Carbon-13 Fourier transform nuclear magnetic resonance. VIII. Role of steric and electric field effects in fatty acid spectra

Abstract

From the carbon-13 spectra of over 40 fatty acids and their derivatives, a set of substituent shift parameters have been derived which uniquely define saturated, unsaturated, polyunsaturated, or cyclopropane fatty acids.Together with model compounds and selectively enriched compounds, lanthanide shift reagents were used to confirm carbon-13 shift assignments. Carbon-13 substituent shifts are interpreted in terms of steric and linear electric field effects and a rationale is given for linear electric field effects. Qualitative agreement between calculated electric field shifts for saturated carbons and the observed shifts has been obtained. Since both steric and linear electric field effects show both angle and distance dependence, they should be considered powerful tools for conformational as well as structural analysis.Carbon-13 chemical shifts are reported for over 40 fatty acids, their methyl esters, triglycerides, and related compounds. Interest in these molecules derives from several sources, among which are their roles as an energy store for animal life and as a major class of tissue components. In their free acid form they are intermediates in fat transport;2 in combined form they represent a very high proportion of fat deposits,2 lipoprotein,2•3 and biological membrane material.4In this study we have been able to derive a set of substituent shift parameters which describes the shifts of all the carbons in fatty acid chains such that the complete structures of unknown fatty acids may now be determined solely from their 13C spectra. Application of these substituent shifts will prove useful in the analysis of fatty acid components in biological tissues.The present study has delineated the significant roles of steric and electric field effects in producing the observed shift differences between alkane and fatty acid spectra. The shifts of the unsaturated carbons, in particular, are strongly dependent on electric field effects and hence on conformation and solvent properties.8 The linear electric field effects can be estimated in a simple way, giving good qualitative agreement with our observed values. We have also, in specific instances, been able to ascribe individual shifts induced by substituent changes to a conformational mechanism. Such information will help to further the application of 13C shift measurements to conformational analysis6 as well as the structural and analytical problems considered here.

Chemical Constituents from Sambucus adnata and Their Protein-Tyrosine Phosphatase 1B Inhibitory Activities

“…The EtOAc fraction was subsequently separated by silica gel, octadecyl silica (ODS) column chromatography and reversed-phase HPLC to afford thirteen compounds 1-13. The known compounds were identified as ursolic acid (2), 14) lupeol 3-acetate (3), 15) oleanolic acid (4), 14) stigmast-4-en-3b-ol-6-one (5), 16) 7-keto-sitosterol (6), 14) sitosterol (7), 17) pinoresinol (8), 18) (Ϯ)-boehmenan (9), 19) isoquercitrin (10), 20) 1H-indol-3-amine (11), 21) methyl p-hydroxybenzoate (12), 22) and stearic acid (13) 23) by detailed NMR spectroscopic analysis and comparison with literature data. Although known in other genera, the isolation of lupane-type triterpene (3), stigmas-type sterols (5, 6), neolignan (9), and indole-type alkaloid (11) was first reported in this study from the genus Sambucus (Fig.…”

13C Fourier transform nuclear magnetic resonance. XII. Structure of the phytol–lecithin bilayer. Application of electric field effects

“…The phytol 13C shifts of a series of fatty acids and their methyl assignments are taken from Goodman et al (12). esters (11). Chemical shifts of phytol carbons at The high field portion of the proton noise-52 and 11 O C are given in Table 2 A noteworthy feature of the 13C spectrum of the lecithin-phytol mixture is the splitting of the main methylene envelope of lecithin into two distinct peaks by the addition of phytol.…”

“…The characteristic upfield-downfield chemical shift for an olefinic pair can be interpreted in terms of an ~ntermolecular linear electric field effect (11,18). The electric field polarizes the double bond causing an equal and opposite electron density change at the two carbons, with the olefinic carbon near the head group bearing the increased charge.…”

Ceramide Constituents from Five Mushrooms.

“…The Et 2 O extract (3.7 g) was chromatographed on a silica gel column using hexane-EtOAc (7 : 3-1 : 7), EtOAc and MeOH, to afford 20 fractions (frs. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Fraction 17 was purified by preparative HPLC to give 3 (1.9 mg) and 4 (1.8 mg).…”