Abstract
The adaptive Cartesian grid-based Poisson–Boltzmann solver (CPB) employs the adaptive Cartesian grid and a singularity-free decomposition of the solution. One of the challenges in implicit solvent-based biomolecular electrostatics modeling is the evaluation of the dielectric-weighted Laplacian at the complex molecular surface where the dielectric constant, and thus also the normal electrostatic gradient, are discontinuous due to the use of dielectric regions with different dielectric permittivities. As a result, the electrostatic field required to evaluate the induced charge and dielectric pressure contributions to the atomic forces, do not converge with grid resolution unless appropriate steps are taken to explicitly resolve these discontinuities. The particular approach implemented in CPB uses standard least squares reconstruction (LSR) methods to accurately estimate the electrostatic potential and its gradients at the surface. The continuity requirements of potential and its gradients at the dielectric boundary are explicitly incorporated into the estimation together with the Poisson–Boltzmann equation itself. The approach is exercised on a collection of systems ranging from a simple sphere model problem and small biomolecules to larger biomolecular assemblies such as viruses. For smooth surfaces such as the solvent excluded surface, the LSR is shown to provide improved estimates of the energy and surface properties. At the re-entrant crevices associated with van der Waals surfaces, however, the Taylor series expansion upon which the reconstruction is based does not apply. Accordingly the reconstruction is locally not valid and, in practice, usually provides little improvement to PBE predictions.
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Abbreviations
- a :
-
Atomic radius; also convergence radius in Taylor series expansion
- {a}:
-
Vector defined in (4.17b)
- [A]:
-
Matrix whose jth row is {a} evaluated at the jth local node (grid point)
- \({a}_\mathrm{p }\) :
-
Solvent probe radius
- e :
-
Electronic charge
- {e \(_{j}\)}:
-
Unit vector whose jth entry is unity and all other entries are zero
- F :
-
Signed function used to implicitly define the molecular surface (\(F=0\))
- \({G}_{\mathrm{el}}\) :
-
Electrostatic energy
- \({k}_\mathrm{B}\) :
-
Boltzmann constant (\(1.3806488 \times 10^{-23}\,\mathrm{m}^{2}\,\mathrm{kg}\,\mathrm{s}^{-2}\,\mathrm{K}^{-1})\)
- n :
-
Outward surface normal vector
- N :
-
Number of grid points or mesh elements
- \({Q}_{k }\) :
-
kth fixed charge
- R :
-
Coordinate in space
- s :
-
Distance from surface
- S(R):
-
Switching function that is 0 when inside the molecule and 1 when outside
- x, y, z :
-
Coordinates in surface-aligned reference frame
- T :
-
Temperature
- \({w}_{j}\) :
-
Neighbor weights used to evaluate the dielectric-weighted Laplacian and defined in (4.23a, b)
- \(\beta \) :
-
Smoothing parameter used in Gaussian surface definition
- \(\delta \)(R):
-
Dirac or delta function
- \(\Delta {G}_{\mathrm{el} }\) :
-
Electrostatic solvation free energy
- \(\Delta \Delta {G}_{\mathrm{el} }\) :
-
Electrostatic binding free energy
- \(\Delta \Delta {G}\) :
-
Total binding free energy
- \(\Delta {s}\) :
-
Grid spacing (also \(\Delta _{0}\) when used at surface)
- \(\varepsilon \) :
-
Dielectric ratio, \({\varepsilon }_{2}/{\varepsilon }_{1}\);
- \(\varepsilon \)(R):
-
Dielectric constant at location, R
- \({\varepsilon }_{i}\) :
-
Dielectric constant in region \(\varOmega _{i}\)
- \(\phi \) :
-
Interior electrostatic potential
- \(\varPhi \) :
-
Electrostatic potential
- {\({\varvec{\upmu }}\)}:
-
Vector of weights whose jth entry is \({\varvec{\upmu }}\!{_{j}}=({Sz}^{2}/2)_{{j}}\)
- \(\tilde{\upmu }\) :
-
Potential quantity defined in (4.20b, c)
- \(\rho \) :
-
Volume charge density defined in (4.2a)
- \({\rho }^\mathrm{f }\) :
-
Fixed charge density
- \({\rho }_{k }\) :
-
Location of kth charge
- \({\rho }^\mathrm{m }\) :
-
Mobile charge density
- \(\mathrm{I}_{1:1 }\) :
-
Ionic strength of 1:1 salt
- \(\kappa \) :
-
Debye–Hückel screening parameter
- {\({\varvec{\uptheta }}\)}:
-
Vector defined of surface potential gradients defined in (4.17a)
- \(\varPsi \) :
-
Exterior electrostatic potential
- \(\tilde{\varPsi }_0 \) :
-
Potential quantity defined in (4.20b, c)
- \(\varOmega _{k }\) :
-
Domains comprising the interior (\(\varOmega _{1})\), exterior (\(\varOmega _{2})\) and Stern layer (\(\varOmega _{3})\) regions
- (\({\bullet })_{0 }\) :
-
Refers to quantity evaluated at the edge-surface intersection point
- (\({\bullet })^\mathrm{a }\) :
-
Refers to an analytically evaluated quantity
- (\({\bullet })^\mathrm{c }\) :
-
Refers to numerically computed quantity
- (\({\bullet })_\mathrm{s }\) :
-
Refers to quantity evaluated at the surface
- (\({\bullet })^\mathrm{T }\) :
-
Denotes matrix transpose
- (\({\bullet }),_\mathrm{x}\) :
-
Comma notation: e.g., \(\phi ,_{x}={\partial }{\phi }/\partial {x}\)
- ACG:
-
Adaptive Cartesian grid
- BEM:
-
Boundary element method
- CPB:
-
Cartesian grid-based Poisson Boltzmann solver
- GB:
-
Generalized Born
- LSR:
-
Least squares reconstruction
- MD:
-
Molecular dynamics
- MM-PBSA:
-
Molecular mechanics Poisson–Boltzmann surface area
- PDB:
-
Protein Data Bank
- PBE:
-
Poisson–Boltzmann equation
- SE:
-
Solvent excluded (molecular surface)
- vdW:
-
van der Waals
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Acknowledgments
This work was supported by NIH grant numbers 5 R44 GM57764-03 and 5R44GM073391-03. One of us (MOF) would like to acknowledge the invaluable contributions of Dr. Robert C. Harris, Dr. Alexander Silalahi, and Mr. Travis Mackoy in the development, validation, testing, and application of the CPB software in a variety of biophysical applications.
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Boschitsch, A.H., Fenley, M.O. (2015). The Adaptive Cartesian Grid-Based Poisson–Boltzmann Solver: Energy and Surface Electrostatic Properties. In: Rocchia, W., Spagnuolo, M. (eds) Computational Electrostatics for Biological Applications. Springer, Cham. https://doi.org/10.1007/978-3-319-12211-3_4
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DOI: https://doi.org/10.1007/978-3-319-12211-3_4
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