Molecular Structure upon Compression and Stability toward Oxidation of Langmuir Films of Unsaturated Fatty Acids: A Vibrational Sum Frequency Spectroscopy Study

Abstract

Vibrational sum frequency spectroscopy (VSFS) has been used to determine the stability toward oxidation in air of a series of unsaturated fatty acids, measuring as a function of time the changes in the chemical structure and conformational order of films spread on a Langmuir trough. The fatty acids studied consisted of a 20-carbon backbone with increasing numbers of cis double bonds in the chain: 11c-eicosenoic acid (20:1 EA, omega-9), 11c,14c-eicosadienoic acid (20:2 EA, omega-6), and 11c,14c,17c-eicosatrienoic acid (20:3 EA, omega-3). Measurements at constant surface pressure show that double bonds are lost from the surface region and that drops in intensity of the vinyl CH stretch are detectable within a few minutes of spreading the monolayer. The results are consistent with the fatty acid peroxidation free radical mechanism. The sum frequency spectra also reveal that what remains on the surface is conformationally more disordered with a larger number of gauche defects. The oxidation kinetics are found to be strongly dependent on the packing density of the monolayer, being more stable at higher pressures. Oxidation can be avoided by purging the system in an inert atmosphere. Finally, the molecular structure upon compression was tracked in unoxidized monolayers. The results suggest that the packing and orientation of the double bond sections of all three unsaturated fatty acids show remarkable similarities, with the direction of the double bonds approximately parallel to each other irrespective of the number of unsaturations in the chain, with the 20:3 EA probably forming "iron-angle" structures. The possibility of unsaturated chains in a "hairpin" configuration is discarded for area per molecules smaller than approximately 50 A(2), which corresponds to the lowest surface pressure measured with VSFS.

Interactions of Na+ Cations with a Highly Charged Fatty Acid Langmuir Monolayer: Molecular Description of the Phase Transition

“…The pressure of the phase transition (Пt) can be increased by reducing the cohesive forces between the fatty acid alkyl chains, or increasing the electrostatic repulsion between the headgroups. The former effect can be achieved by increasing the temperature of the subphase 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8 (Figure 1b), shortening the length of the alkyl chains, 28,[34][35] or adding an unsaturation, 36 while the latter is reached by further deprotonating the monolayer with increasing pH (Figure 1a). 27 Although as discussed above the fatty monolayers have structurally been extensively characterized, direct molecular information regarding the carboxylic acid headgroup along the compression isotherm at high pH is much more limited.…”

Self-assembly of long chain fatty acids: effect of a methyl branch

“…The r FR + peak is strongest in SSP but also seems to appear as a small peak in PPP where it is less commonly resolved. 6 The symmetric methylene stretch, d + , at 2845 cm À1 is almost absent in the spectra, and only gives a small contribution to the SSP combination. Finally, the antisymmetric methylene stretch (d À ) partly contributes to the broad feature around 2900 cm À1 in PPP.…”

“…4 The reasons for the enrichment of 18-MEA on the hair surface are poorly understood. The main differences compared to corresponding straight chain fatty acids appears to be the lower melting temperature without the inherent oxidation instability which double bonds would engender, 2,5,6 together with the different packing structure due to the space requirements of the methyl branch. 1 In addition, the inhibition of bilayer formation and a better match of the area per molecule to the limited number of cysteine residues available for anchoring the fatty acids to the macromolecular structure have been proposed.…”

Propofol adsorption at the air/water interface: a combined vibrational sum frequency spectroscopy, nuclear magnetic resonance and neutron reflectometry study

“…For alkyl chains, the aCH 3 peak is normally significantly weaker than the sCH 3 peak in the SSP polarisation combination due to the chain orientation. [63][64][65] However, for propofol the opposite is found. The reason is that the total VSF signal has contributions from all four closely spaced methyl groups (see Figure 1), and due to different directions of the transition dipole moments for the symmetric and antisymmetric methyl stretches, their signal strength will be differently enhanced.…”