4 research outputs found
Can We Execute Reliable MM-PBSA Free Energy Computations of Relative Stabilities of Different Guanine Quadruplex Folds?
The self-assembly and stability of
DNA G-quadruplexes (GQs) are
affected by the intrinsic stability of different GpG base steps embedded
in their G-quartet stems. We have carried out MD simulations followed
by MM-PBSA (molecular mechanics Poisson–Boltzmann surface area)
free energy calculations on all the experimentally observed three-quartet
intramolecular human telomeric GQ topologies. We also studied antiparallel
GQ models with alternative <i>syn</i>-<i>anti</i> patterns of the G-quartets. We tested different ions, dihedral variants
of the DNA force field, water models, and simulation lengths. In total,
∼35 μs of simulations have been carried out. The systems
studied here are considerably more complete than the previously analyzed
two-quartet stems. Among other effects, our computations included
the stem–loop coupling and ion–ion interactions inside
the stem. The calculations showed a broad agreement with the earlier
predictions. However, the increase in the completeness of the system
was associated with increased noise of the free energy data which
could be related, for example, to the presence of long-lived loop
substates and rather complex dynamics for the two bound ions inside
the G-stem. As a result, the MM-PBSA data were noisy and we could
not improve their quantitative convergence even by expanding the simulations
to 2.5 μs long trajectories. We also suggest that the quality
of MM-based free energy computations based on MD simulations of complete
GQs is more sensitive to the neglect of explicit polarization effects,
which, in real systems, are associated with the presence of multiple
closely spaced ions inside the GQs. Thus, although the MM-PBSA procedure
provides very useful insights that complement the structural-dynamics
data from MD trajectories of GQs, the method is far from reaching
quantitative accuracy. Our conclusions are in agreement with critical
assessments of the MM-PBSA methodology available in contemporary literature
for other types of problems
Effect of Monovalent Ion Parameters on Molecular Dynamics Simulations of G‑Quadruplexes
G-quadruplexes (GQs)
are key noncanonical DNA and RNA architectures
stabilized by desolvated monovalent cations present in their central
channels. We analyze extended atomistic molecular dynamics simulations
(∼580 μs in total) of GQs with 11 monovalent cation parametrizations,
assessing GQ overall structural stability, dynamics of internal cations,
and distortions of the G-tetrad geometries. Majority of simulations
were executed with the SPC/E water model; however, test simulations
with TIP3P and OPC water models are also reported. The identity and
parametrization of ions strongly affect behavior of a tetramolecular
d[GGG]<sub>4</sub> GQ, which is unstable with several ion parametrizations.
The remaining studied RNA and DNA GQs are structurally stable, though
the G-tetrad geometries are always deformed by bifurcated H-bonding
in a parametrization-specific manner. Thus, basic 10-μs-scale
simulations of fully folded GQs can be safely done with a number of
cation parametrizations. However, there are parametrization-specific
differences and basic force-field errors affecting the quantitative
description of ion-tetrad interactions, which may significantly affect
studies of the ion-binding processes and description of the GQ folding
landscape. Our d[GGG]<sub>4</sub> simulations indirectly suggest that
such studies will also be sensitive to the water models. During exchanges
with bulk water, the Na<sup>+</sup> ions move inside the GQs in a
concerted manner, while larger relocations of the K<sup>+</sup> ions
are typically separated. We suggest that the Joung-Cheatham SPC/E
K<sup>+</sup> parameters represent a safe choice in simulation studies
of GQs, though variation of ion parameters can be used for specific
simulation goals
Reference Simulations of Noncanonical Nucleic Acids with Different χ Variants of the AMBER Force Field: Quadruplex DNA, Quadruplex RNA, and Z-DNA
Refinement of empirical force fields for nucleic acids
requires
their extensive testing using as wide range of systems as possible.
However, finding unambiguous reference data is not easy. In this paper,
we analyze four systems that we suggest should be included in standard
portfolio of molecules to test nucleic acids force fields, namely,
parallel and antiparallel stranded DNA guanine quadruplex stems, RNA
quadruplex stem, and Z-DNA. We highlight parameters that should be
monitored to assess the force field performance. The work is primarily
based on 8.4 μs of 100–250 ns trajectories analyzed in
detail followed by 9.6 μs of additional selected backup trajectories
that were monitored to verify that the results of the initial analyses
are correct. Four versions of the Cornell et al. AMBER force field
are tested, including an entirely new parmχ<sub>OL4</sub> variant
with χ dihedral specifically reparametrized for DNA molecules
containing <i>syn</i>-nucleotides. We test also different
water models and ion conditions. While improvement for DNA quadruplexes
is visible, the force fields still do not fully reproduce the intricate
Z-DNA backbone conformation
Reference Simulations of Noncanonical Nucleic Acids with Different χ Variants of the AMBER Force Field: Quadruplex DNA, Quadruplex RNA, and Z-DNA
Refinement of empirical force fields for nucleic acids
requires
their extensive testing using as wide range of systems as possible.
However, finding unambiguous reference data is not easy. In this paper,
we analyze four systems that we suggest should be included in standard
portfolio of molecules to test nucleic acids force fields, namely,
parallel and antiparallel stranded DNA guanine quadruplex stems, RNA
quadruplex stem, and Z-DNA. We highlight parameters that should be
monitored to assess the force field performance. The work is primarily
based on 8.4 μs of 100–250 ns trajectories analyzed in
detail followed by 9.6 μs of additional selected backup trajectories
that were monitored to verify that the results of the initial analyses
are correct. Four versions of the Cornell et al. AMBER force field
are tested, including an entirely new parmχ<sub>OL4</sub> variant
with χ dihedral specifically reparametrized for DNA molecules
containing <i>syn</i>-nucleotides. We test also different
water models and ion conditions. While improvement for DNA quadruplexes
is visible, the force fields still do not fully reproduce the intricate
Z-DNA backbone conformation