and 2-D NOESY spectra aided analysis of spatial orientation. Carbon-13 and proton-coupled P-31 NMR spectra of dT6pd4T were also obtained. These analyses indicate: C5=C6 of dT6p- is saturated and the -pd4T base is more aromatic; the dT6p- base possesses a configuration of 5R, 6S; dT6p- and -pd4T have anti-type glycosidic conformations; furanose conformation of dT6p- is mainly C3'-endo and that of -pd4T exists in a C3'-endo in equilibrium C3'-exo; exocyclic bonds gamma (C5'-C4'), beta (05'-C5') and epsilon (C3'-03') are non-classical rotamers; dihedral angle about epsilon (C3'-03') is smaller relative to dTpdT.

Selector for structural isomers of neutral molecules.

Abstract: During the last decades, the properties of biomolecules in the gas phase have been studied in ever greater detail [1‐3]. Although the study of biomolecules outside of their natural environment was met with skepticism in the beginning, spectroscopic studies on isolated species in a molecular beam have proven to be very powerful to understand their intrinsic properties. Moreover, their native environment can be mimicked by adding solvent molecules one by one. These studies on well-defined biomolecular systems are particularly relevant to benchmark theoretical calculations. Even in the cold environment of a molecular beam, biomolecules exist in various conformational structures. The existence of multiple conformers (structural isomers) has been observed in the study of jetcooled glycine for the first time [4] and in numerous experiments since then. In many cases, the individual conformers are identified via their different electronic spectra [5, 6]. This has been exploited in multiple-resonance techniques to measure, for instance, conformer-specific infrared spectra from which the conformational structures can be deduced [7, 8].

Lithium chloride perturbation of cis-trans peptide bond equilibria: effect on conformational equilibria in cyclosporin A and on time-dependent inhibition of cyclophilin

Abstract: The cyclic undecapeptide cyclosporin A (CsA) is a slow-binding inhibitor of the peptidylprolyl cis-trans isomerase (PPIase) cyclophiiin. Both the initial inhibitory activity and the subsequent time-dependent inhibition are sensitive to the solvent system (DMSO, THF, LiCl-THF) in which CsA is dissolved prior to the assay. NMR experiments demonstrate that in tetrahydrofuran the MeLeu'-MeLeu10 peptide bond has a cis conformation (Kessler, H.; et al. Biochem. Pharmacol. 1990, 40, 169-173). The cis conformer is inactive as a PPIase inhibitor. The same peptide bond adopts a trans conformation when lithium chloride is present as an additive in THF or when CsA is bound to cyclophiiin (Fesik, S. W.; et al. Science 1990, 250, 1406-1409). The trans conformer is a tight-binding inhibitor of PPIase activity (K1 = 20 nM), and the inhibition increases over time (K1 = 7 nM after 30 min). Detailed kinetic analysis of this transition indicates the presence of at least two interconverting forms of unbound CsA, and a slow structural change in the enzyme-inhibitor complex. The kinetic and structural data taken together suggest that the sequence MeLeu9-rraMj-MeLeu10-MeVal" is responsible for efficient binding in the active site of cyclophiiin, while the corresponding cis conformer is not recognized at all. The interconversion between the two conformers is kinetically expressed in the time-dependent binding of the drug.

Metallatriangles and metallasquares: the diversity behind structurally characterized examples and the crucial role of ligand symmetry.

Abstract: Reports on spontaneous self-assembly processes between metal fragments and organic ligands frequently tend to ignore the fact that the product isolated and structurally characterized in most cases is only one out of a more or less large series of feasible ones. This is true even for rings containing as few as three or four metal ions. Here we shall review metallatriangles and metallasquares containing predominantly cis-square-planar metal entities and a range of bidentate bridging ligands. The most significant features contributing to the number of possible stereoisomers appear to be ligand symmetry and flexibility, viz. rotation of two halves of a ligand about a single bond, or rotation of the whole ligand about the metal–donor atom bonds. With low-symmetry bidentate ligands the number of isomers increases dramatically with ring size as a consequence of an increase in possible connectivity patterns, hence linkage isomers, and an increase in possible rotamer states of the bridging ligands. In this tutorial review it is demonstrated how complexity increases as the symmetry of the bridging ligands is lowered from D∞h and D2h to C∞v, C2v, C2h and Cs. Special attention will be paid to cyclic tri- and tetranuclear complexes of substituted pyrimidine ligands (C2v and Cs symmetries) as well as the flexible 2,2′-bipyrazine, which can adopt states of either C2v or C2h symmetry. Uses of these complexes and ways to reduce the number of isomers will be pointed out.