FTIR spectroscopy characterization of fatty-acyl-chain conjugates

Abstract

FTIR spectroscopy is used to identify poly-l-lysin fatty-acyl-chain (PLL-FAC) conjugates based on structural differences found between FAC species. Twenty-one PLL-FAC models were used, from C8 to C24, and with up to 5 unsaturation levels (C20:5). Curve fitting of the 3050–2800 cm⁻¹ spectral interval permitted extraction of IR bands belonging to the stretching vibration modes of methyl, methylene, and alkene groups. Based on molecular structure models in 3D, the number and position of methyl bands could be set according to chain length and unsaturation level. Band positions for ν-(C = C < H), ν_as(CH₃), and ν_as(CH₂) groups did not follow the maximum intensity shift of spectrum curve; it is the underlying band's intensity that is modifying maximum intensity of spectrum curve with respect to chain length and unsaturation level. We thus propose to use FTIR spectroscopy for the production monitoring and the quality control of PLL-FAC conjugates used as nutritional complements, and this should be extended to analysis of fatty acid compounds in general.

Graphical abstract

Legend: Fatty-acyl-chain FTIR spectra bands assignation according to curve-fitting methods and cross-validated by molecular structure modeling. Series of fatty-acyl-chain conjugates of different length and with increased unsaturation levels allow determining the position of ν(-C = C < H), ν_as(CH₃), and ν_as(CH₂) groups. It is also demonstrated that ν_as(CH₃) groups from polypeptidic chain or fatty acids do not raise and absorption band at the same location. Finally, ν_as(CH₂) groups raise different absorption bands according to their position in the fatty acyl chain, at primer position (close to ester or amide bond), inner or terminal positions. The unsaturations of fatty acyl chains give rise to at least two ν(-C = C < H) absorption bands with second unsaturation group.

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