ACYL CHAIN CONFORMATIONAL ORDERING IN 1,2 DIPALMITOYLPHOSPHATIDYLETHANOLAMINE - INTEGRATION OF FT-IR AND H-2 NMR RESULTS

The extent of trans-gauche isomerization at the 4 and 4' positions of the acyl chains of fully hydrated 4,4,4',4'-d4 1,2-dipalmitoylphosphatidylethanolamine (4-d4 DPPE) bilayers was quantitatively evaluated from the infrared (IR) intensity of the CD2 rocking modes. About 20% gauche conformers were observed at 72-degrees-C (above T(m)), while at 23-degrees-C, well below T(m), about 4% were noted. The order parameter S(C-D) was determined from H-2 nuclear magnetic resonance (NMR) quadrupolar splittings. S(C-D)) is the product of a segmental order parameter (S(gamma)), which depends on conformational order, and a chain order parameter (S(alpha)) which depends on slower motions such as chain wobble. The IR-determined percentage of gauche forms was converted into a segmental order parameter and factored out of the measured value for S(C-D) to yield an estimate of S(alpha) = 0.59 for L(alpha) phase DPPE. A comparison with S(alpha) for 1,2 dipalmitoylphosphatidylcholine (DPPC) suggests that increased wobble is responsible for enhanced motional averaging of the quadrupolar splittings in the latter at a similar reduced temperature. The extent of conformational disordering [at the 4(4') position] is essentially unchanged between the two molecules. The current study demonstrates the advantage of integrating quantitative IR with H-2 NMR data, for elucidation of the contributions of the individual motions that average the NMR quadrupolar splittings.