Alanine, phenyl-, labelled with

This is demonstrated here with DFT-MD simulations to extract the vibrational spectrum of the Ac-Phe-Ala-NH2 peptide (capped peptide composed of phenyl and alanine residues, labeled FA in the remainder of the text see Fig. 9) in the far-IR domain, in relation to IR-UV ion-dip experiments [111]. With dynamical spectroscopy, one has to keep in mind that the length of the trajectories has to be commensurate with the domain investigated. Hence, to sample the vibrational modes and then-couplings in the 100-1,000 cm region, we have accumulated trajectories of 30 ps and make an average for the final theoretical specfrum over two ttajectories (i.e., 60 ps sampling in total). 

As an example of a molecule with diastereotopic ligands, consider the amino acid L-phenylalanine. The two protons at C-3 are diastereotopic, since substitution of either of them would generate a molecule with two chiral centers. Because the chiral center already present is 5, the two diastereomers would be the 2S,3R and the 25,35 stereoisomers. As in the case of enantiotopic protons, diastereotopic protons are designated pro-R or pro-S. The enzyme phenylalanine ammonia lyase catalyzes the conversion of phenylalanine to trans-cinnamic acid by a process involving anti elimination of the amino group and the 3-pro-S hydrogen. This stereochemical course has been demonstrated using deuterium-labeled L-phenyl-alanine as shown" ... 

L-Phenylalanine in the cytoplasm serves protein and alkaloid biosynthesis. Excess L-phenyl-alanine is accumulated in the vacuoles and can be reused later in alkaloid formation but not in protein biosynthesis. Hence, with respect to alkaloid formation there are two channels for L-phenylalanine a direct, low capacity pathway via the peripheral pool (primary labeling of alkaloids) and an indirect, high capacity pathway from the expandable pool (secondary labeling of the alkaloids). The relative contributions of these two channels vary with the concentration of L-phenylalanine, the time of incubation, etc. Under all experimental conditions, however, in contrast to protein biosynthesis, about 90 % of the L-phenylalanine incorporated into the alkaloids is recruited from endogenous sources, i.e., de novo synthesis and protein degradation. 

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