Conformation NMR studies

Furthermore, the estimation of the rotational barriers of ortho-substituted 6-aryl-1,1,5-trimethylindanes (Fig. 1a) [24] by NMR as well as conformational NMR-studies of (9-anthryl)carbinol derivatives (Fig. 1b) [25] have shown the CF3 substituent to have steric effects that are comparable to those of an isopropyl group (Fig. 1). 

For 2,6-disubstituted 3,6-dihydro-2FZ- 1,2-oxazines there are four energetically different half-chair conformations reflecting both ring and W-inversion whereas for 3-substituted 3,4-dihydro-l//-2,3-benzoxazines there are only two conformers. NMR studies do not... 

Agonists - The existence of two receptor populations for histamine raises the interesting question of whether the chemical mechanism of histamine interaction differs between the two receptor types. Some indications of the chemical properties which may differentiate receptor action come from studies of histamine chemistry and from structure-activity considerations of congeners. Histamine in aqueous solution is a mixture of equilibrating species, viz. ionic forms, tautomers and conformers nmr studies confirm earlier pK work indicating a N -H N -H (structures 1 and 2) tautomer ratio of approximately 4 1 for histamine monocation, and a comparable ratio for histamine base. The latter result contrasts with crystal structure data and molecular orbital predictions, and may indicate an influence of solvent on tautomer stability. Recent studies of properties pertinent to consideration of ligand-receptor interactions are conformation (MO calculations and infra-red comparison of solid state and chloroform solutions of histamine base ), electronic charge distribution, metal complexation, and phospholipid inter-... 

It appears that phosphorylation of CheY causes it to change its conformation. NMR studies of CheY in its active phosphorylated state find significant chemical shifts which involve residues that must be on or near the surface of the molecule that interacts with CheA and with motor switch components [61]. The half-life of the aspartyl phosphate group on the protein is short, less than a minute, making an X-ray diffraction analysis difficult. 

Extensive discussions have focused on the conformation of the alkyl chains in the interior ". It has been has demonstrated that the alkyl chains of micellised surfactant are not fully extended. Starting from the headgroup, the first two or three carbon-carbon bonds are usually trans, whereas gauche conformations are likely to be encountered near the centre of tlie chain ". As a result, the methyl termini of the surfactant molecules can be located near the surface of the micelle, and have even been suggested to be able to protrude into the aqueous phase "". They are definitely not all gathered in the centre of tire micelle as is often suggested in pictorial representations. NMR studies have indicated that the hydrocarbon chains in a micelle are highly mobile, comparable to the mobility of a liquid alkane ... 

You can often use experimental data, such as Nuclear Overhauser Effect (NOE) signals from 2D NMR studies, as restraints. NOE signals give distances between pairs of hydrogens in a molecule. Use these distances to limit distances during a molecular mechanics geometry optimization or molecular dynamics calculation. Information on dihedral angles, deduced from NMR, can also limit a conformational search. 

An appreciable amount of information concerning the conformational preferences of substituted heterocycles has accrued, largely through dipole moment and NMR studies. However, the earliest appreciation of this topic apparently arose out of the extension of studies of restricted rotation in biphenyls to heterocyclic analogues. 

Long loop regions are often flexible and can frequently adopt several different conformations, making them "invisible" in x-ray structure determinations and undetermined in NMR studies. Such loops are frequently involved in the function of the protein and can switch from an "open" conformation, which allows access to the active site, to a "closed" conformation, which shields reactive groups in the active site from water. 

The only selenium diimide to be structurally characterized in the solid state, Se(NAd)2 (Ad = adamantyl), ° adopts the cis, trans conformation consistent with conclusions based on H and C NMR studies for Se(N Bu)2. ... 

Characteristic H NMR data of (4a/ ,55)- and (4n5,5R)-2-substituted 5- [A-(/e/ /-butoxycarbonyl)-L-tryptophyl]amino perhydropyrido[l,2-c]pyri-midine-l,3-diones were tabulated (01JMC2219). C CPMASS NMR data of 4-(4-methoxyphenyl)perhydropyrido[l,2-c]pyrimidine were reported (00JST73). C NMR data were reported for eight 4-aryl-2,3,5,6,7,8-hexahydro-l//-pyrido[l,2-c]pyrimidin-l,3-diones in the solid state and in CDCI3 solution (00JPO213). The structure of 4-aryl-3,4-dihydro-2//-pyrido [l,2-c]pyrimidine-l,3-diones and their 2,3,5,6,7,8-hexahydro derivatives were characterized by H and C NMR data (99JHC389). Conformational analysis of 6-methyl-2,3,4,6,7,ll/)-hexahydro-l//-pyrimido[6,l-n]isoquino-lin-2-ones 138 and 139 were carried out by H and C NMR studies (97LA1165). 

The conformational preference of 1,3,5-trithiane-l-oxide has been determined in solution and in the solid state168. 13C and dynamic 1H NMR studies indicated that the S=0 bond is equatorial (182) in solution, as did molecular mechanics calculations. Surprisingly, the axial conformation (183) is preferred in the crystalline state. 

The previously discussed conformational study of 3-substituted thietane oxides using lanthanide shift reagents185 corroborates the conclusions derived from other NMR studies and suggests that all rrans-3-substituted thietane oxides prefer an equatorial oxygen conformation when the thietane oxides are bound to shift reagents. 

The two -OH groups in l,2 5,6-di-0-cyclohexylidene-wii/o-inositol and its di-O-isopropylidene analog are trans. The X-ray crystal structure of the latter compound suggests that the ring is in a skew conformation with the 0-3 and 0-4 -OH groups both in ax positions, but NMR studies and ab initio calculations indicate that a mixture of the skew and chair conformations, with 0-3 and 0-4 both in eq positions, is present. Formation of a dibutylstannylene acetal presumably locks these two compounds in the latter conformation. 

Despite the vast amount of data on the pharmacological properties, very little about the conformation of the proteins has b n known until recent NMR studies. 2D-NMR results have provided detailed information about the secondary structure of several related anemone toxins. ATX I from Anemonia sulcata (3,4) and AP-A from Anthopleura xanthogrammica (5,6) have been studied by Gooley and Norton, and more recently Widmer et al. have further purified the A. sulcata toxins and obtained complete sequence specific assignments for ATX la (7). Our laboratory, on the other hand, has studied the structures of RpII and RpIII from RadiarUhiis paumotensis (8). 

X-ray crystallography and variable temperature H NMR studies show that the conformation of the coordinated imidazolidin-2-ylidene, in both the neutral and cationic complexes 70, is anti, anti with respect to the Ph of the backbone of the NHC, exclusively in the solid state and predominantly in solution at lower temperatures (-75°C). At room temperature in solution, possible conformer interconversion by the rotation around the phenyl-N bond of the NHC substituent is apparent from the broadness of the peaks in the NMR spectra. Hydrosilylation of acetophenone by Ph SiH catalysed by 70 at room temperature or at -20°C results in maximum ee of 58%. However, at lower temperatures the reaction rates are much slower [55]. 

The tveak and reversible binding of these complexes to calf-thymus DNA (ct DNA) suggests a dominant electrostatic mode of interaction nevertheless, relevant conformational distortions of the double helix are caused [50]. A multinuclear NMR study of the reactivity of [Au(en)Cl2]Cl and [Au(en)2]Cl3 vith guanosine 5 -monopho-sphate (5 -GMP) reveals that in an aqueous solution only [Au(en)Cl2]Cl binds very weakly to 5 -GMP via N(7) to give a 1 1 adduct [48]. 

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