光機能性蛋白質にみられるカラーチューニング機構に関する理論的研究
Color Tuning Mechanism in Photofunctional Proteins:
A SAC-CI Theoretical Study
Jun-ya Hasegawa
Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University
The present talk summarizes some recent SAC-CI theoretical studies on the photoabsorption
spectral tuning mechanism in photobiological systems [1-5]. The role of light is roughlyclassified into the energy resource and the information. In the former case, green plants andsome bacteria have the photosynthetic units in which the photon energy is efficiently
converted into the chemical energy [1]. In the latter case, since light is defined by spectrum(wavelength and intensity), controlling photo-absorption energy is essential forphoto-functional proteins in our vision, plants, and some bacterial ion pumps. Fluorescentproteins have got significant attention in the field of molecular imaging. It is therefore very
important to investigate how these proteins control the excitation energies of the chromophore
included.
In our recent studies, SAC-CI calculations successfully reproduced the absorption energy ofvarious retinal proteins, and the color-tuning mechanism was clarified for rhodopsin,bacteriorhodopsin, sensoryrhodopsin, and visual cone pigment [2]. For the emission fromfluorescent proteins, role of protein environment was investigated [3]. For firefly luciferin, theyellow-green luminescence had long been recognized as the fluorescence from theenol-oxyluciferin. A recent experiment and our calculations suggested that keto-oxyluciferin
can produce the yellow-green luminescence with the help of the protein environment [4].
Depending on the protein environments, some chromophores, retinal and oxyluciferin forexamples, significantly change their photoabsorption energies. On the basis of our theoreticalanalysis, we found a common feature in the character of the electronic transitions and theprotein electrostatic environment. On the basis of the proposed color-tuning mechanisms, we
also performed theoretical mutagenesis [4].
References[1] J. Hasegawa and H. Nakatsuji, “Exploring Photo-Biology and Bio-Spectroscopy with the SAC-CI(Symmetry-Adapted Cluster-Configuration Interaction) Method”, in “Radiation Induced Molecular
Phenomena in Nucleic Acid: A Comprehensive Theoretical and Experimental Analysis”, Ed., by M. Shukla
and J. Leszczynsk, pp. 93-124, Springer, 2008; M. Ehara, J. Hasegawa, H. Nakatsuji, “SAC-CI Method
Applied to Molecular Spectroscopy”, in "Theory and Applications of Computational Chemistry: The First 40Years, A Volume of Technical and Historical Perspectives", Ed. by C. E. Dykstra, G. Frenking, K. S. Kim,and G. E. Scuseria, pp. 1099-1141, Elsevier, Oxford, 2005.
[2] K. Fujimoto, S. Hayashi, J. Hasegawa, and H. Nakatsuji, “Theoretical Studies on the Color-TuningMechanism in Retinal Proteins”, J. Chem. Theo. Comp. vol. 3, pp. 605-618, 2007; K. Fujimoto, J. Hasegawa,S. Hayashi, and H. Nakatsuji, “On the color-tuning mechanism of Human-Blue visual pigment: SAC-CI andQM/MM study”, K. Fujimoto, J. Hasegawa, S. Hayashi, and H. Nakatsuji, Chem. Phys. Letters, vol. 432, pp.
252-256, 2006; K. Fujimoto, J. Hasegawa, H. Nakatsuji, “Origin of color tuning in human red, green, andblue cone visual pigments: SAC-CI and QM/MM study”, Chem. Phys. Letters, vol. 462, pp. 318-320, 2008;
K. Fujimoto, J. Hasegawa, and H. Nakatsuji, to be submitted.
[3] J. Hasegawa, K. Fujimoto, B. Swerts, T. Miyahara, H. Nakatsuji, “Excited States of GFP Chromophore andActive Site Studied by the SAC-CI method: Effect of Protein-environment and Mutations”, J. Comp. Chem.,
vol. 28, pp. 2443-2452, 2007.
[4] N. Nakatani, J. Hasegawa, H. Nakatsuji, J. Am. Chem. Soc. 129, 8756-8765 (2007); N. Nakatani, J.
Hasegawa, and H. Nakatsuji, submitted.