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Quantum chemical modeling of photoexcitations in biology
Ville R. I. Kaila Theoretical Biophysics Section @ Laboratory of Chemical Physics
NIDDK, National Institutes of Health, USA Ville R. I. Kaila
Winter School in Theoretical Chemistry, Helsinki, Dec. 13., 2011
Contents
Methodology Cluster modeling of proteins & enzymes Methods applicable for bio-modeling Benchmarking the methods on biochromophores
Short Introduction to Photobiology
Case-study Protein environment & photoexcitations in retinal
Phototrophs: Energy transduction
Chemotrophs: Photoreceptors/ Photosensors
= organisms using light as a primary energy source
Introduction to Photobiology
= organisms using chemical coumpounds as a primary energy source
Bioluminescence
Photobiology in energy transduction
Introduction to Photobiology
Example I: Biological energy transduction Photosystem II converts the energy of the sunlight into electrochemical energy
Cardona et al. BBA (2012)
Introduction to Photobiology
H2O → 2H+ + 2e- + ½O2
Q + 2H+ + 2e- → QH2
Introduction to Photobiology
Example II: Biological energy transduction Bacteriorhodopsin (bR) converts light energy into an electrochemical proton gradient
Freier et al. PNAS 2011
all-trans → 13-cis
Photochemical reaction: all-trans retinal to 13-cis isomerization
Photoreceptors
Introduction to Photobiology
Introduction to Photobiology
Six main types of photoreceptors
Cryptochrome FAD
Blue-light sensing FAD (BLUF) FAD
Light, oxygen, voltage (LOV) FMN
Photoactive yellow protein (PYP) pCA
Rhodopsin retinal
Phytochrome bilin
Electron transfer/ “catalytic triplet state”
Photoisomerization
Chromophore Photoreceptor Photochemical process
Example III: Biological photoreseptors Rhodopsin is a G-protein coupled receptor
Introduction to Photobiology
Sensor of visible light in vertebrates Photochemical reaction: 11-cis retinal to all-trans isomerization
Triggers vision process
Introduction to Photobiology
Example IV: Biological sensors Photoactive yellow protein (PYP) is a blue-light receptor in chemotrophic bacteria
Ihee et al. PNAS 2008
Importance in phototaxis Photochemical reaction: Trans-cis isomerization of p-coumaric acid
Introduction to Photobiology
Example V: Bioluminescense Green fluorescent protein (GFP) emits green light in Aequoria jellyfish
Fluorescence observed only in protein environment but not in solution
Function in nature unclear, biotechnology as a labeling tool
p-hydroxybenzylidene imidazolinone
Quantum chemical methodology
Quantum chemical cluster modeling of proteins and enzymes
Quantum chemical methodology
Protein (X-rays) structure QM-model
Hydrogen atoms are not resolved Missing residues? Missing waters/ions/etc.?
Quantum chemical methodology
How to construct a cluster model?
Residues based on proximity (to chromophore/site of interest) Physico-chemical aspects? Charged/polar/aromatic/non-polar/H-bonding residues?
How many atoms can we afford? Excited state DFT modeling 150 - 200 atoms
Bioinformatic consideration? Evoluationary conserved residues?
Where should I cut the residues? Proteins are polymers!
Where should I cut the residues? Where should I cut the residues?
Include protein backbone of residues n ± 1 if important for H-bonding
Model residue size: 33 atoms
Where should I cut the residues? Where should I cut the residues?
Cut at Cα atom if conformational flexibility of side-chain is particularly important
Model residue size: 19 atoms
Where should I cut the residues? Where should I cut the residues?
Cut at Cβ atom minimial model where protein strain can easily be included
Model residue size: 16 atoms
Where should I cut the residues? Where should I cut the residues?
