ISSN:
1432-1017
Keywords:
Phospholipids
;
FT-IR spectroscopy
;
2H-NMR spectroscopy
;
Chain conformations
;
Wobbling motions
;
Order parameters
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Physics
Notes:
Abstract FT-IR spectroscopy has been used to evaluate the acyl chain conformational ordering of DMPC, DMPE, DMPA (pH 6 and 12), DMPG (pH 1 and 7), and DPPC, DPPE, DPPA (pH 6). The frequencies of the symmetric and antisymmetric methylene stretching vibrations were determined as a function of temperature. In the liquid-crystalline phase the frequencies show a qualitative dependence on the amount of chain disorder. Quantitative data for trans-gauche isomerization were obtained from the integral intensities of the conformation sensitive methylene wagging absorptions at ca. 1368 cm−1 (gtg′ and gtg sequences), 1356 cm −1 (double gauche) and 1342 cm−1 (end gauche). The integral band intensities were converted to the number of gauche conformers per acyl chain using the calibration factors published by Senak et al. (1991). At 69°C the highest number of gauche conformers excluding contributions from single gauche conformers and jogs (g′tttg) are found for PCs (DMPC: 2.6; DPPC: 2.4), followed by DMPG− (2.0), phosphatidylethanolamines (DMPE: 1.4; DPPE: 2.0), protonated DMPG (1.5), and phosphatidic acids (DPPA−: 1.7; DMPA−: 1.4, DMPA2−: 1.7). From 2H-NMR measurements of perdeuterated samples of DMPC, DMPA, DPPC, and DPPA the quadrupolar splittings Δν Q⊥i and the order parameter S CDi of the CD2-segments close to the chain ends could be determined whereas splittings in the plateau region of the chains could not be resolved. The quadrupolar splittings are affected by trans-gauche isomerization, long axis rotation, and restricted wobbling motions of the acyl chains. In the simplest assumption, the order parameter SCD can be expressed as a product of a segmental order parameter Sγ and a lhain order parameter S α. For comparison of the different lipids we used average order parameters SCD, obtained by averaging over all $$S_{CDi^ - }$$ values, and Sγ determined from the total number of gauche conformers per chain by FT-IR-spectroscopy, to calculate an empirical average chain order parameter Sα. The combination of 2H-NMR and FT-IR results allows the estimation of the relative extent of chain wobbling for the different lipid molecules. Sα is lowest for PCs (Sα ≈ 0.475) while PEs (Sα≈ 0.51) and PAs (Sα≈0.52) show less chain wobbling.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00188656
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