dielectric boundary force in biomolecular solvation · dielectric boundary force in biomolecular...
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Dielectric Boundary Force in Biomolecular Solvation
Bo Li
Department of Mathematics and Quantitative Biology Graduate Program
UC San Diego
Funding: NSF and NIH
IMA Workshop 22nd Century Mathematics and Mechanics: Seven Decades
and Counting … Eugene, Oregon, October 23 – 25, 2015
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protein folding molecular recognition
solvation
conformational change water
water
solute solute
solute
water
receptor ligand
binding
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ΔG = ?
Solvation
solvent
solute
solvent solute
Biomolecular Modeling: Explicit vs. Implicit
MD simulations
Statistical mechanics
mi!!ri = −∇riV (r1,…, rN )
A =1Z
A(p, r)∫∫ e−βH ( p,r )dpdr = Atime
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! Solute-solvent interfacial property
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γ 0
R
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γ 0 = 73mJ /m2
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γ = γ 0 1− 2τH( )
Curvature effect
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τ = 0.9
Huang et al., JPCB, 2001.
A!
Solute-solvent interface: vapor-liquid interface Widom 1969, Weeks 1977, Chandler 2010.
Solute
Water
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ULJ (r) = 4ε σr( )12 − σ
r( )6[ ]σ rO
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−ε
The Lennard-Jones (LJ) potential ! Excluded-volume effect and van der Waals (vdW) dispersion
! Electrostatic interactions
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∇ ⋅εε0∇ψ = −ρPoisson’s equation: solvent
solute
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ε =1
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ε = 80
ρ = ρ f + ρi
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Dielectric boundary
Hasted, Ritson, & Collie, JCP, 1948.
Surface energy
PB/GB calcula1ons
Commonly used, surface based, implicit-solvent models
solvent accessible surface (SAS)
probing ball
vdW surface
solvent excluded surface (SES)
Possible issues
! Hydrophobic cavities ! Curvature correction ! Decoupling of polar and
nonpolar contributions
5 Koishi et al., PRL, 2004. Sotomayor et al., Biophys. J., 2007.
PB = Poisson-Boltzmann GB = Generalized Born
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1. The Poisson-Boltzmann (PB) Theory 2. Dielectric Boundary Force 3. Stability of a Cylindrical Dielectric Boundary 4. Implications to Molecular Recognition
Dzubiella, Swanson, & McCammon, PRL, 2006; JCP, 2006.
Free-energy functional
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r i
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Ωm
Γ
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Qi
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Ωw
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c j∞,
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q j , wρ
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G[Γ] = Pvol(Ωm ) + γ 0 (1− 2τH)dSΓ
∫+ρw ULJ ,i
i∑
Ωw
∫ (| !r − "ri |)dV
Variational Implicit-Solvent Model (VISM)
+Gelec[Γ]
BphC p53/MDM2
This talk: Two paraffin plates
Cheng, Wang, Zhou, Guo, Setny, Che, Dzubiella, Li, & McCammon, JCP, JPCB, JCTC, PRL, J. Comput. Phys., J. Comput. Chem., SIMA, SIAP, Nonlinearity, 2007-2015.
Hadwiger’s Theorem
Pvol(Ωm )+γ0area(Γ)− 2γ0τ H dSΓ∫ +cK K
Γ∫ dS( )
Let C = the set of all convex bodies, M = the set of finite union of convex bodies. If is
! rotational and translational invariant, ! additive:
! conditionally continuous: ),()(,, UFUFUUCUU jjj →⇒→∈
RMF →:
,,)()()()( MVUVUFVFUFVUF ∈∀∩−+=∪
.)()()( MUKdSdHdScUbAreaUaVolUFUU
∈∀++∂+= ∫∫ ∂∂
then
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Application to solvation of nonpolar molecules Roth, Harano, & Kinoshita, PRL, 2006. Harano, Roth, & Kinoshita, Chem. Phys. Lett., 2006.