Cut at Cγ atom if model is otherwise too large
Model residue size: 13 atoms
Quantum chemical methodology
Simulating protein backbone strain - freezing terminal carbons
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Quantum chemical methodology
The low dielectric environment of the protein (ε ~ 4) can be described by continuum solvation models e.g. the COnductor-like Screening MOdel (COSMO)
The “solvent” is approximated by a polarizable cavity, defined by the shape of the molecular cluster
A. Klamt & G. Schuurman J. Chem. Soc. Trans. Perkin Trans. 2 (1993)
The solvation effects are obtained from interaction between the charges polarized on the continuum shell and the charge distribution of the molecule
Quantum chemical approaches
X-ray structure
Active site + conserved surr.
~150-300 atoms
Freeze terminal carbons + optimize B3LYP/
def2-SVP
Properties: large basis sets, Dielectric constant
B3LYP/ def2-TZVP/ε=4
Quantum chemical methodology
QM/MM approaches
MM
QM
An alternative approach: QM-model + full protein environment described by classical force fields (cf. Menucci’s talk)
link atoms
QM/MM approaches
Consideration for QM/MM: including large QM region might be important
JCTC (in press)
Protein shifts with classical non-polarizable forcefield not large enough
Quantum chemical methodology
Methods applicable for biochemical modeling
Excited state properties
Linear-response Time-Dependent Density Functional Theory (cf. Norman’s lectures) (N4 scaling)
Solve:
To obtain: transition vectors, XY excitation energies, ω Where matrix elements,
a,b – occupied orbitals i,j – virtual orbitals
two-electron integrals exchange potential orbital energies of φa & φi
Excited state properties
Example of drawbacks in LR-TDDFT: The charge transfer problem
CIS
BHLYP
B3LYP
SVWN LB94
Dreuw & Head-Gordon: Chem. Rev. 2005
Wrong charge-transfer excited states due to erranous 1/R behavior of TDDFT Underestimation of excitation energies
Excited state properties
Example of drawbacks in LR-TDDFT: The charge transfer problem
Kaila et al. (in progress)
Particularly problematic in many biological systems, as they might comprise weakly interacting, non-covalently bound fragments
CCSD, N6
Excited state properties
Approximate second order coupled cluster theory, CC2 (N5 scaling)
|ui> manifold of excitations from |HF> T1 single excitations T2 double excitations
F+U+V Hamiltonian, H F Fock operator V one-electron operator U fluctuation operator
Excited state properties
Benchmarking the TDDFT and CC2 for biochromophores
Send, Kaila, Sundholm JCTC 7 (2011)
Excited state properties
Benchmarking the TDDFT and CC2 for biochromophores
Retinal (rhodopsin) Lowest-excited states in agreement
0
1
1 2 3 4 5
f
2 2.5
3 3.5
4 4.5
5E
[eV]
DFTCC2
Send, Kaila, Sundholm JCTC 7 (2011)
Excited state properties
Benchmarking the TDDFT and CC2 for biochromophores
0 0.05
1 2 3 4 5
f
2 2.5
3 3.5
4 4.5
5E
[eV]
DFTCC2
TDDFT has problems in describing anionic molecules (pCA’’-, PYP chromophore) Send, Kaila, Sundholm JCTC 7 (2011)
Excited state properties
Benchmarking the TDDFT and CC2 for biochromophores
0 0.05
1 2 3 4 5
f
2 2.5
3 3.5
4 4.5
5E
[eV]
DFTCC2
DFT pc
TDDFT has problems in describing anionic molecules (pCA’’-, PYP chromophore)
but can be resolved by stabilizing the chromophore with external point changes
Excited state properties
Conclusions from benchmarking the TDDFT and CC2 for biochromophores
CC2 deviation (GFP and Rhodopsin chromophores) 0.15 eV
TD-B3LYP deviation (GFP and Rhodopsin chromophores) 0.2-0.3 eV
Avoid anionic DFT models (due to DFT-continuum problem?)
