Successive energy transfer within multiple photosensitizers assembled in a hexameric hemoprotein scaffold

Abstract

An assembly of multiple photosensitizers is demonstrated by development of a hexameric hemoprotein (HTHP) scaffold as a light harvesting model to replicate the successive energy transfer occuring within photosensitizer assemblies of natural systems. In our model, six zinc protoporphyrin IX (ZnPP) molecules are arrayed at the heme binding site of HTHP by supramolecular interactions and five fluorescein (Flu) molecules and one Texas Red (Tex) molecule as donor and acceptor photosensitizers, respectively, are attached to the HTHP protein surface with covalent linkages. The flow of excited energy from photoexcited Flu to Tex occurs via two pathways: direct energy transfer from Flu to Tex (path 1) and energy transfer via ZnPP (path 2). Steady state and time-resolved fluorescence measurements reveal that the energy transfer ratio of these pathways (path 1 : path 2) is 39 : 61. These findings indicate that the excited energy originating at five Flu and six ZnPP molecules is collected at one Tex molecule as a funnel-like bottom for light harvesting. The present system using the hexameric hemoprotein scaffold is a promising candidate for construction of an artificial light harvesting system having multiple photosensitizers to promote efficient use of solar energy.

Thermoresponsive Micellar Assembly Constructed from a Hexameric Hemoprotein Modified with Poly(N-isopropylacrylamide) toward an Artificial Light-Harvesting System

“…The UV−vis absorption spectrum of ZnPP (6/6) HTHP V44C is similar to that of reconstituted HTHP WT with ZnPP as reported in our previous work. 50,51 As a reference sample, a protein (ZnPP (1/6) HTHP V44C ) where only one heme pocket is occupied by ZnPP was also prepared by mixing apoHTHP V44C and ZnPP (6/6) HTHP V44C with a ratio of 5:1 (Figure 9b). These obtained reconstituted proteins are described as ZnPP (n/6) HTHP V44C , where n shows the apparent number of ZnPP for HTHP having six heme pockets.…”

“…Excess Zn protoporphyrin IX, ZnPP, as a photosensitizer was added into a solution of apoHTHP V44C under neutral-pH conditions, yielding the reconstituted HTHP V44C (ZnPP (6/6) HTHP V44C ), where 6/6 shows the complete binding of ZnPP into the six heme pockets of apoHTHP V44C (Figure a). The UV–vis absorption spectrum of ZnPP (6/6) HTHP V44C is similar to that of reconstituted HTHP WT with ZnPP as reported in our previous work. , As a reference sample, a protein (ZnPP (1/6) HTHP V44C ) where only one heme pocket is occupied by ZnPP was also prepared by mixing apoHTHP V44C and ZnPP (6/6) HTHP V44C with a ratio of 5:1 (Figure b). These obtained reconstituted proteins are described as ZnPP ( n /6) HTHP V44C , where n shows the apparent number of ZnPP for HTHP having six heme pockets.…”

“…Thus, the smaller r 0 value suggests that energy migration occurs more rapidly. The initial r 0 value in the fluorescence anisotropy decay of ZnPP (6/6) HTHP V44C is found to be smaller than that of ZnPP (1/6) HTHP V44C due to efficient energy migration occurring within the ZnPP array in the hexameric structure of HTHP . Furthermore, the ZnPP (6/6) micelle has a remarkably small r 0 value compared to other samples.…”

“…The initial r 0 value in the fluorescence anisotropy decay of ZnPP (6/6) HTHP V44C is found to be smaller than that of ZnPP (1/6) HTHP V44C due to efficient energy migration occurring within the ZnPP array in the hexameric structure of HTHP. 51 Furthermore, the ZnPP (6/6) micelle has a remarkably small r 0 value compared to other samples. This finding indicates that rapid energy migration occurs on the surface of the ZnPP (6/6) micelle.…”

A Supramolecular Assembly of Hemoproteins Formed in a Star-Shaped Structure via Heme–Heme Pocket Interactions

“…It is considered as an interesting building block for artificial protein assemblies due to its symmetric structure and thermal stability with a denaturation midpoint, T m , above 130 • C. Chemical modifications of HTHP, via an engineered cysteine residue, enable construction of various assemblies such as a stacked dimer, a two-dimensional sheet, and a micelle-like structure [19][20][21]. In addition to these features, HTHP allows the replacement of heme with artificial cofactors [21][22][23]. Thus, HTHP is one of the useful building blocks for the generation of supramolecular assemblies.…”