Permethylated molecules

Data from f.a.b.-m.s., and also f.d.-m.s., revealed the existence of naturally occurring, large cyclic polysaccharides. The first indication that a molecule may be cyclic comes from its precise molecular-weight determination. Cyclic molecules are 18 mass units less than their linear counterparts. Loss of water may, of course, occur in a number of ways, for example, by dehydration or lactonization, and conclusive evidence for the presence of a cyclic molecule can only be obtained from f.a.b.-m.s. of suitable derivatives, such as the permethyl derivative. Cyclic and dehydrated linear polymers are distinguishable after permethylation, as the cyclic polymer will incorporate one methyl group less than the linear molecule. 

Finally, the presence of the substituents on the nitrogen donors also influences the reactivity of the corresponding complexes significantly. Thus, utilization of the permethylated derivative in place of H2L23 drastically alters the Lewis-acidity of the metal ions, that is the coligands in the [Ni2(L19)(L )]+ compounds become more polarized than in the [Ni2(L23)(L )]+ complexes. In the latter complexes, the Lewis-acidity of the divalent nickel(II) ions is in large part consumed for the polarization of the six secondary NH functions. This explains why only the complexes of the permethylated macrocycle are able to activate and transform small molecules such as H20 or C02 (210,239,241). 

Table 16 Rate constants k X 10 cm molecule s and selectivities kr>lki) of selected amino acids with CD and linear oligosaccharides. All hosts are permethylated...

The stability of the polypyridyl rhenium(I) compounds mentioned above stimulated applications of this coordination chemistry. Thus, new heterotopic bis(calix[4]arene)rhenium(I) bipyridyl receptor molecules have been prepared and shown to bind a variety of anions at the upper rim and alkali metal cations at the lower rim. A cyclodextrin dimer, which was obtained by connecting two permethylated /3-cyclodextrins with a bipy ligand, was used for the preparation of a luminescent rhenium(I) complex. The system is discussed as a model conipound to study the energy transfer between active metal centers and a bound ditopic substrate. The fluorescence behavior of rhenium(I) complexes containing functionalized bipy ligands has been applied for the recognition of glucose. ... 

A chiral selector can also be dissolved in the IL solvent and be subsequently coated on the capillary wall [38]. In this approach, the achiral [C4CiIm]Cl was used to dissolve permethylated p-cyclodextrin (p-PM) and dimethylated P-cyclodextrin (p-DM). The chromatographic separations obtained from these two columns were compared to two commercially available CSPs based on p-PM and p-DM dissolved in polydimethylsiloxane. From a set of 64 chiral molecules separafed by fhe commercial p-PM column, only 21 of the molecules were enantioresolved by the IL-based p-PM column. Likewise, from a collecfion of 80 analytes separated by the p-DM column, only 16 analytes could be separated on the IL-based p-DM column. The authors also noted a considerable enhancement in the separation efficiency of fhe IL-based CSPs. This resulf, coupled to fhe loss of enantioselecfivify for mosf separations, suggests that the imidazolium cation may occupy the cavity of the cyclodextrin preventing the analyte-cyclodextrin inclusion complex-ation that is crucial for chiral recognition. The ability for ILs to form inclusion complexes wifh cyclodextrin molecules has been recently studied by Tran and coworkers using near-infrared spectromefry [39]. 

Figure 10.1 Analysis of racemic 2,5-dimethyl-4-hydroxy-3[2H]-furanone (1) obtained from a strawberry tea, flavoured with the synthetic racemate of 1 (natural component), using an MDGC procedure (a) dichloromethane extract of the flavoured strawberry tea, analysed on a Carbowax 20M pre-column (60 m, 0.32 mm i.d., 0.25 pan film thickness carrier gas H2, 1.95 bar 170 °C isothermal) (b) chirospecific analysis of (1) from the strawberry tea extract, transferred for stereoanalysis by using a permethylated /1-cyclodextrin column (47 m X 0.23 mm i.d. carrier gas H2, 1.70 bar 110°C isothermal). Reprinted from Journal of High Resolution Chromatography, 13, A. Mosandl et al., Stereoisomeric flavor compounds. XLIV enantioselective analysis of some important flavor molecules , pp. 660-662, 1990, with permission from Wiley-VCH.

Lacto-N-fucopentaose II also contains an a-L-fucosyl residue linked to a lacto-iV-tetraose molecule, but in this case it is linked to C4 of the 2-acet-amido-2-deoxy-D-glucose unit. The sequence and linkages were again established by hydrolysis of the permethylated saccharide alcohol.279... 

Analysis by positive ion chemical ionization of the permethylated ethylglycosides leads to spontaneous fragmentation of the molecular species. As shown in Figure 8.54, this fragmentation produces oxonium ions (resulting from the loss of ethanol) at m/z 219 (for non-deuterated hexoses). The oxonium ions then can lose a methanol molecule to yield a fragment 32 mass units lower (m/z 187 for the non-deuterated hexoses). 

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