CC2 deviation (PYP chromophores) 0.3-0.4 eV
TD-B3LYP deviation (PYP chromophores) 0.1-0.5 eV
Send, Kaila, Sundholm JCTC 7 (2011)
Excited state properties
Frozen core-approximation in CC-methods saves computational time and has a small effect on the results
Excited state properties
Restricted virtual space approximation
Send, Kaila, Sundholm JCP 134 (2011)
CP
U [h
] Error [eV
]
Freeze also virtual orbitals above a certain energy treshold
Excited state properties
Restricted virtual space approximation has the same effect on the chromophore in gas-phase and embedded in the protein
Send, Kaila, Sundholm JCP 134 (2011)
Erro
r [eV
]
RVS treshold [eV]
Excited state properties
Check also out: Laplace-transformed spin-opposite scaled CC2 (LT-SOS-CC2), N4 scaling
Algebraic diagrammatic correction ADC(2), ~3 x faster than CC2, (cf. Dreuw’s lecture)
The effect of protein environment on photoexcitation properties of retinal
Ville R. I. Kaila, Robert Send, and Dage Sundholm (in press)
Some abbreviations and concepts
SBH+ - protonated Schiff base retinal
SB - deprotonated Schiff base retinal
BC - blue-cone pigment
Rh - rhodopsin
Glu - will refer here to glutamate-113
For protein models SBH+ - will imply Glu- SB - will imply GluH
Some abbreviations and concepts
Blueshift Redshift
E E
Vertebrate vision
Human vision: rod pigment for dim-vision (498 nm)
Three cone pigments for color vision: pigments sensitive for blue (414 nm), green (553 nm), and red light (564 nm)
414 498 553 564
Vertebrate vision
How is the color-tuning achieved by the protein environment?
Gas-phase 610 nm (2.03 eV) Rhodopsin
498 nm (2.48 eV)
10.5 kcal/mol 0.45 eV
Bluecone 414 nm (3.00 eV)
11.6 kcal/mol 0.5 eV
We are not the first to address spectral tuning of retinal
Steric tuning mechanism: e.g. Blatz & Liebman 1973
Electrostatic tuning mechanism e.g. Honig et al. 1976 Photoexcitations leads to strong increase in retinal dipole moment
-
+
- hv
From experimental work on cyanines and polyenes:
Long λ → small bond lenght altenation → increased π-electron delocalization Short λ → large bond lenght altenation → decreased π-electron delocalization
QM/MM studies
Semi-empirical Warshel, A.; Chu, Z. T. J. Phys. Chem. B 2001, 105, 9857–9871. Hayashi, S.; Ohmine, I. J. Phys. Chem. B 2000, 104, 10678–10691. Matsuura, A. et al. J. Comp. Chem. 2006, 27, 1623–1630. Hoffmann, M.; et al. M. J. Am. Chem. Soc. 2006, 128, 10808–10818.
HF Yamada, A. et al. Chem. Phys. Letters 2002, 366, 670–675.
CIS Rajamani, R.; Gao, J. L. J. Comp. Chem. 2002, 23, 96–105. Trabanino, R. J.; et al. J. Phys. Chem. B 2006, 110, 17230–17239.
SAC-CI Fujimoto, K. et al. Chem. Phys. Letters 2005, 414, 239–242. Sakurai, M.; et al. J. Am. Chem. Soc. 2003, 125,3108–3112.
SORCI Hoffmann, M. et al. J. Am. Chem. Soc. 2006, 128, 10808–10818.
DFT/TDDFT Gascón, J. A. et al. Acc. Chem. Res. 2006, 39, 184–193. Röhrig, U. F.; Sebastiani, D. J. Phys. Chem. B 2008, 112, 1267–1274. Altun, A.; et al. J. Phys. Chem. A 2009, 113, 11685–11692.
CASSCF Hayashi, S.; et al. J. Phys. Chem. B 2001, 105, 10124–10131. Hayashi, S.; et al. Biophys. J. 2009, 96, 403–416.
CASPT2 Ferré, N.; Olivucci, M. J. Am. Chem. Soc. 2003, 125, 6868–6869. Andruniów, T.; et al. Proc. Natl. Acad. Sci. USA 2004, 101, 17908–17913.
MCQDPT2, MRCISD, MR-SORCI+Q, MR-DDCI+Q Altun, A.; et al. J. Phys. Chem. A 2009, 113, 11685–11692.
