Microwave-Induced Structural Changes in Bacteriorhodopsin: Studies by Optical and Fourier Transform Infrared Difference Spectroscopy

Abstract

Abstract—Using optical and Fourier transform infrared (FTIR) difference spectroscopy, microwave radiation was found to affect the bacteriorhodopsin (BR) structure in films at a 30% relative humidity. This study is the first to demonstrate that a transition from the dark-adapted basal state BR₅₆₀ to a state similar to the light-adapted state BR₅₆₈, which is a mixture of BR₅₆₈ and other isomeric forms, occurs in the absence of external photoexcitation on exposure to microwave radiation with a frequency range of 8 ≤ f ≤ 18 GHz and an intensity lower than 10 mW/cm². The initial response of dark-adapted BR to microwave radiation included collective motion of a large portion of protein atoms, producing relatively strong signals in the regions 3700–3300, 2400–2300, and 800–600 cm^–1 of the FTIR difference spectrum. Relatively weak amplitude responses were detected in the frequency range characteristics of the retinal chromophore and amide bands (Amide I and Amide II). The effects reflected changes in the retinal chromophore structure and local changes in the chromophore microenvironment.