The science of the mechanism of vision as well as of the light driven proton pumping in visual membranes has been the centre of a great attention by biochemistry and biophysics. The light-triggered processes go through clearly identified changes of the molecular structure of the retinylidene chromophore which is covalently bonded to the opsin protein by a protonated Schiff base (see Figure 1). In the case of bacteriorhodopsin (bR), at specific molecular steps of the photo-cycle, H+ are taken up from the cytoplasmic medium and ejected into the extracellular medium, thus acting as a very efficient light-driven proton pump. For investigating the peculiar molecular structure of the chromophore in bR with Raman spectroscopy, several molecular models have been considered: free retinal, unprotonated Schiff base (SB), Schiff base protonated with HCl (SB·HCl). Raman spectra have been interpreted with the help of Density Functional Theory (DFT) calculations and the Effective Conjugation Coordinate (ECC) theory [1]. In this contribution we explore the molecular origin of the so called “opsin shift”, which is defined as the difference between the spectroscopic properties of a molecular model of protonated Schiff base (such as SB·HCl) and the spectroscopic properties of the retinylidene chromophore within the opsin protein. The opsin shift has been matter of many investigations [2], mainly from the point of view of electronic absorption. In this work we focus our attention on the Raman side of the opsin shift. Based on the results from calculations and experiments we conclude that the opsin shift is mainly due to the interplay existing between the π electrons of the retinylidene chromophore and the local environment of the proton attached to the Schiff base nitrogen atom. Simple theoretical molecular models with different counter ions interacting with the Schiff base proton display a remarkable tuning of the Raman spectra which positively compares with experimental trends and allows a sound interpretation of the data within the framework provided by ECC theory. These results pave the way for the application of Raman scattering to visual membranes in the fields of biochemistry and biophysics, even on samples in awkward experimental conditions for which Raman scattering can be still recorded in a reasonably quick and easy way.

A Raman study of Schiff base protonation in bacteriorhodopsin and related molecular models

TOMMASINI, MATTEO MARIA SAVERIO;LUCOTTI, ANDREA;ZERBI, GIUSEPPE
2010

Abstract

The science of the mechanism of vision as well as of the light driven proton pumping in visual membranes has been the centre of a great attention by biochemistry and biophysics. The light-triggered processes go through clearly identified changes of the molecular structure of the retinylidene chromophore which is covalently bonded to the opsin protein by a protonated Schiff base (see Figure 1). In the case of bacteriorhodopsin (bR), at specific molecular steps of the photo-cycle, H+ are taken up from the cytoplasmic medium and ejected into the extracellular medium, thus acting as a very efficient light-driven proton pump. For investigating the peculiar molecular structure of the chromophore in bR with Raman spectroscopy, several molecular models have been considered: free retinal, unprotonated Schiff base (SB), Schiff base protonated with HCl (SB·HCl). Raman spectra have been interpreted with the help of Density Functional Theory (DFT) calculations and the Effective Conjugation Coordinate (ECC) theory [1]. In this contribution we explore the molecular origin of the so called “opsin shift”, which is defined as the difference between the spectroscopic properties of a molecular model of protonated Schiff base (such as SB·HCl) and the spectroscopic properties of the retinylidene chromophore within the opsin protein. The opsin shift has been matter of many investigations [2], mainly from the point of view of electronic absorption. In this work we focus our attention on the Raman side of the opsin shift. Based on the results from calculations and experiments we conclude that the opsin shift is mainly due to the interplay existing between the π electrons of the retinylidene chromophore and the local environment of the proton attached to the Schiff base nitrogen atom. Simple theoretical molecular models with different counter ions interacting with the Schiff base proton display a remarkable tuning of the Raman spectra which positively compares with experimental trends and allows a sound interpretation of the data within the framework provided by ECC theory. These results pave the way for the application of Raman scattering to visual membranes in the fields of biochemistry and biophysics, even on samples in awkward experimental conditions for which Raman scattering can be still recorded in a reasonably quick and easy way.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11311/655630
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