QM studies of retinal models
TDDFT/DFT Blomgren, F.; Larsson, S. J. Phys. Chem. B 2005, 109, 9104–9110. Send, R.; Sundholm, D. J. Phys. Chem. A 2007, 111, 27–33. Rostov, I. V.; et al. J. Phys. Chem. B 2010, 114, 5547–5555.
CC2 Send, R.; Sundholm, D. Phys. Chem. Chem. Phys. 2007, 9, 2862–2867. Szymczak, J. J. et al. J. Phys. Chem. A 2009, 113, 11907–11918.
Quantum Monte Carlo Valsson, O.; Filippi, C. J. Chem. Theory Comput. 2010, 6, 1275–1292.
CASSCF/CASPT2 Sekharan, S.; Weingart, O.; Buss, V. Biophys. J. 2006, 91, L07–L09. Andruniów, T.; et al. Proc. Natl. Acad. Sci. USA 2004, 101, 17908–17913.
We are not the first to address spectral tuning of retinal
What’s new?
Large QM models and chromophore-protein interactions treated at a QM-level
We are not the first to address spectral tuning of retinal
Methods
DFT cluster models of the rhodopsin (PDB ID: HZX) and blue-cone (PDB ID:1KPN)
Methods
DFT cluster models of the rhodopsin (PDB ID: HZX) and blue-cone (PDB ID:1KPN) Models comprised chromophore + 10 residues, terminated at Cβ, 160-171 atoms
Methods
DFT cluster models of the rhodopsin (PDB ID: HZX) and blue-cone (PDB ID:1KPN) Models comprised chromophore + 10 residues, terminated at Cβ, 160-171 atoms Two models: a) Schiff-base proton coordinated to retinal or b) Glu-113
Methods
DFT cluster models of the rhodopsin (PDB ID: HZX) and blue-cone (PDB ID:1KPN) Models comprised chromophore + 10 residues, terminated at Cβ, 160-171 atoms Two models: a) Schiff-base proton coordinated to retinal or b) Glu-113
B3LYP/def2-SVP optimized Single point and vertical excitation energies using: TD-B3LYP/def2-TZVP, TD-BHLYP/def2-TZVP, RVS20-ADC(2)/def2-TZVP First excited state optimized at the TD-B3LYP/def2-SVP level
Vertebrate vision
Results
Vertebrate vision
Bond-length alternations
Bond-length alternation
Bond length altenation of retinal in gas-phase
SB has stronger BLA relative to SBH+→ blueshifted in gas-phase?
Bond-length alternation
Protein increases the bond length altenation of retinal of SBH+ Smaller effect on SB
SBH+ more blueshifted by protein?
Vertebrate vision
Proton affinity of retinal embedded in the protein
Ground-state properties
Blue cone pigment: proton affinity of Schiff base ~ proton affinity of Glu-113
0 kcal/mol 0 kcal/mol
Glu-113
Ground-state properties
Rhodopsin: proton affinity of Schiff base 4 kcal mol-1 higher than for Glu-113
0 kcal/mol 4 kcal/mol
Glu-113
Vertebrate vision
Ground-state charge distributions
Ground-state charge distribution
Small protein polarization effect for protonated retinal (SBH+/Glu-)
BC
Qret = +0.97e
Qprot = -0.97e
Rh
Qret = +0.91e
Qprot = -0.91e
Ground-state charge distribution
Transfer of 0.5e upon proton transfer to Glu-113 (SB/GluH)
BC
Qret = +0.43e
Qprot = -0.44e
Rh
Qret = +0.44e
Qprot = -0.44e
Vertebrate vision
Vertical excitations
Vertical excitations at TD-B3LYP/def2-TZVP level
553 nm (1.12) gas-phase:
SBH+
Vertical excitations at TD-B3LYP/def2-TZVP level
553 nm (1.12)
491 nm (1.10)
gas-phase:
Rh
