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ANALYTICAL CURRENTS
Macrocyclic antibiotics used as chiral selectors Because the enantiomeric forms of optically active drugs can have very different effects, regulations regarding the marketing of such drugs—which require separation of the enantiomeric forms—have recently been enacted. Several approaches to achieving enantiomeric separation using CE have been developed, including adding a chiral selector to the free solution or immobilizing it in a gel or other suitable packing in the capillary. Cyclodextrins and their derivatives, the most commonly used chiral additives, are of the right size to form inclusion complexes with a significant number of chiral compounds but research on tn ore
diverse and potentially more powerful chiral selectors continues Timothy J Ward and colleagues at Milsaps College have used the macrocyclic antibiotics rifamycin B and rifamycin SV as chiral selectors for the separation by CE of selected chiral drugs including epinephrine norepinephrine octapamine and hexabarbital Rifamycins have a characteristic ring structure or chromophore spanned by an
Luminescent square complex for molecular sensing
charged solutes. Solutes containing single aromatic groups appear to be especially suited for chiral resolution using rifamycin B, whereas solutes containing more than one ring are also amenable to separation. The researchers also found that the choice of detection wavelength significantly afSeparation of 500 jjg/mL of (+)-isoproterenol and 0.50 ug/mL fects sensitivity (-)-isoproterenol using indirect UV detection at 350 nm. (Adapted and that monitorwith permission of Elsevier Science.) ing one of the aliphatic chain. They differ in the type anc wavelengths that coincide with the absorption minima of the chiral selector imlocation of the substituents on their naphthohydroquinone ring, and they con- proves selectivity. Resolution can be enhanced by keeping the amount of analyte tain groups capable of providing the mulinjected on the column as low as possible. tiple interactions necessary to achieve By using indirect detection, the researchchiral recognition between enantiomers. ers were able to detect as little as 0 1% of Ward and co-workers found that rifamycii one enantiomer in the presence of anB is enantioselective toward positively other. (/. Chromatogr A 1995 715 337charged compounds, whereas rifamycin 44) SV is enantioselective toward negatively
tection of guest inclusion and offers the possibility of electronic excited-state reactivity as well as manipulation of reactivity by encapsulated guests. Rigid macrocycles based on cis bridging The electrochemical and luminescence ligation of transition metals are a new class data obtained on the complex offer insight of compounds that have tremendous pointo the viability of alternative electron tential in host-guest, inclusion, and molec- transfer quenching pathways, and the ular recognition chemistry. Most of these combined potential data are energetically systems are derived from Pt(II) or Pd(II) consistent with an oxidative quenching triflate species arranged in square or mechanism. Emission from the square is boxlike geometries with metal atoms at enhanced by added tetraethylammonium the corners. Joseph T. Hupp and colperchlorate, presumably via ClOj binding leagues at Northwestern University have in the tetracationic host cavity. The reincorporated visible-light-addressable lu- searchers found that a fit of the concentraminescent metal-ligand components tion-dependent luminescence intensity within a square assembly. They suggest changes yields a guest-host association that induction of photoluminescent charac- constant of 900 M"1 and the absence of an teristics is useful for molecular sensing emission lineshape or energy change in applications because it suggests an alterthe ion-binding experiment implies that native to proton NMR spectroscopy for de- the guest affects the luminescence effi-
ciency indirectly. (J. Am. Chem. Soc. 1995,117111813-14)
Light-emitting square complex prepared by combining the chromophore \ac-Re(CO)3CI(4,4'-bipyridine)2 with Pd 1,3-(diphenylphosphino)propane (triflate)2 in dry methylene chloride.
Analytical Chemistry News & Features, February 1, 1996 79 A
