# Calculating Salt Molarity in an Explicit Water System

By Abigail Held^{1} and Maria Nagan^{2}

^{1}from Bill Miller III's lab, Truman State University, ^{2}Stony Brook University

# Learning Outcomes

# Introduction

Ion concentration and ionic strength need to be properly set for MD simulations. The concentration of NaCl in cells is approximately 150 mM, so we will be calculating how may sodium and chloride ions are required for this concentration in the protein system from__Tutorial 1.4__. By convention, these ions are added beyond what is required to neutralize the solute.

1. Find the Volume of the box from leap.log file.

Open the leap.log file generated from the

*tleap*script without a buffer present for the volume of the box in Å

^{3}. The relevant portion of the leap.log output for the 2YX8 system from tutorial 1.4 is shown below. You can technically calculate the volume of the truncated octahedral box but LEaP also does this for you. Find the line with "Volume".

> solvateOct ramp OPCBOX 10.0 Scaling up box by a factor of 1.413369 to meet diagonal cut criterion Solute vdw bounding box: 45.400 31.108 30.346 Total bounding box for atom centers: 73.667 73.667 73.667 (box expansion for 'iso' is 55.9%) Solvent unit box: 18.865 18.478 19.006 The number of boxes: x= 4 y= 4 z= 4 Volume: 208141.839 A^3 (oct) Total mass 110188.212 amu, Density 0.879 g/cc Added 5569 residues.

The volume of the truncated octahedron is 208141.839 Å

^{3}.

2. Convert the volume of the system in Å

^{3}to liters using the following conversion factors.

Be sure to distribute the exponents when performing the calculations.

3. Determine how many chloride ions are present in one liter of solution at a concentration of 150 mM.

4. Determine how many chloride ions are needed in the system.

Multiply the volume of the box by the concentration of Cl

^{-}ions and round to the nearest whole number.

5. Determine the number of sodium ions needed for 150 mM NaCl.

The amount of chloride ions and sodium ions needed are the same. If 19 Cl

^{-}ions are added, then 19 Na

^{+}ions should also be added to the system.

__Note 1:__Upon equilibration of the system, the volume of the box will decrease as the solute-solvent interactions come into effect. Therefore, the equilibrated salt concentration is larger than the starting cocentration (150 mM NaCl) for NPT simulations.

__Note 2:__

__Matias Machado__has proposed a possibly better way to calculate charges and buffers, the instructions for which can be found on

__this page__of the AMBER mailing list archive.

This tutorial was last updated July 16, 2021.