Interconversions with resolution

An effect observed with a number of compounds which have apparent chiral centers on elements other than carbon. Eor example, secondary and tertiary amines have a pyramidal structure in which the unshared pair of electrons is at the top of the pyramid. If the three substituents hnked to the nitrogen are all different, one might suspect that the tertiary amine would give rise to optical activity and be resolvable. However, rapid oscillation of the unshared pair of electrons on one side of the nitrogen to the other (hence, pyramidal inversion) in effect causes interconversion of the two enantiomers and prevents resolution. If the nitrogen is at a bridgehead, this umbrella effect is inhibited and optical isomers can be isolated. 

Resolution Effects with Peptides Undergoing Interconversion in Reversed-... 

When the process pN- n-> pv in the mobile phase or stationary phase can be represented by first-order or pseudo-first-order interconversion kinetics and as a reversible binding event, the resolution of the interconverting species can be evaluated319 by treating the column as a chemical reactor with properties specified by the corresponding Damkohler number Da and the corresponding interconversion rate constants derived. Thus,... 

Over the years, a very large amount of static and time-resolved spectroscopy of various kinds, as well as studies of site-specific mutants of bacteriorhodopsin, has generated kinetic models for the transport cycle ( photocycle ) and identified the side chains of importance (Haupts et al., 1999 Lanyi and Varo, 1995 Oesterhelt, 1998). The results had begun to identify the molecular events that underlie the interconversions of the spectroscopically distinct intermediate states termed J, K, L, M, N, and O. Together with low-resolution 3D maps of the protein and some of the intermediate states from cryoelectron microscopy of 2D crystals, these results suggested the beginnings of a mechanistic model... 

It is well known that the O2 reduction site of bovine heart cytochrome c oxidase in the fuUy oxidized state exhibits variable reactivity to cyanide and ferrocytochrome c, which is dependent on the method of purihcation (Moody, 1996). Some preparations react with cyanide extremely slowly at an almost immeasurable rate and are known as the slow form. Other preparations, which react at a half-Ufe of about 30 s, are known as the fast form (Brandt et al., 1989). Electronic absorption spectra of the slow-and fast-form preparations exhibit Soret bands at 418 and 424 nm, respectively. The two forms often coexist in a single preparation (Baker et al., 1987). Both forms exhibit an identical visible-Soret spectrum in the fully reduced state. The slow-form preparation can be converted to the fast form by dithionite reduction followed by reoxidation with O2. The fast form thus obtained returns to the slow form spontaneously at a rate much slower than the enzymatic turnover rate. Thus, the slow form is unlikely to be involved in the enzymatic turnover (Antoniniei a/., 1977). It should be noted that no clear experimental evidence has been reported for direct involvement of the fast form in the enzyme turnover, although its direct involvement has been widely accepted. The third species of the fully oxidized O2 reduction site, which appears in the partially reduced enzyme, reacts with cyanide 10 —10 times more rapidly than the fast form (Jones et al., 1984). In the absence of a reducing system, no interconversion is detectable between the slow and the fast forms (Brandt et al., 1989). Thus, the heterogeneity is expected to inhibit the crystallization of this enzyme. In fact, the enzyme preparations providing crystals showing X-ray diffraction at atomic resolution are the fast form preparation. 

Deracemization. In this type of process, one enantiomer is converted to the other, so that a racemic mixture is converted to a pure enantiomer, or to a mixture enriched in one enantiomer. This is not quite the same as the methods of resolution previously mentioned, although an outside optically active substance is required. To effect the deracemization two conditions are necessary (7) the enantiomers must complex differently with the optically active substance (2) they must interconvert under the conditions of the experiment. When racemic thioesters were placed in solution with a specific optically active amide for 28 days, the solution contained 89% of one enantiomer and 11 % of the other. In this case, the presence of a base (Et3N) was necessary for the interconversion to take place. Biocatalytic deracemization processes induce deracemization of chiral secondary alcohols. In a specific example, Sphingomonas paucimobilis NCIMB 8195 catalyzes the efficient deracemization of many secondary alcohols in up to 90% yield of the (R)-alcohol. ... 

---