MOLECULAR CONFORMATIONAL-CHANGES IN THE TRIGLYCERIDE AND METHYL-ESTER OF STEARIC-ACID AS STUDIED BY TEMPERATURE-DEPENDENT C-13 CHEMICAL-SHIFTS

The temperature dependence of the 13C chemical shifts in tristearin and methyl stearate has been investigated in both the melt and solution phases. Intramolecular conformational changes dominate the observed behaviour and there is little evidence for intermolecular interactions even in the melt phase of tristearin. The terminal methyl carbons of methyl stearate and tristearin and the C17, C2, and glyceryl carbons of tristearin exhibit a temperature dependence consistent with there being only two rotamers of significant population. The calculated enthalpy difference between the terminal methyl and C17 rotamers is of the same order of magnitude as would be expected for tt and tg± rotamers in hydrocarbon chains. For the glyceryl carbons the rotamer energy difference is very large and only one of the rotamers is significantly populated at room temperature. The remaining carbons (C16, C17, C15, C6, C4 and C3) show a general drift to high fields with increasing temperature but the observed temperature dependence requires the existence of more than two rotamers. In the absence of an acceptable mechanism for the chemical shifts of 13C nuclei in hydrocarbon chains it is not possible to use this data to investigate conformational changes along the hydrocarbon chain. © 1979.