Posts Tagged ‘Phenylalanine’

Water Absorption Effects on Amino Acids Structure

Author: Danut Dragoi, PhD

Biological process utilize amino acids almost uniquely one chiral form or the other. In terrestrial biology, only the L-amino acids is common in biological processes. If signature of life existed elsewhere in the D form it then be concluded that life had evolutionary beginning on that body. Detection of an enantiomeric excess of L over D would also be a powerful sign that life had existed on that body at one time, see link in here .

As we know powder XRD pattern of both the L- or D-amino acids are basically identical. This is primarily due to the fact that the chirality of the amino acids does not favor a specific orientation on the sample holder in the powder form. However, when the amino acids are crystallized in moisture environment, water molecule enters in the unit cell of the amino acid by changing its structure. In order to determine structural changes using x-ray diffraction, two samples of both L- and D-phenylalanine (min 98% and >99% purity, respectively; purchased from Sigma Aldrich Co. St. Louis Mo) directly on the x-ray sample holder, alumina single crystal (0001) face, see link in here. The picture below shows an example of the diffraction pattern for two specimens of D- and L- phenylalanine, fresh crystallized in pure water under the same conditions.

L and D Phenilalanine-XRD

Image SOURCE: http://webapp1.dlib.indiana.edu/virtual_disk_library/index.cgi/1185879/FID1164/ABSTRACTS/1401-1800/1682.pdf.

As we see from the picture above, the two peaks L and D do not have same position. For raw phenylalanine D and L enantiomers the laboratory x-ray diffraction pattern matches, despite the fact that Powder Diffraction File (PDF) that correspond to PDF# 11-0827 for D form and PDF# 37-1771 indicates detectable differences. This is due to a structure factor, which is invariant for both D and L forms. For fresh crystallized specimens of D and L forms, a difference can be distinguished. The diffraction peak D of D-Phenylalanine has a less d-spacing suggesting a more compact structure than L-Phenylalanine.

Comparing diffraction pattern for the entire range produced by L and D crystallized Phenylalanine in pure water, shifts occur in peak position and intensity. These differences are possibly due to stereo-specific interactions of D- and L- phenylalanine molecules with water molecules. The x-ray diffraction pattern for D and L form, a small part shown in the picture above, reflects new structural configurations such as rotation, slight deformation and/or translation of the phenylalanine molecule inside the unit cell. It is possible that D form crystallizes with more molecules of water than L form. Structural solution for freshly crystallized phenylalanine can be found using Rietveld refinement technique.

The implications of water effect on the structure of the amino acids can lead to similar effects on DNA structure as well as proteins and cells in human body.



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