Abstract

We present a combined quantum mechanics/molecular mechanics (QM/MM) molecular dynamics–statistical approach for the interpretation of nuclear magnetic resonance (NMR) chemical shift patterns in phycocyanobilin (PCB). These were originally associated with colour tuning upon photoproduct formation in red/green-absorbing cyanobacteriochrome AnPixJg2 and red/far-red-absorbing phytochrome Cph12. We pursue an indirect approach without computation of the absorption frequencies since the molecular geometry of cofactor and protein are not accurately known. Instead, we resort to a heuristic determination of the conjugation length in PCB through the experimental NMR chemical shift patterns, supported by quantum chemical calculations. We have found a characteristic correlation pattern of C chemical shifts to specific bond orders within the -conjugated system, which rests on the relative position of carbon atoms with respect to electron-withdrawing groups and the polarisation of covalent bonds. We propose the inversion of this regioselective relationship using multivariate statistics and to apply it to the known experimental NMR chemical shifts in order to predict changes in the bond alternation pattern. Therefrom the extent of electronic conjugation, and eventually the change in absorption frequency, can be derived. In the process, the consultation of explicit mesomeric formulae plays an important role to qualitatively account for possible conjugation scenarios of the chromophore. While we are able to consistently associate the NMR chemical shifts with hypsochromic and bathochromic shifts in the P and P, our approach represents an alternative method to increase the explanatory power of NMR spectroscopic data in proteins.

Highlights

  • Tetrapyrroles showcase an overwhelming abundance in living species, accompanied by an impressive functional versatility [1]

  • No crystal structures of the photoproduct states of Cph1∆2 and AnPixJg2 exist that could build the basis of theoretical structure models, but we show that relying on the two Phytochromes switch between red-absorbing (Pr) states, structural fluctuations can profitably be correlated with calculated spectroscopic patterns and increase the explanatory power of nuclear magnetic resonance (NMR) spectroscopy towards conjugation effects in the Pg and Pfr state, respectively

  • Extensive molecular dynamics (MD) sampling under the quantum mechanics/molecular mechanics (QM/MM) regime allows for the inclusion of thermal fluctuations of the molecular structure on the femtosecond and picosecond time scale

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Summary

Introduction

Tetrapyrroles showcase an overwhelming abundance in living species, accompanied by an impressive functional versatility [1]. They can be classified into cyclic and open-chain forms, the former comprising the well-known porphyrins that build the precursors of hemes and chlorophylls, renowned for their important role in oxygen transport, oxidoreductase enzymes, and photosynthesis [2]. The second group, open-chain tetrapyrroles, referred to as bilins, are ubiquitous in living species as well. Organised in biliproteins, they take the role of a chromophore. Variants of (bacterio)phytochromes were found, genuine to cyanobacteria, that are called cyanobacteriochromes (CBCRs) [12,13]

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