Free-radical reactions in the synthesis of alpha -amino acids and derivatives

Natural Occurrence, Syntheses, and Applications of Cyclopropyl-Group-Containing α-Amino Acids. 1. 1-Aminocyclopropanecarboxylic Acid and Other 2,3-Methanoamino Acids

A recently developed quantum mechanical approach devoted to the study of unstable species in solution was applied to the radicals resulting from the homolytic breaking of the Cα−Hα bond of glycine in aqueous solution at different pH values.The computational protocol includes density functional calculations, simulation of the solvent by a mixed discrete-continuum approach, and averaging of spectroscopic properties over the most important vibrational motions. In vacuo computations provide reliable results for the zwitterionic form when using hybrid Hartree−Fock/density-functional methods with purposely tailored basis sets. Under the same conditions, disappointing results are obtained for the magnetic properties of neutral and, especially, anionic forms. Although the modifications of the structure and the magnetic properties of these species induced by the solvent are well reproduced by either a continuum model or a supermolecule approach, quantitative results can be obtained only by a mixed discrete-continu...

Structure and Magnetic Properties of Glycine Radical in Aqueous Solution at Different pH Values

High level ab initio molecular orbital calculations have been used to examine the ring opening of the cyclopropylcarbinyl radical and its heterosubstituted analogues. The applicability of various theoretical techniques to this ring-opening reaction has been investigated. A variant of the recently introduced CBS-RAD procedure is found to give good agreement with the experimental thermochemistry. The hybrid density functional method B3-LYP is found to perform well for various geometry- and frequency-dependent quantities and to provide a possible economical alternative for the reliable prediction of the energetics. We find that heterosubstitution by nitrogen at the 1-position has very little effect on the kinetics of ring opening. On the other hand, heterosubstitution by nitrogen or oxygen at the 2-position results in a significant rate enhancement for an already rapid reaction. In both these latter cases, the kinetically preferred ring-opening pathway is predicted not to lead to the thermodynamically prefer...

Ring Opening of the Cyclopropylcarbinyl Radical and Its N- and O-Substituted Analogues: A Theoretical Examination of Very Fast Unimolecular Reactions

The enzyme that oxidatively converts 1-aminocyclopropanecarboxylic acid (ACC) to ethylene, a key plant growth hormone, has been classified, on the basis of a comparison of homologous protein sequences (derived from the cDNA sequences), as a member of a family of non-heme iron proteins that includes plant and bacterial oxidative enzymes. This knowledge has facilitated the purification of the relatively abundant ethylene-forming enzyme to homogeneity from apple tissue. The properties of the enzyme are consistent with two other recent reports that describes its purification by different protocols, lending credence to the assertion that the key protein has been isolated. New characterizations of the protein have been conducted. Electrospray mass spectrometry shows that its molecular weight (35 331.8 +/- 5 amu) is approximately 50 amu higher than that predicted from the cDNA sequence, identifying the blocking group at the N-terminus as acetyl. The enzyme is activated by bicarbonate at low concentration but is inhibited at high concentration, with the maximum activation occurring at 5 mM. The iron concentration leading to half-maximal activity is 1 microM. The enzyme self-inactivates during turnover. The availability of the purified enzyme will permit definitive studies of the mechanism by which ethylene is produced and provide opportunities to discover molecules that inhibit the process.

Purification and properties of the apple fruit ethylene-forming enzyme.

The conversion of a series of 2,3-dimethylated 1-aminocyclopropanecarboxylates by apple tissues into mixtures of cis- and trans-butenes is reported; the results are in accord with a stepwise enzymatic mechanism of cyclopropane ring opening.

Pete D. Baird

Papers

On the biosynthesis of ethylene: further evidence for stepwise enzymatic cyclopropane ring cleavage