Length of the DHA Chain

Docosahexaenoic acid's conformation remains a matter of conjecture. An early computer simulation by Applegate and Glomset (Applegate & Glomset, 1991) predicted that DHA would assume a rigid, extended "angle iron" shape. In fact, the "angle iron" conformation does predict better packing efficiency by forming a DHA, sn-2 groove into which the saturated sn-1 chain fits. However, this static model would likely pertain only to the gel state. The "angle iron" conformation is also at odds with experimental observations demonstrating that ROS membranes with very high levels of DHA are quite thin (27-28 A) (Dratz et al., 1985). In comparison, bilayers made from phospholipids with much shorter 18:0 saturated chains are about 29-30 A (Lewis & Engelman, 1983). In fact, nuclear magnetic resonance (NMR) studies predict that DHA's chain length is similar to that of palmitic acid (Mitchell et al., 1998).

These observations indicate that whatever DHA's conformation is, it must be compact. A second model, based on the ROS membrane, predicts a much different structure for DHA. A "molecular spring model" predicts a helical structure where DHA lengthens and shortens to accommodate conformation changes in rhodopsin (Dratz & Holte, 1992). Conformational energy calculations suggest DHA can lengthen or shorten over a range of 3-4 A with a small input of energy (Dratz et al., 1985). NMR order parameters and spin lattice relaxation times support the idea that DHA performs rapid structural transitions between extended and looped conformations (Holte et al., 1998; Koenig et al., 1997; Mitchell et al., 1998). It is therefore likely that whatever DHA's structural role in membranes is, it does not support a thick membrane; in fact, DHA's conformation is quite compact.

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