Stretching methylene

Fig. 15.4. Averaged Raman spectra from within the SC (top) and lipid (bottom) regions as noted in Fig. 15.3b and c as factors 1 and 2, respectively, a The 590-780cm-1 region with bands at 605 and 700cm-1 assigned to cholesterol as marked, b The 800-1380cm-1 region displays three major bands (1058, 1130, and 1296 cm-1) characteristic for ordered (all-bans) lipid chains, c The C-H stretching (2800-3000 cm-1) region with symmetric (2850 cm-1) and asymmetric (2880 cm-1) methylene stretching modes noted. The frequencies of the bands are consistent con-formationally ordered chains...

Fig. 15.5. Factor analysis results for the C-H stretching region (2800-3050 cm 1 region) in human skin and in cultured skin model (Epiderm ). Data from human skin (8 x 12 pixels) and cultured skin (7 x 12 pixels) have been concatenated. Pixels marked with x s were excluded from the analysis, a Factor loadings for the methylene stretching region. The dashed vertical line marks 2876 cm-1 and emphasizes the shift in frequency between factors 1 and 2. b Score plots for factor 1 are depicted for human skin in the left set of 8 X 12 pixels and for cultured skin in the right set of 7 x 12 pixels, c Score plots for factor 2 are depicted for human skin in the left set of 8 x 12 pixels and for cultured skin in the right set of 7 x 12 pixels...

This band often appears as a doublet probably resulting from the accidental degeneracy of an overtone of another low-lying vibration mode (Fermi resonance). 7a, 6 8) The C—H stretching vibrations also undergo a high-frequency shift and appear at 3050 cm-1, a position generally characteristic of the methylene stretch in cyclopropyl systems.67)... 

The second issue upon which the current results depend, namely the physical state of the exogenous lipids upon permeation, was addressed through the well-known sensitivity of the methylene stretching modes to chain conformational order. Varying views pertaining to this issue have been propounded and have been reviewed recently by Bouwstra el al.42... 

The in-situ near-normal external reflection results on adsorption of cadmium ions to mixed monolayers are given in Fig. 2b. Since the background spectra are taken for the close-packed mixed monolayers on pure water at each composition, the methylene stretching bands are nearly cancelled out. By contrast, the asymmetric carboxylate band appears with reasonable S/N ratios even without a polarizer-modulator set-up [7], It is noted that the near-normal incidence of the infrared beam resulted in negative signals on the contrary to that of the grazing-angle incidence. 

An interesting feature of the IR data is the peak at 2868 cm which correlates with the aromatic component at 1600, although it is believed to represent a combination of methyl and methylene stretches. Painter et al. (8) also found this behavior in IR spectra of coal macerals. Our data strongly suggest a reinterpretation of this peak assignment. 

Horswell et al. [80] made the first attempt to apply PM IRRAS to determine tilt angles of acyl chains of DM PC in a bilayer of this phospholipid deposited at an Au(lll) electrode surface in 2002. No correction of the measured spectra for the PEM response function was made at that time, and the tilt angle was determined using the relative method. It was assumed that the transition dipole of the asymmetric VaslCHs) stretch was randomly oriented. The tilt angle was then calculated from the ratio of the integrated intensity of the asymmetric Vas(CH2) and symmetric Vs(CH2) methylene stretches to the integrated intensity of the Vas(CH3) band using Eq. (43). 

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