The impact that the position of double bonds has upon the properties of membranes is investigated using solid-state H-2 NMR and MD simulations to compare positional isomers of 1-palmitoyl-2-octadecenoylphos-phatidylcholine (16:0-18:1PC) bilayers that are otherwise identical apart from the location of a single cis double bond at the Delta(6), Delta(9), Delta(12), or Delta(15) position in the 18:1 sn-2 chain. Moment analysis of H-2 NMR spectra recorded for isomers perdeuterated in the 16:0 sn-1 chain reveals that average order parameters (S) over bar (CD) change by more than 35% and that the temperature for chain melting T-m varies by 40 degrees C. At equal temperature, the (S) over bar (CD) values exhibit a minimum, as do T-m values, when the double bond is in the middle of the 18:1 sn-2 chain and increase as it is shifted toward each end. Order parameter profiles generated from depaked ("dePaked") spectra for the 16:0 sn-1 chain all possess the same shape with a characteristic "plateau" region of slowly decreasing order in the upper portion before progressively decreasing more in the lower portion. The NMR results are interpreted on the basis of MD simulation results obtained on each of the four systems. The simulations support the idea that the order parameter changes reflect differences in molecular surface areas, and furthermore that the molecular areas are a function of the strength of the acyl chain attractions.