SBH+
Vertical excitations at TD-B3LYP/def2-TZVP level
553 nm (1.12)
491 nm (1.10) 485 nm (1.07)
gas-phase:
Rh BC
SBH+
Vertical excitations at TD-B3LYP/def2-TZVP level
553 nm (1.12)
491 nm (1.10) 485 nm (1.07)
gas-phase:
408 nm (1.47) gas-phase:
Rh BC
SBH+
SB
Vertical excitations at TD-B3LYP/def2-TZVP level
553 nm (1.12)
491 nm (1.10) 485 nm (1.07)
gas-phase:
408 nm (1.47)
454 nm (1.28)
Rh
gas-phase:
Rh BC
SBH+
SB
Vertical excitations at TD-B3LYP/def2-TZVP level
553 nm (1.12)
491 nm (1.10) 485 nm (1.07)
gas-phase:
408 nm (1.47)
454 nm (1.28) 441 nm (1.20)
Rh BC
gas-phase:
Rh BC
SBH+
SB
Excited-state charge distribution
+ hv -
Excitation leads to intra-molecular CT in retinal
gas-phase BC Rh
0.73e 0.74e 0.78e
0.08e 0.05e 0.07e
retinyl β-ionine gas-phase BC Rh
0.55e 0.53e 0.61e
0.26e 0.16e 0.25e
retinyl β-ionine
Excited-state charge distribution
Charge-transfer in excited state for Rhodopsin (SBH+/Glu-)
BC
Qret = +0.94e
Qprot = -0.94e
Rh
Qret = +0.78e
Qprot = -0.78e
DFT artifacts?
Charge-transfers
1) Based on oscillator strengths, TD-B3LYP suggest that the second excited state has a strong intensity
System Model ES BC SBH+ 1
2
B3LYP [nm]/osc. 506 (0.00) 485 (1.07)
BHLYP [nm]/osc. 415 (1.58) 293 (0.25)
ADC(2) [nm]/osc. 420 (1.67) 300 (0.32)
2a) Based on osc. stregths, the order of the states are flipped when increasing the amount of HF exchange 2b) ES2 is significantly blueshifted at the BHLYP level (indicator of CT problem)
3) ADC(2) results support that the weak state at B3LYP has CT problems
DFT artifacts?
Support from flipping the order of states from dominant orbital contributions
System Model ES BC SBH+ 1
2
B3LYP [nm]/osc. 506 (0.00) 485 (1.07)
BHLYP [nm]/osc. 415 (1.58) 293 (0.25)
ADC(2) [nm]/osc. 420 (1.67) 300 (0.32)
System Model ES BC SBH+ 1
2
B3LYP orbital character |c|2 HOMO-1→LUMO 99.9
HOMO → LUMO 92.6
BHLYP orbital character |c|2 HOMO→LUMO 95.9
HOMO-3 → LUMO 90.1
1 2
ADC(2) orbital character |c|2 HOMO→LUMO 95.3
HOMO-3 → LUMO 89.4
1 2
Excited-state charge distribution
What about Rh SBH+ with excited state charge transfer? TD-B3LYP artifact?
Rh
Qret = +0.78e
Qprot = -0.78e
DFT artifacts?
Similar charge-density distribution upon photoexcitation at B3LYP and BHLYP-levels
BHLYP/ES2
467 nm (0.08) 309 nm (0.02)
However, excitation energy shifts is blueshifted, possible CT problem ADC(2): 302 nm (0.32)
B3LYP/ES2
DFT artifacts?
ADC(2) calculations predict 25.5% orbital contributions from Trp-265 orbitals
Orbitals @ ADC(2) |c|2 HOMO-3 → LUMO 64.6 HOMO-1 →LUMO 25.5
Charge transfer to rhodopsin might be a ”real” effect
DFT artifacts?
Conformational vs. electrostatic tuning?
Approach:
Chromophore optimized in protein
Chromophore fixed in protein conformation
Chromophore optimized in gas-phase
“Electrostatic” “Strain”
Conformational vs. electrostatic tuning?
Constructing a thermodynamic cycle
Electrostatics: 59.0-47.3 kcal/mol = 11.7 kcal/mol (protein blueshifts)
Vacuum
Energies in kcal/mol (eV)
Conformational vs. electrostatic tuning?
