Thermochromism of Bacteriorhodopsin and Its pH Dependence

Abstract

Purple membranes (PMs), which consist of the photochromic membrane protein bacteriorhodopsin (BR) and lipids only, show complex thermochromic properties. Three different types of reversible temperature-dependent spectral transitions were found, involving spectral states absorbing at 460, 519, and 630 nm. These thermochromic absorption changes were analyzed in the range from 10 to 80 degrees C. In dependence on the bulk pH value, hypsochromic or bathochromic shifts in the BR absorption spectra are observed in BR gels as well as in BR films. The thermochromic changes between both purple and blue or purple and red were quantified in the CIE color system. The molecular changes causing these effects are discussed, and a model is presented in terms of intramolecular protonation equilibriums. The thermochromic properties of BR may be of interest in applications like security tags, as this feature may complement the well-known photochromic properties of BR.

Temperature Independence of Ultrafast Photoisomerization in Thermophilic Rhodopsin: Assessment versus Other Microbial Proton Pumps

“…As shown by Neebe et al, the absorption spectrum of BR undergoes blue shifting by ∼40 nm accompanied by broadening upon heating (Figure S1). 38 The absorption changes are reversible, and the original spectrum is retained upon cooling. No such large shift is observed for TR upon heating, even above 70 °C.…”

“…An irreversible change is observed above the melting temperature of ∼90 °C. 38 During this elevation in temperature, the absorption spectrum of bacteriorhodopsin is blue-shifted, broadened, and reduced in peak absorbance. These changes have been tentatively assigned to altered tertiary structure of the protein, but details of these changes have yet to be determined.…”

Development and Characterization of Titanium Dioxide Gel with Encapsulated Bacteriorhodopsin for Hydrogen Production

“…17,30 The presence of free retinal is associated with the denaturation of BR as the chromophore is stripped from the protein during the denaturation process whereas the BR-bound retinal absorbance is indicative of a properly folded BR structure. 30 The degree of BR denaturation observed with addition of 1.24 M methanol (5.34 M total alcohol), 2.6 M methanol (6.7 M total alcohol), or 2.6 M ethanol (5.9 M total alcohol) was comparable, as shown in Figure 7. Gels produced with the inclusion of 25 mM methanol (far left methanol data point) resulted in insignificant levels of denaturation, similar to 0 M added ethanol gels (4.1 M total alcohol).…”

“…The ratio of free retinal (360–380 nm) to retinal-associated BR (560–570 nm) absorbance was used to estimate the extent of denaturation of BR as absorbance at these locations has previously been used to indicate denaturation of BR in polyacrylamide gels. , The presence of free retinal is associated with the denaturation of BR as the chromophore is stripped from the protein during the denaturation process whereas the BR-bound retinal absorbance is indicative of a properly folded BR structure . The degree of BR denaturation observed with addition of 1.24 M methanol (5.34 M total alcohol), 2.6 M methanol (6.7 M total alcohol), or 2.6 M ethanol (5.9 M total alcohol) was comparable, as shown in Figure .…”

Chromic Platinum Complexes Containing Multidentate Ligands

“…Chromic phenomena are abundant in nature [11–15] . For instance, chameleons display remarkable color changes during courtship and male‐male contests due to the tuning of guanine nanocrystal spacing within their skin [16] .…”