A Multipronged Bioengineering, Spectroscopic and Theoretical Approach in Unravelling the Excited-State Dynamics of the Archetype Mycosporine Amino Acid

Abstract

Mycosporine glycine (MyG) was produced by the fermentation of a purposely engineered bacterial strain and isolated from this sustainable source. The ultrafast spectroscopy of MyG was then investigated in its native, zwitterionic form (MyG_zwitter), via femtosecond transient electronic absorption spectroscopy. Complementary nonadiabatic (NAD) simulations suggest that, upon photoexcitation to the lowest excited singlet state (S₁), MyG_zwitter undergoes efficient nonradiative decay to repopulate the electronic ground state (S₀). We propose an initial ultrafast ring-twisting mechanism toward an S₁/S₀ conical intersection, followed by internal conversion to S₀ and subsequent vibrational cooling. This study illuminates the workings of the archetype mycosporine, providing photoprotection, in the UV-B range, to organisms such as corals, macroalgae, and cyanobacteria. This study also contributes to our growing understanding of the photoprotection mechanisms of life.

Figure 1

(a) Structures of zwitterionic (left) and protonated MyG (resonance forms, center and right). (b) pSW002 plasmid containing mysABC genes under the control of SP44 promoter. Lambda t0 terminator was included to increase transcription rate. [Legend: hygR = hygromycin resistant gene, ampR = ampicillin-resistant gene. pSAM2 is an integrative element that helps integrate the plasmid onto the Streptomyces chromosome.]

Figure 2

LC-MS analysis of culture supernatants of S. albidoflavus engineered to express mysABC genes. (Top) Extracted ion chromatogram at 246.09 (±0.5) for the mysABC (blue trace) and wild-type (red trace) strains. (Middle) UV spectrum of MyG detected in the engineered strain with λ_max at 308 nm (∼310 nm) at a retention time of 4 min. (Bottom) Mass/charge (m/z) of MyG detected in positive-ion mode at 245.97.

Figure 3

Normalized UV-vis spectra of the supernatant of S. albidoflavus WT (dotted purple), S. albidoflavus with mysABC cluster (solid purple), and purified MyG at pH 1.0 (red) and 5.0 (green); peak absorption wavelengths (λ_max) for each pH (dashed red and green) and the “pump” wavelength selected for use in our transient absorption spectroscopy experiments (solid black) are also shown. Inset shows a magnified range of 280–320 nm, highlighting the difference in absorption maxima for three MyG-containing solutions (purple, red, and green numbers).

Figure 4

(a) Structure and atomic numbering scheme of the most stable protonated isomer of MyG. (b) optimized geometry of MyGH⁺, (c) optimized geometry of MyG_zwitter, determined at the RI-MP2 theoretical level. For the determination of MyG_zwitter, an implicit water solvent model was used.

Figure 5

(a) Collected TAS and (b) selected time slices for MyGH⁺ at pH 5.0 following photoexcitation at 310 nm. (c) PE profiles of the ground (black) and S₁ excited (red) states of MyGH⁺ calculated at the MS-CASPT2/SA-CASSCF(6,6)/cc-pVDZ level of theory along the LIIC reaction path; the inset represents the optimized geometry of the S₁/S₀ CI₁.