A Thermostable Protein Matrix for Spectroscopic Analysis of Organic Semiconductors

Abstract

Advances in protein design and engineering have yielded peptide assemblies with enhanced and non-native functionalities.Here, various molecular organic semiconductors (OSCs), with known excitonic up-and down-conversion properties, are attached to a de novo-designed protein, conferring entirely novel functions on the peptide scaffolds. The protein-OSC complexes form similarly-sized, stable, water-soluble nanoparticles that are robust to cryogenic freezing and processing into the solidstate. The peptide matrix enables the formation of protein-OSC-trehalose glasses that fix the proteins in their folded states under oxygen-limited conditions. The encapsulation dramatically enhances the stability of protein-OSC complexes to photodamage, increasing the lifetime of the chromophores from several hours to more than 10 weeks under constant illumination. Comparison of the photophysical properties of astaxanthin aggregates in mixed-solvent systems and proteins shows that the peptide environment does not alter the underlying electronic processes of the incorporated materials, exemplified here by singlet exciton fission followed by separation into weakly-bound, localized triplets. The adaptable protein-based approach lays the foundation for spectroscopic assessment of a broad range of molecular OSCs in aqueous solutions and the solid-state, circumventing the laborious procedure of identifying the experimental conditions necessary for aggregate generation or film formation. The non-native protein functions also raise the prospect of future bio-compatible devices where peptide assemblies could complex with native and non-native systems to generate novel functional materials.

Engineering excitonically coupled dimers in an artificial protein for light harvesting via computational modeling

“…We start the analysis by considering the BT6 protein (Farid et al, 2013; Lishchuk et al, 2018; Sutherland et al, 2018; Tsargorodska et al, 2016), which has four histidine residues (Figure 1a), and two mutants in which either two (Kodali et al, 2016) (2H → 2A variant) or all four (Sutherland et al, 2020) (4H → 4A) His were replaced by alanine (Ala, see sequences in Table 1). With light‐harvesting functionality in mind, we have chosen Zn 13 2 ‐OH‐methylpheophorbide a (Figure 1b), a chlorophyll a derivative, as the chromophoric cofactor.…”

Twisted Carotenoids Do Not Support Efficient Intramolecular Singlet Fission in the Orange Carotenoid Protein

“…Before doing so, we make several observations about the carotenoids involved in SF in LHCs of purple bacteria: (1) the S 0 → S 2 absorbance spectra of the carotenoids in light-harvesting antenna are similar to those of their all- trans forms in organic solvent and depend sensitively on the carotenoid conjugation length. , A full break in conjugation along the chain would lead to a dramatic blue shift of the carotenoid absorption feature that is not observed. (2) The carotenoid T 1 → T n excited-state absorption feature seen in transient absorption of LHCs ,,, is very similar to that seen in aggregated carotenoids of comparable conjugation lengths forming triplets by intermolecular SF. − , The T 1 → T n feature is also sensitive to the carotenoid conjugation length, ,, and a conjugation break along the chain would similarly lead to a blue shift that is not observed. (3) The dipolar D and E parameters of the SF-generated triplets in LHCs from transient electron parametric resonance (EPR) spectroscopy are similar to full-chain triplet D and E parameters rather than to their half-chain alternatives .…”

“…To avoid the problems associated with using spectroscopy to probe a light-activated conformational switch, we prevent the conformational change by trapping the protein in either its OCPo or OCPr conformation in a trehalose−sucrose glass as previously described. 70 This glass matrix prevents OCPo ⇄ OCPr conversion, as demonstrated in Figure 2a,b, and allows us to probe each conformation in isolation at room temperature without altering its conformation or photophysics. 89 To confirm the twisted/planar conformations of CAN in the OCPo/OCPr glass films, we turn to resonance Raman spectroscopy.…”

Engineering Excitonically-Coupled Dimers in an Artificial Protein for Light Harvesting via Molecular Dynamics Simulations

“…We start the analysis by considering the BT6 protein, 19,[26][27][28] which has 4 histidine residues (Figure 1a), and two mutants in which either two 21 (2H→2A variant) or all four 29 (4H→4A) His were replaced by alanine (Ala, see sequences in Table 1). With light-harvesting functionality in mind, we have chosen Zn 13 2 -OH-methylpheophorbide a (Figure 1b), a chlorophyll a derivative, as the chromophoric cofactor.…”