Characterization by NMR

The secondary stmcture elements are then identified, and finally, the three-dimensional protein stmcture is obtained from the measured interproton distances and torsion angle parameters. This procedure requites a minimum of two days of nmr instmment time per sample, because two pulse delays are requited in the 3-D experiment. In addition, approximately 20 hours of computing time, using a supercomputer, is necessary for the calculations. Nevertheless, protein stmcture can be assigned using 3-D nmr and a resolution of 0.2 nanometers is achievable. The largest protein characterized by nmr at this writing contained 43 amino acid units (51). However, attempts ate underway to characterize the stmcture of interleukin 2 [85898-30-2] which has over 150 amino acid units. 

A well-studied case of tautomerism is that of l,2-diazepin-4-ones tautomers 47a and 47b were characterized by NMR, the former being the most stable (85JOC2141). 3,5-Dihydro-4//-2,3-benzodiazepin-4-ones exist as 48a and not as 4-hydroxy tautomers 48b (74JHC401). 

Diorganotin(IV) complexes 109 were characterized by NMR spectroscopy (96MI4). The downfield chemical shift of 6-H in 2-fluoroalkyl-4//-pyrido[l,2-n]pyrimidin-4-ones 111 is attributed to the anisotropic effect of the 4-carbonyl group (97JCS(P1)981). 

Yang and Jenekhe [186,187] reported a successful solubilization of aromatic polyimines in organic solvents via their soluble coordination complexes, which facilitated their solution characterization by NMR and processing films and coatings by spin coating and other techniques. This has created opportunities for various studies of the aromatic polyimines. 

Singlet ground state was characterized by NMR, IR, UV spectroscopy and single-crystal X-ray [162]... 

Kim et al. synthesized similar dendritic pseudorotaxanes as shown in Fig. 16 [58]. Reaction of triply-branched amine 42 with pseudorotaxane 43 incorporating CB[6] threaded on a string with a carboxylic acid terminus in DMF in the presence of EDC produces branched [4]pseudorotaxane 44, which is a G-1 dendritic pseudorotaxane. Coupling between 42 and pseudorotaxane dendritic wedge 45 under similar conditions yields G-2 dendritic pseudorotaxane 46, which contains 9 beads threaded on a dendritic framework ([lOjpseudorotax-ane). These dendritic pseudorotaxanes have been characterized by NMR spectroscopy, but further characterization remains to be done. 

More recently, Kim et al. synthesized dendritic [n] pseudorotaxane based on the stable charge-transfer complex formation inside cucurbit[8]uril (CB[8j) (Fig. 17) [59]. Reaction of triply branched molecule 47 containing an electron deficient bipyridinium unit on each branch, and three equiv of CB[8] forms branched [4] pseudorotaxane 48 which has been characterized by NMR and ESI mass spectrometry. Addition of three equivalents of electron-rich dihydrox-ynaphthalene 49 produces branched [4]rotaxane 50, which is stabilized by charge-transfer interactions between the bipyridinium unit and dihydroxy-naphthalene inside CB[8]. No dethreading of CB[8] is observed in solution. Reaction of [4] pseudorotaxane 48 with three equiv of triply branched molecule 51 having an electron donor unit on one arm and CB[6] threaded on a diaminobutane unit on each of two remaining arms produced dendritic [ 10] pseudorotaxane 52 which may be considered to be a second generation dendritic pseudorotaxane. 

For ruthenium, special precursors are required to synthesize defined bidentate diphosphine complexes. With Taniaphos for instance, it is possible to synthesize such complexes starting from unusual rathenium(ll) species. The complexes were characterized by NMR and single crystal analysis. 

This complex is formed with more than 1.0 equivalents of (C2H5)2A1C1 with concomitant formation of Li2AICI2. The open and chelated structures have been characterized by NMR.90 The chelated structure is substantially more reactive than the open complex, which accounts for the increase in enantioselectivity with more than 1.0 equivalents of catalyst. 

Subsequently, other members of the family of siienes (Me3Si)2Si= C(OSiMe3)R have been prepared, where R = Me, Et, i-Pr, CH2Ph, bicyclooctyl, CEt3, 1-methylcyclohexyl, and Mes. The first four siienes listed were not stable in inert solvents, and hence were not observable by NMR spectroscopy, since they rapidly reacted intermolecularly to give linear and/or cyclic head-to-head dimers.87 This is illustrated in Eq. (14) for the benzyl compound where the initially formed silene 5 yielded the cyclic head-to-head dimer 6 as well as the linear head-to-head dimer 7. The latter four siienes were all relatively stable and were characterized by NMR spectroscopy.105... 

The first moderately stable phosphasilene derivatives 12 were synthesized by Bickelhaupt et al. in 1984 (Eq. 2).l0a They possess limited stability (up to 60°C), and were characterized by NMR spectroscopy (see Section IV.C) and chemical reactions (see Section IV.E). 

This process has also been extended to include the Peterson olefination reaction (eq 5) of 12 (where E = Me3Si) to give a series of substituted vinyl derivatives 13. Both 12 and 13 have typically been obtained in high yield and have been fully characterized by NMR spectroscopy and elemental analysis. 

These new derivatives were isolated in good yields (60-94%) as high boiling liquids and were fully characterized by NMR spectroscopy (1H, 13C, and 11B) and elemental analysis. The proton NMR of the starting material 1 shows a well-resolved multiplet and quintet for the trimethylene bridge. Upon monosubstitution, however, three complex multiplets are observed, indicative of the unsymmetrical structures of these derivatives. Also, the nonequivalence of the N-C carbon atoms is clearly apparent in the 13C NMR spectra of 2-4. 

A study of sequential Arbuzov-type demethylation reactions of the platinum(II)-phosphite complex [Pt P(OMe)3 4]2+, in which the phosphite ligands are converted to phosphonates, has been described.285 Many of the products of the reaction were characterized by NMR spectroscopy and X-ray crystallography.286 The X-ray structures are the first reported for trialkylphosphite complexes of platinum(II). 

The zinc complex of the saturated macrocycle (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19, 20,21,22-docosahydrodibenzo-[B,I][l,4,8,l 1] tetra-azacyclotetradecine), which is a 14[ane]N4 (cyclam) ring incorporating two cyclohexyl rings into the macrocycle, has been prepared and characterized.688 Two isomers of the ligand were separately complexed with zinc and characterized by NMR. 

Summary Several lithium l,3-diphospha-2-sila-allyl complexes 3a-f and the diphosphino-dichlorosilane 2 have been prepared and characterized. The hydrolysis and substitution reactions of these compounds are described yielding a number of phosphino- and diphosphino-silaphosphenes 5a-d, 4a,b and 6. The compounds have been characterized by NMR and by X-ray analyses in the cases of 2, 3a-c and 4a. 

---