Abstract
The solid-state photo-chemically induced dynamic nuclear polarization (photo-CIDNP) effect generates non-Boltzmann nuclear spin magnetization, referred to as hyperpolarization, allowing for high gain of sensitivity in nuclear magnetic resonance (NMR). Well known to occur in photosynthetic reaction centers, the effect was also observed in a light-oxygen-voltage (LOV) domain of the blue-light receptor phototropin, in which the functional cysteine was removed to prevent photo-chemical reactions with the cofactor, a flavin mononucleotide (FMN). Upon illumination, the FMN abstracts an electron from a tryptophan to form a transient spin-correlated radical pair (SCRP) generating the photo-CIDNP effect. Here, we report on designed molecular spin-machines producing nuclear hyperpolarization upon illumination: a LOV domain of aureochrome1a from Phaeodactylum tricornutum, and a LOV domain named 4511 from Methylobacterium radiotolerans (Mr4511) which lacks an otherwise conserved tryptophan in its wild-type form. Insertion of the tryptophan at canonical and novel positions in Mr4511 yields photo-CIDNP effects observed by 15N and 1H liquid-state high-resolution NMR with a characteristic magnetic-field dependence indicating an involvement of anisotropic magnetic interactions and a slow-motion regime in the transient paramagnetic state. The heuristic biomimetic design opens new categories of experiments to analyze and apply the photo-CIDNP effect.
Highlights
The solid-state photo-chemically induced dynamic nuclear polarization effect generates non-Boltzmann nuclear spin magnetization, referred to as hyperpolarization, allowing for high gain of sensitivity in nuclear magnetic resonance (NMR)
Aiming for designed molecular spinmachines producing light-induced nuclear hyperpolarization, we have designed a series of protein mutants, which will be presented in parts (i)—(iii) (Table 1). (i) So far, the occurrence of the solid-state photo-CIDNP effect was limited to cysteine-lacking LOV domains of phototropin[39,40,41]; for this reason, here we explored other potential LOV-based light-induced hyperpolarization generators
We show that a photo-CIDNP effect originating from the spin-correlated radical pair (SCRP) of flavin mononucleotide (FMN) and tryptophan can be produced in artificially designed flavoproteins
Summary
The solid-state photo-chemically induced dynamic nuclear polarization (photo-CIDNP) effect generates non-Boltzmann nuclear spin magnetization, referred to as hyperpolarization, allowing for high gain of sensitivity in nuclear magnetic resonance (NMR). The first successful solid-state photoCIDNP effect observation was demonstrated for a frozen cysteine-lacking LOV domain of phototropin from the green alga Chlamydomonas reinhardtii (Crphot-LOV1-C57S)[41,44] In this case, the photo-excited FMN undergoes inter-system crossing to the triplet state, causes intra-protein electron transfer from a tryptophan to the FMN and gives rise to formation of a transient SCRP. By using a field-cycling system, the magnetic field-dependencies of the 1H, 13C, and 15N hyperpolarization effects have been obtained revealing that the magnetic field for maximal photoCIDNP depends on the nuclear gyromagnetic ratio Such a behavior is a characteristic feature of the solid-state photo-CIDNP effect[36,39], closely resembling that of Crphot-LOV1-C57S. We propose that, like in phototropin, the anisotropic magnetic interactions might play a decisive role in photo-CIDNP formation in the LOV domain of aureochrome in solution
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