RESEARCH
Optical Activity Is Clue to Isomerization Cis-Co(L-proline)3+3 isomerizes via bond rupture path, Illinois work shows
LIKELY. J. A. Stanko (left) and Dr. R. G. Denning have found, with the late Dr. Piper, that the three mechanisms for isomerization of tris-substituted metal complexes can be distinguished by analyzing the rearrangement products
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C&EN
OCT.
4,
1965
The three most likely mechanisms for the isomerization of some tris-substituted metal complexes can be distinguished by analyzing the products after rearrangement. The cis and trans geometrical isomers of such complexes (in which the two ends of the chelate ring are not identical) may exist in either lambda (left-hand screw) or delta (right-hand screw) configurations. The late Dr. Piper of the University of Illinois (Urbana) saw that, in isomerization of any one such isomer (the cis-lambda, for example), the three most likely mechanisms give different products. Thus, by analyzing the products of the rearrangement, the mechanisms could be detected, Dr. R. G. Denning, a colleague of Dr. Piper's at Illinois, told the Division of Inorganic Chemistry during the 150th ACS National Meeting. Appearing before the Symposium on Optical Activity in Inorganic Chem-
istry (dedicated to Dr. Piper), Dr. Denning disclosed some recent, unpublished work, which he had done with Dr. Piper. Inversion. He explained that the trigonal-twist mechanism provides for inversion of an isomer of tris-substituted metal complexes, but not for cistrans isomerization. Thus, a cislambda molecule will isomerize to a cis-delta configuration. For the rhombic twist, however, inversion and isomerization occur together. A cislambda molecule therefore becomes a trans-delta molecule. The third mechanism—bond rupture or a dissociative mechanismprovides a path for any of the isomers to isomerize to all of the others. However, these isomerizations need not take place at the same rate or have equal activation energies. To test the hypothesis, Dr. Denning prepared the four isomers of cobalt(III) with L-alanine, L-leucine, and L proline. Tris complexes of cobalt (III) with an optically active amino acid ligand are fairly easily separated because a lambda molecule is not chemically identical to a delta isomer. The experiments were hampered, Dr. Denning says, by the low solubility of the complexes in convenient solvents and by their tendency to solvolyze before rearrangement. However, circular dichroism measurements show two rearrangements. The cis-delta-tris. (L-proline) complex isomerized in aqueous solution at 90° C. to equilibrium with the transdelta complex. The absence of an inversion in this reaction rules out both trigonal and rhombic twist mechanisms, Dr. Denning says, leaving only a dissociative path. Supporting this conclusion, he adds, is the fact that no reaction occurs under the same conditions in 0.1N perchloric acid solution. The mechanism for this isomerization reaction probably is an S N 1CB type. Besides determining the mechanism of isomer rearrangement, Dr. Denning found evidence—the "first conclusive proof"—that red amino acid complexes
are cis isomers and violet complexes are trans. The evidence is based on circular dichroism and absorption spectra of the complexes, and, where feasible, nuclear magnetic resonance spectra. The methyl proton resonances of two isomers of the L-alanine complex were studied. Both have net negative rotational strength in the visible absorption band, but one belongs to the red series and the other to the violet. The C 3 axis of the red cis complex makes the methyl groups equivalent, accounting for a simple doublet (with J = 7.0 cycles per second). The violet trans complex (which has no symmetry elements) has three doublets. Two doublets have similar chemical shifts while one is different; but all have J = 7.0 cycles per second. Steric. The different steric distributions of the alkyl groups in the lambda and delta molecules help distinguish absolute configurations of some of the tris-substituted isomers. Taking the four possible isomers having an L-amino acid ligand, for example, Dr. Denning pointed out that the delta molecules have pseudo-axial alkyl groups. But in the lambda molecules, these groups are pseudo-equatorial with respect to the C 3 (or the pseudo-C 3 ) axis of the molecule. The significance of this distinction is apparent, he notes, from the much greater solubility of the isomers of one absolute configuration, compared to the solubility of their counterparts with the opposite configuration. One isomer, the trans-lambda, involves a much closer distance of approach of two alkyl groups than in any other isomer. Dr. Denning calls this the "clash." The clash is greatly accentuated in the L-proline complex. It is accentuated to such an extent that the steric hindrance seems prohibitive. In fact, the Illinois chemists have been able to prepare only three isomers with L-proline, one of the trans isomers being absent. With the other amino acids, equal yields of both trans isomers are invariably obtained. Dr. Denning concludes that it is the sterically hindered lambda molecule which is absent. "This allows us," he says, "to correlate the absolute configurations of the remaining isomers by means of their circular dichroism spectra. Since we are dealing with the same ring sizes and coordinating groups, this should be a reliable procedure." NMR spectra of the leucine complexes
