# NMR-Derived Salt Bridges in Insulin Analogue: Resolving Artifactual Overbinding in Molecular Dynamics via Charge Scaling

## TL;DR

This paper uses charge scaling in molecular dynamics simulations to accurately model salt bridges in an insulin analogue, resolving issues of overbinding seen in traditional methods.

## Contribution

The novel use of scaled-charge prosECCo75 in MD simulations provides a reliable description of salt bridges in proteins.

## Key findings

- ff19SB induces a non-native salt bridge in insulin analogue simulations.
- prosECCo75 provides a biologically reasonable dissociation barrier of 1 kcal mol–1.
- CHARMM36m and ff19SB show high salt bridge strengths of up to 4 and 5 kcal mol–1, respectively.

## Abstract

Salt bridges are ionic interactions that are of great
importance
in protein recognition. However, their structural description using
X-ray crystallography or NMR may be inconclusive. Classical molecular
dynamics (MD) used for the interpretation neglects electronic polarization,
which results in artifactual overbinding. Here, we resolve the problem
via charge scaling, which accounts for electronic polarization in
a mean-field way. We study three salt bridges in insulin analogue.
New NMR ensembles are generated via NOE-restrained MD using ff19SB
and CHARMM36m force fields and the scaled-charge prosECCo75. Tens
of μs of unrestrained MD show in a statistically converged manner
that ff19SB induces a non-native salt bridge. This behavior is quantified
via umbrella sampling of salt bridge dissociation, which indicates
a rather high strength of up to 4 and 5 kcal mol–1 for CHARMM36m and ff19SB, respectively. In contrast, prosECCo75
gives a biologically reasonable dissociation barrier of 1 kcal mol–1. Our results indicate that a physically justified
description of charge–charge interactions within a nonpolarizable
MD framework reliably describes aqueous biomolecular systems.