Free
energy landscapes (FELs) of proteins are indispensable for
evaluating thermodynamic properties. Molecular dynamics (MD) simulation
is a computational method for calculating FELs; however, conventional
MD simulation frequently fails to search a broad conformational subspace
due to its accessible timescale, which results in the calculation
of an unreliable FEL. To search a broad subspace, an external bias
can be imposed on a protein system, and biased sampling tends to cause
a strong perturbation that might collapse the protein structures,
indicating that the strength of the external bias should be properly
regulated. This regulation can be challenging, and empirical parameters
are frequently employed to impose an optimal bias. To address this
issue, several methods regulate the external bias by referring to
system energies. Herein, we focused on protein structural information
for this regulation. In this study, a well-established structural
indicator (the G-factor) was used to obtain structural
information. Based on the G-factor, we proposed a
scheme for regulating biased sampling, which is referred to as a G-factor-based external bias limiter (GERBIL). With GERBIL,
the configurations were structurally validated by the G-factor during biased sampling. As an example of biased sampling,
an accelerated MD (aMD) simulation was adopted in GERBIL (aMD-GERBIL),
whereby the aMD simulation was repeatedly performed by increasing
the strength of the boost potential. Furthermore, the configurations
sampled by the aMD simulation were structurally validated by their G-factor values, and aMD-GERBIL stopped increasing the strength
of the boost potential when the sampled configurations were regarded
as low-quality (collapsed) structures. This structural validation
is regarded as a “Brake” of the boost potential. For
demonstrations, aMD-GERBIL was applied to globular proteins (ribose
binding and maltose-binding proteins) to promote their large-amplitude
open–closed transitions and successfully identify their domain
motions.