Infrared spectroscopic evidence for a conformational alteration of plant plasma membranes upon exposure to the growth hormone analog, 2,4-dichlorophenoxyacetic acid.

Infrared spectroscopy of highly purified fractions of plasma membrane vesicles from hypocotyls of etiolated soybean (Glycine max L.) seedlings revealed changes in bands assigned to proteins and phospholipids upon exposure to the growth hormone analog, 2,4-dichlorophenoxyacetic acid (2,4-D). The changes included a concentration dependent broadening of amide I absorbance and a change in the absorbance ratios of amide I and amide II indicative of a change in protein conformation. Band broadening of amide I was observed at 2,4-D concentrations as low as 10(-8) M, and the optimal 2,4-D concentration to evoke the change was 1 microM whereas the amide peak ratios (amide II/amide I) declined steadily over the range of concentrations (10(-8) to 10(-3)M) tested. An alteration in hydrocarbon chains (CH2 scissoring) was seen only at 1 mM (10(-3) M) 2,4-D. In contrast, the vibrational frequency of the choline stretch declined proportionally over the range 10(-6) to 10(-3). The findings provide evidence for a conformational change in the plasma membrane in response to the hormone demonstrable in a cell-free system.