Geometrical part:
Consider an ionic solution ! : local ionic concentrations ! dielectric coefficient ! fixed charge density
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∇⋅εε0∇ψ −B '(ψ) = −ρ f
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B(ψ) = β−1 c j∞ e−βq jψ −1( )j=1
M∑
G =12∫ ρψdV = −
εε02|∇ψ |2 +ρ fψ −B(ψ)
$
%&'
()∫ dV
1. The Poisson-Boltzmann (PB) Theory
Poisson equation
Boltzmann distributions Charge density
∇⋅εε0∇ψ = −ρ
ρ = ρ f + ρi = ρ f + qicii=1
M∑
ci = ci∞e−βqiψ
! Linearized PBE ! Sinh PBE
∇⋅εε0∇ψ −κ2ψ = −ρ f
∇⋅εε0∇ψ − 2c∞ sinh(βψ) = −ρ f
ci = ci (x) (i =1,...,M )ε :ρ f :
PBE
PB free energy
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Theorem (Li, Cheng, & Zhang, SIAP, 2011) has a unique minimizer, bounded in and uniformly with respect to It is the unique solution to the PBE.
I[•]
Proof. ! Existence and uniqueness of a minimizer in by direct
methods in the calculus of variations and the convexity of . ! Uniform bound by comparison. ! Regularity theory and routine calculations. Q.E.D.
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H1
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L∞
∇⋅εε0∇ψ −B '(ψ) = −ρ f
B(ψ) = β −1 ci∞ e−βqiψ −1( )i=1
M∑
I[ψ]= εε02|∇ψ |2 −ρ fψ +B(ψ)
#
$%&
'(∫ dV
PBE
Charge neutrality o s
B
Define
−B '(0) = ci∞qi = 0i=1
M∑
Hg1(Ω) = {φ ∈ H1(Ω) :φ = g on ∂Ω}
ε ∈ [εmin,εmax ].
Hg1(Ω)
L∞(Ω)I[•]
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Electrostatic free-energy functional F[c]= 1
2ρψ +β −1 ci ln(Λ
3ci )i=1
M
∑ − µicii=1
M
∑$
%&
'
()∫ dV
ρ = ρ f + qicii
M∑
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∇ ⋅εε0∇ψ = −ρ
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δiF[c] = 0 ci = ci∞e−βqiψ
O s
slns
Theorem (B.L., SIMA, 2009). ! has a unique minimizer . ! There exist such that for all ! All satisfy the Bolzmann distributions. ! The corresponding is the unique solution to PBE.
F[c] c = (c1,…,cM )θ1,θ2 > 0 θ1 ≤ ci ≤θ2 i =1,…,M.
ψ
(+ B.C.)
ci
Proof. ! Existence and uniqueness of a minimizer by direct methods, using
convexity of and the superlinear growth of ! Uniform bounds for equilibrium concentrations by Lemma below. ! Regularity theory and routine calculations. Q.E.D.
F[c] s! s ln s.
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€
r i
€
Ωm
Γ
€
Qi
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Ωw
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c j∞,
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q j , wρ€
εm =1
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εw = 80
The PB theory applied to molecular solvation
! Dielectric coefficient ! Fixed charge density ! All
ε = εΓ =εm in Ωm
εw in Ωw
ρ f = Qiδrii=1
N∑
ci = 0 (i =1,...,M ) in Ωm
∇⋅εε0∇ψ − χwB '(ψ) = − Qiδrii=1
N∑
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B(ψ) = β−1 c j∞ e−βq jψ −1( )j=1
M∑
Gelec[Γ]=12
Qiψreac (!ri )−
12
B '(ψ)ψreac dVΩw∫i=1
N∑
PBE
PB free energy
Reaction field
Reference potential
ψreac =ψ −ψref
ψref (!r ) = Qi
4πεmε0 |!r − !ri |i=1
N∑
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A shape derivative approach Perturbation defined by
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V :R3 → R3 :
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˙ x = V (x)
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x(0)= X{
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x = x(X,t) = Tt (X)
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Γt PBE:
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ψt
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Gelec[Γt ]
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δΓGelec[Γ] =ddt$
% &
'
( ) t= 0
Gelec[Γt ]
2. Dielectric Boundary Force (DBF):
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Fn = −δΓGelec[Γ]
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r i
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Ωm
Γ
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Qi
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Ωw
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c j∞,
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q j , wρ€
εm =1
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εw = 80
Structure Theorem
Shape derivative
∇⋅εΓε0∇ψ − χwB '(ψ) = −ρ f
Gelec[Γ]= −εΓε02|∇ψ |2 +ρ fψ − χwB(ψ)
$
%&'
()∫ dV
PBE
PB free energy
n
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Che, Dzubiella, Li, & McCammon, JPCB, 2008. Li, Cheng, & Zhang, SIAP, 2011. Luo et al., PCCP, 2012 & JCP, 2013.
