Function of Membrane Protein in Silica Nanopores:  Incorporation of Photosynthetic Light-Harvesting Protein LH2 into FSM

Abstract

A high amount of functional membrane protein complex was introduced into a folded-sheet silica mesoporous material (FSM) that has nanometer-size pores of honeycomb-like hexagonal cylindrical structure inside. The photosynthetic light-harvesting complex LH2, which is a typical membrane protein, has a cylindrical structure of 7.3 nm diameter and contains 27 bacteriochlorophyll a and nine carotenoid molecules. The complex captures light energy in the anoxygenic thermophilic purple photosynthetic bacterium Thermochromatium tepidum. The amount of LH2 adsorbed to FSM was determined optically and by the adsorption isotherms of N2. The FSM compounds with internal pore diameters of 7.9 and 2.7 nm adsorbed LH2 at 1.11 and 0.24 mg/mg FSM, respectively, suggesting the high specific affinity of LH2 to the interior of the hydrophobic nanopores with a diameter of 7.9 nm. The LH2 adsorbed to FSM showed almost intact absorption bands of bacteriochlorophylls, and was fully active in the capture and transfer of excitation energy. The LH2 complex inside the FSM showed increased heat stability of the exciton-type absorption band of bacteriochlorophylls (B850), suggesting higher circular symmetry. The environment inside the hydrophobic silica nanopores can be a new matrix for the membrane proteins to reveal their functions. The silica-membrane protein adduct will be useful for the construction of new probes and reaction systems.

Structural Stability of Light-harvesting Protein LH2 Adsorbed on Mesoporous Silica Supports

“…1). 10 The results of spectral measurements confirm a slight ellipsoidal deformation of LH2 on the MPS supports. Since the pore size of MPS does not influence the thermal deformation behavior of LH2 on MPS support, it can be considered that the spatial confinement of LH2 in size-matches pore do not improve the structural stability of LH2.…”

“…Since these interactions are sensitive to changes in the protein structure, the B800 and B850 band properties can provide information on the LH2 structure. 9 Recently, Oda et al 10 studied irreversible heat degradation of LH2 adsorbed on mesoporous silica (MPS) by monitoring the B850 band intensity, and found that the degradation rate of LH2 was slower in mesoporous silica than in bulk water. They suggested that this slower degradation rate to be due to restrictions on the fluctuation of LH2 inside the silica mesopores.…”

“…On the other hand, LH2 locates on both the outer surface and the inside the pore when it is adsorbed to MPS with a pore diameter of greater than 7.3 nm. 10,11 Thus, the LH2 structures adsorbed on MPS supports with a series of pore diameter can provide information on the influence of the spatial confinement.…”

“…tepidum on FSM type mesoporous silica with a hexagonal pore arrangement, and the loading of LH2 into silica pores with 7.9 nm in BJH pore diameter was confirmed by a nitrogen adsorption isotherm measurement. 10 Ikemoto et al studied the adsorption of LH2 from Rps. acidophila on SBA type mesoporous silica with a 10.7 or 17.3 nm in BJH pore diameter, and confirmed the loading of LH2 into these silica pores by means of wide-field fluorescence microscopy.…”

Determination of the Molar Extinction Coefficients of the B800 and B850 Absorption Bands in Light-harvesting Complexes 2 Derived from Three Purple Photosynthetic Bacteria Rhodoblastus acidophilus, Rhodobacter sphaeroides, and Phaeospirillum molischianum by Extraction of Bacteriochlorophyll a

“…[11][12][13][14][15][16][17][18][19][20] Such confusion would originate from poor experimental evidence and/or chain citations; some reports just referred to the previous publications for the ε values of LH2. Therefore, it is necessary to reconsider the ε values of the B800 and B850 bands of the LH2 proteins based on precise experimental measurements.…”