Amino acid enantiomers conformations

In removing the oxazoline auxiliary from the products, Richards and coworkers have demonstrated the use of the diastereoselective ferrocenyloxazoline lithiation in the synthesis of conformationally constrained amino acid derivatives (Scheme 146) °. Amination of 305 was achieved by nitration, reducing to the amino group after removal of the oxazoline under standard conditions. Using the trick of silylating the more reactive diastereotopic site, it was possible to make either enantiomer of 321 from the same oxazoline starting material. 

A careful analysis of conformational energy maps (Ramachandran plots) revealed that both enantiomers of P-tetralin amino acids were compatible with the right handed a-helical conformation.109 This was an important prerequisite for the development of N- and C-caps since the ( -configuration of the P-tetralin amino acid is needed for N-terminal helix induction, whereas the (R)-enantiomer was used in the C-cap series. The fact that both configurations were compatible with a-helical conformations made these amino acids our first choice as building blocks for... 

SHMT also catalyzes racemization of alanine and transamination of both its enantiomers. The particular reaction catalyzed by SHMT is mainly determined by the structure of the amino acid substrate. In the case of serine or glycine, the true substrates, SHMT does not catalyze any of the alternate reactions. The currently accepted model attributes this reaction specificity to the existence of open and closed active-site conformations. The physiological substrates generate the closed conformation, whereas alternate substrates react while the enzyme remains in the open conformation, which permits reaction paths leading to decarboxylation, transamination, and racemization. ... 

It was stated [111] that the sign of the energy difference between the enantiomers of the a-amino acids and of the polypeptides in the regular a-helix and /3-sheet conformation indicated, that the members of the L-series were preferentially stabilised by the parity violating weak interaction. 

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