δΓGelec[Γ]=ε02
1εm
−1εw
#
$%
&
'( |εΓ∂nψ |
2 +ε02εw −εm( ) (I − n⊗ n)∇ψ 2
+B(ψ).
Theorem (Li, Cheng, & Zhang, SIAP, 2011). Let point from to . Then
n Ωm Ωw
Consequence: Since we have The dielectric boundary force always points to solutes!
Chu, Molecular Forces, based on Debye’s lectures, Wiley, 1967. “Under the combined influence of electric field generated by solute charges and their polarization in the surrounding medium which is electrostatic neutral, an additional potential energy emerges and drives the surrounding molecules to the solutes.”
εw > εm, −δΓGelec[Γ]< 0.
∇⋅εΓε0∇ψ − χwB '(ψ) = −ρ f
Gelec[Γ]= −εΓε02|∇ψ |2 +ρ fψ − χwB(ψ)
$
%&'
()∫ dV
PBE
PB free energy
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3. Stability of a Cylindrical Dielectric Boundary Cheng, Li, White, & Zhou, SIAP, 2013.
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µw∇2u−∇pw − nw∇Uext +∇⋅Σ = 0 in
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Ωw (t)
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∇ ⋅ u = 0 in
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Ωw (t)
pm,i (t)Ωm,i (t) = NikBT =Cm
at
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Γ(t)
Fluctuating solvent fluid:
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∇ ⋅εε0∇ψ − χwB'(ψ) = −ρ f
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r i
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Ωm
Γ
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Qi
€
Ωw
Solvent Fluid Dielectric Boundary Model
Σij (x, t)Σkl (x ', t ') = 2µwkBTδ(x − x ')δ(t − t ')(δikδ jl +δilδ jk )
Interface motion Vn = u ⋅n
Electrostatics
Force balance
− fele =ε02
1εm
−1εw
"
#$
%
&' |ε∂nψ |
2 +ε02εw −εm( ) (I − n⊗ n)∇ψ 2
+B(ψ)
2µwD(u)n+ (pm − pw − 2γ0H + nwUvdW + fele )n = 0
M. White, Ph.D. thesis, UCSD, 2013. Li, Sun, and Zhou, SIAP, 2015. Sun et al., JSC, 2015. Luo et al., Chem. Phys. Lett., 2014.
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Dispersion relation
Li, Sun, and Zhou, SIAP, 2015.
Stability of a cylindrical dielectric boundary: Effect of geometry, electrostatics, and hydrodynamics
Viscosity slows down the decay of perturbations.
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4. Implications to Molecular Recognition ! Water molecules inside a protein are unhappy – broken bounds.
(Yin, Hummer, & Rasaiah, JACS, 2007. Yin, Feng, Clore, Hummer, & Rasaiah, JPCB, 2010, Baron & McCammon, Annu. Rev. Phhy. Chem., 2013.)
! Instability and fluctuations lead to drying near a pocket on protein surface, allowing a diffusional ligand to bind to the protein.
! Dielectric boundary force in the transient complex theory for the binding kinetics of two molecules in water: electrostatic steering.
J. A. McCammon, PNAS, 2009.