1. Structural Analysis of the Oxygen Evolving Complex Using Low Frequency SERS
Andrew J. Wilson and Prashant K. Jain
Department of Chemistry
University of Illinois at Urbana-Champaign
21 June 2017
International Symposium on Molecular Spectroscopy
Clusters/Complexes
Urbana-Champaign, IL

2. Photosystem II (PSII)
Yano et al., Chem. Rev., 2001, 114,
Umena et al., Nature, 2011, 473, 55-60
Govindjee et al., eLS, 2001
PSII is a protein complex which initiates solar-to-chemical energy conversion in photosynthesis
The oxygen evolving complex (OEC) splits water with a small overpotential and produces little reactive intermediates

3. Oxygen Evolving Complex (OEC)
Redox Intermediates S0 S1 S2 S3 S4 e- H+ O2
H2O
Net reaction:
2H2O  4H+ + 4e- + O2
Umena et al., Nature, 2011, 473, 55-60
What is the protonation of the OEC in S1?
What are the intermediate Si structures in operando?
Kok et al., Photochem. Photobio., 1970, 11,

4. Surface-enhanced Raman scattering (SERS) microscopy
Pushing resolution in catalysis S0 S1 S2 S3 S4
Current challenges
X-ray beam damage / H-insensitivity
Crystals (time/space average, T, physiology)
Impure S-state preparation
Spatial averaging over samples
Surface-enhanced Raman scattering (SERS) microscopy
Reduce spatial averaging (~ 0.5 μm)
Observe bond breaking/formation in operando
Track OEC water ligands
air
H2O
10 μm

5. Photosystem II cluster
PSII SERS measurement
Photosystem II cluster
Ag nanoparticle monolayer
glass
10 μm
100X
514.5 nm cw laser excitation
beam splitter
0.2-1 s / frame
Long pass filter
spectrometer

6. Dynamic PSII SERS spectra
air
air
H2O
H2O
Low Frequency Dynamics

7. PSII SERS in Water Isotopologues
672 cm-1
664 cm-1
N = 2250
N = 2250
S1 dark-stable state
Protonation
W2: H2O or OH?
O5: O or OH?
Use DFT to calculate Raman frequencies using 16OH2/18OH2 ligands
Umena et al., Nature, 2011, 473, 55-60

8. S1 Protonation State: Raman from DFT Model
W2: OH
O5: O
W2: H2O
O5: O
W2: OH
O5: OH
W2: H2O
O5: OH
16OH2
18OH2
Exp.

9. S1 Model and SERS
SERS
16OH2
18OH2
O5=O, W2= H2O model reproduces 16OH2/18OH2 shift for 672 cm-1: (β HO(W1)-Mn4)
DFT Raman spectrum predicts several modes observed in SERS snapshots
Observe OEC sample the S1 state
Use the protonation of S1 to generate models of S0-S3

10. S0 Model SERS, S0 Raman (DFT) S0 - H+, - e- S1
S0 Interatomic distances: Pal et al., Biochemistry, 2013, 52,

11. S2 Model Open Isomer Closed Isomer S1 - e-
Interatomic distances: Pantazis et al., Angew. Chem. Int. Ed. 2012, 51,

12. S3 Model LH Wx Isomer RH Wx Isomer S2 -H+, -e- +H2O
Interatomic distances: Askerka et al., Biochemistry, 2016, 55,

13. Proposed IntermediatesH+ H+
H2O
Summary
Stay tuned! S4
In operando OEC dynamics
S1 protonation using DFT/SERS
S0-S3 models S1 e- H+ e- S3
H2O S2
“RH Wx”
“open” e- H+

14. Acknowledgements ▪ Prof. Prashant K. Jain ▪ Dinumol Devasia Funding
▪ Arnold and Mabel O. Beckman Foundation
▪ Springborn Postdoctoral Fellowship