give additional evidence for the clash. The circular dichroism spectra of the tris-substituted complexes are of particular use in determining their absolute configurations. For example, Dr. Denning points out, the lambda and delta configurations of any one geometrical isomer do not give mirror image circular dichroism (CD) curves because of the additional asymmetry of the alpha carbon atoms (described by Dr. Bodie Douglas, University of Pittsburgh, as the vicinal or neighboring effect). The Illinois group couldn't devise any theoretical model to explain how this additional asymmetry is communicated to the metal chromophores. However, the over-all asymmetry of the three chelate rings may be influenced simply by the axial and equatorial distribution of the alkyl groups and from the distortions they cause. Perturbation. The cis isomers show three circular dichroism components in the high-energy absorption band. Under a C 3 crystal field perturbation (an elongation or contraction along the C 3 axis), the T 2 energy level is resolved into A1 and E components. These may not be further resolved without an additional perturbation. Spin-orbital coupling may be ruled out in this context, Dr. Denning says, because of its small effect in the cobalt(III) ion. He believes that the Ax component gains rotational strength by vibronic borrowing from the strong E a component. The validity of such a mechanism has been shown by Dr. Oscar Weigang, Jr., of Tulane. If the A1 component has a different band width and opposite rotational strength to the E b component, then the sum of these two would readily explain the composite spectrum, according to Dr. Denning. The situation is further complicated, however. Sign reversals (due to the operation of more than one vibrational mixing mechanism within that band) may occur within a single electronic circular dichroism band. Dr. Denning concludes that assignments of molecular symmetry based on the observation of the apparent number of components in a CD spectra are not necessarily reliable. The Illinois experiments show that the net rotational strength in the first absorption band of a delta molecule is negative. The same is true for the ions tris (ethylenediamine) cobalt (III) and tris (L-propy lenediamine) cobalt(III).
Instruments Speed Nucleoside Analysis Chromatograph-ion exchange method takes less than one hour Complete analysis of the major nucleic acid bases can now be accomplished in less than one hour. With some recent instrumental modifications, elution ion exchange chromatography can be used to analyze for the nucleosides at nanomole (10~9 mole) levels, Dr. Waldo E. Cohn of Oak Ridge National Laboratory told the Division of Biological Chemistry during the 150th ACS National Meeting. The sequence of bases along nucleic acid chains is a significant factor controlling the biological function of the chains. To examine the many kinds of fragments obtained from these chains, it's necessary to determine the relative amounts of each basic fragment. Dr. Cohn, recipient of the 1963 American Chemical Society Award in Chromatography and Electrophoresis, introduced column chromatography by ion exchange for this purpose in 1949. This technique is the most exact way to analyze for the mixtures of bases obtained from the hydrolysis of nucleic acids or the oligonucleotides obtained from them. But it is a long, laborious process which takes six to 18 hours. Moreover, each analytical setup and procedure needs much attention and material, for each column fraction must be measured manually in a spectrophotometer. By contrast, paper chromatography and especially paper electrophoresis—although just as laborious in measuring the individual separated components—have the advantage that many analyses can be made in parallel. Advances. Several advances have made ion exchange competitive with paper chromatography and paper electrophoresis, in both time per analysis and amounts of material required, Dr. Cohn points out. One advance is the "nucleotide analyzer" developed by Dr. Norman G. Anderson and his coworkers at Oak Ridge National Laboratory. This instrument records quantitatively the ultraviolet absorbance of each peak emerging from a column. Tedious manual assays are not needed. Another recent advance is the "scale expander," developed at ORNL by Dr. Cohn's co-worker, Dr. Mayo. OCT. 4, 1965 C & E N
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