Hormone (3-indoleacetic acid)-phospholipid interaction: 1H, 13C and 31P nuclear magnetic resonance studies

The interaction between the plant hormone, 3-indoleacetic acid (IAA), and some phospholipids in CDCL₃ has been studied by ¹H, ¹³C and ³¹P nuclear magnetic resonance (NMR) spectroscopy. Upon interaction with IAA, significant changes occurred in resonance positions of the phospholipid head group nuclei. Alteration of the fatty acid composition influenced the effects of IAA on these nuclei. These effects were observed in the ethanolamine and phosphate groups of the phosphatidylethanolamines, and in the choline, phosphate and glycerol groups of the phosphatidylcholines. Changes in resonance positions of the phospholipid head group nuclei were used for the determination of dissociation constants (K_d). In all cases, K_d values were approx. 10^?2 molal for 1 : 1 interaction. The NMR results suggest an interaction orientation in which the aromatic ring system of IAA interacts with the quaternary nitrogen function of the head group, and the phosphate group becomes hydrogen-bonded to the NH or carboxyl proton of 1AA.