Metal ion catalyzed racemization and .alpha.-proton exchange of N,N-dicarboxymethyl-D-phenylglycine

Hydration of serine–metal cation complexes: implication for the role of water in the origin of homochirality on the Earth

“…It is evident that hydration of heterochiral complexes is more efficient than hydration of homochiral complexes Another problem is that amino acids may be racemized in water. Furthermore, the metal cations may enhance the racemization of amino acids (Reddy et al 1989;Smith et al 1983;Stadtherr and Angelici 1975). However, the metal cations which enhance the racemization of amino acids are transition metal cations.…”

Controlling the Chiral Inversion Reaction of the Metallopeptide Ni-Asparagine-Cysteine-Cysteine with Dioxygen

“…38 Binding of a transition metal to the residues allows the process to occur under milder conditions. 18–24 Metal-bound peptides are able to undergo racemization over the course of hours to days at temperatures between 35 and 40 °C and at a pH above 9. In both cases, the racemization or chiral inversion proceeds by deprotonation of an α carbon.…”

“…Reprotonation by solvent gives either the D- or L-isomer. 24 Coordination of the amino acid or peptide to a metal center renders the deprotonation event more thermodynamically favorable, 18 leading to racemization or chiral inversion under milder conditions. In most cases where metal-facilitated chiral inversion occurs instead of racemization, steric factors promote the formation of the D-isomer over the L-form.…”

“…Metal-facilitated peptide modifications include ligand oxidation, spontaneous disulfide bond formation, decarboxylation, hydroxylation, and racemization. ,− It was recognized in the 1960s that peptide binding to transition metals, such as Co II , Co III , Cu II and Ni II , can lead to amino acid racemization under conditions of elevated pH and/or temperature. − The rates of these racemization reactions show a first-order dependence on OH – concentration, and carbanion intermediates have been detected using acetaldehyde as a trapping agent . The initiating step in peptide racemization was therefore proposed to be deprotonation of the Cα–H group to generate a carbanion intermediate that is subsequently quenched by a proton.…”

The effect of nickel(II) and cobalt(III) and other metal ions on the racemization of free and bound L-alanine

“…( 4) Addition of pivalic acid, f-BuCOOH, which is a much stronger acid than either i-PrOH or EtOH and is known to oxidize tungsten(O) to the 4+ oxidation state in reactions involving W(CO)6,12 reacts smoothly with W2(0-f-Bu)6 in hydrocarbon solvents to give W2(02C-f-Bu)6, which may be isolated as a yellow crystalline solid.5 This compound has not been structurally characterized by an X-ray study because its spectroscopic properties indicate a structure analogous to that found for W2-(G2CNMe2)6. 13 In the mass spectrometer, the ion of highest mass corresponds to W2(02C-t-Bu)64 and many daughter W2-containing (10) Crystal data for W2(OEt)6(MeN(H)CH2CH2N(H)Me) at -61 °C: space group P2fn, a = 19.050 (14) A, b = 15.627 (9) A, c = 8.793 (4) A, ß = 104.27 (4)°, Z = 4, defied = 1.901 g cm"3. Of the 2667 reflections collected with use of Mo Ka radiation, 6°< 2 < 40°, the 1739 reflections having F > 2.33 F were used in the full-matrix refinement.…”