Temperature Jump-Induced Secondary Structural Change of the Membrane Protein Bacteriorhodopsin in the Premelting Temperature Region: A Nanosecond Time-Resolved Fourier Transform Infrared Study

AbstractThe secondary structural changes of the membrane protein, bacteriorhodopsin, are studied during the premelting reversible transition by using laser-induced temperature jump technique and nanosecond time-resolved Fourier transform infrared spectroscopy. The helical structural changes are triggered by using a 15°C temperature jump induced from a preheated bacteriorhodopsin in D2O solution at a temperature of 72°C. The structural transition from αII- to αI-helices is observed by following the change in the frequency of the amide I band from 1667 to 1651cm−1 and the shift in the frequency of the amide II vibration from 1542cm−1 to 1436cm−1 upon H/D exchange. It is found that although the amide I band changes its frequency on a time scale of <100ns, the H/D exchange shifts the frequency of the amide II band and causes a complex changes in the 1651–1600cm−1 and 1530–1430cm−1 frequency region on a longer time scale (>300ns). Our result suggests that in this “premelting transition” temperature region of bacteriorhodopsin, an intrahelical conformation conversion of the αII to αI leads to the exposure of the hydrophobic region of the protein to the aqueous medium