Constructing a thermodynamic cycle
Vacuum
Electrostatics: 59.0-47.3 kcal/mol = 11.7 kcal/mol (protein blueshifts) Strain: 47.3-51.8 kcal/mol = -4.5 kcal/mol (protein redshifts)
Conformational vs. electrostatic tuning?
BC Rh Electrostatic: 11.7 kcal/mol (0.51 eV) 8.0 kcal/mol (0.35 eV) Strain: -4.5 kcal/mol (0.20 eV) -1.5 kcal/mol (0.07 eV)
Conformational vs. electrostatic tuning?
BC Rh Electrostatic: -4.8 kcal/mol (0.21 eV) -4.9 kcal/mol (0.21 eV) Strain: -0.4 kcal/mol (0.02 eV) -2.2 kcal/mol (0.10 eV)
Conformational vs. electrostatic tuning?
Electrostatic effects blueshift SBH+ by ~8-12 kcal/mol (TDDFT) ~19 kcal/mol at ADC(2) level
Electrostatic effects redshift SB by ~5 kcal/mol (TDDFT) ~6 kcal/mol at ADC(2) level
Strain effects redshift SBH+ by ~2-5 kcal/mol (TDDFT) ~2-3 kcal/mol at ADC(2) level Strain effects redshift SB by ~0.4-2 kcal/mol (TDDFT) ~0.5-3.5 kcal/mol at ADC(2) level
Electrostatics: 70-80% Strain: 20-30%
Blueshift the bluecone pigment?
TDDFT calculations suggest that the blueshift in BC might be caused by a deprotonted Schiff base: 485 nm (SBH+)
441 nm (SB)
ADC(2) calculations: 423 nm (SBH+) 372 nm (SB)
TDDFT shift: 44 nm ADC(2) shift: 51 nm
Absolute spectra support SB for TDDFT and SBH+ for ADC(2) Exp. 414 nm
What happens upon relaxation of the excited state?
Excited state structure
The excited state structure
Smaller BLA → single bonded character → easier for isomerization?
Almost no change in dihedral angle (C10-C11-C12-C13) ~ 2 degrees
The excited state structure
Thermal activation of rhodopsin is also possible
Dark-activation barrier comparable to the photon energy (ca. 60 kcal/mol) or early photo-intermediates (ca. 40 kcal/mol)?
Ea ~ 23 kcal mol-1
Baylor et al. 1980
The excited state structure
Energy relaxation of the excited state ca. 30 kcal mol-1
Large scale TD-DFT cluster calculations correctly predict that the protein environment blueshifts the absorption spectra of retinal
Summary I
TD-DFT and ADC(2) calculation suggest that deprotonation of retinal in protein blushifts the absorption maximum by ca. 50 nm
Mechanism of spectral tuning: Electrostatics: 70-80% Strain: 20-30%
(Supported at both TD-B3LYP and ADC(2) levels)
Absolute TD-B3LYP spectra: Rh Glu-/SBH+ 491 nm (exp. 498 nm ) BC GluH/SB 441 nm (exp. 414 nm )
Absolute ADC(2) spectra: Rh Glu-/SBH+ 430 nm (exp. 498 nm ) BC Glu-/SBH+ 420 nm (exp. 414 nm )
Benefits and drawbacks with TD-DFT
Summary II
+ Large-scale cluster modeling possible - Charge-transfer problems
Low-order coupled-cluster method increasingly applicable for biomolecular simulations
Restricted virtual space approimation Laplace transformed SOS-CC2 ADC(2)
Acknowledgements
Dr. Robert Send BASF Germany
EMBO – Long Term Fellowship
Biowulf clusters at NIH and Center of Scientific Computing (CSC), Finland
Helsinki Bioenergetics Group, University of Helsinki
Department of Chemistry, University of Helsinki
Laboratory of Chemical Physics, US National Institutes of Health
Prof. Dage Sundholm Department of Chemistry, Uni. Helsinki, Finland