GROMACS Protein Analysis

This skill provides comprehensive workflows for analyzing protein molecular dynamics simulation results from GROMACS. It covers five major analysis types commonly used in protein dynamics studies.

Prerequisites

• •GROMACS simulation completed with trajectory files (.xtc/.trr) and topology file (.tpr)
• •Understanding of GROMACS commands (call gromacs-skills when needed)
• •For visualization: DuIvyTools skill (call duivytools-skills when needed)

Analysis Types

1. Periodic Boundary Condition (PBC) Correction

Correct trajectory for PBC artifacts to prevent molecules from crossing box boundaries and ensure proper alignment for downstream analysis.

Purpose: Remove PBC artifacts, center protein/ligand, eliminate overall translation/rotation

Input: Trajectory file (.xtc), topology file (.tpr), index file (.ndx)

Output: Corrected trajectory (fit.xtc), corrected topology (fit.tpr)

When to use: Before any analysis when molecules cross box boundaries or when RMSD shows abrupt jumps

Detailed workflow: See PBC Correction Guide

2. Root Mean Square Deviation (RMSD)

Calculate RMSD to measure structural stability and assess simulation convergence.

Purpose: Monitor structural stability, identify equilibration phase, assess simulation convergence, compare structures

Input: Trajectory file (.xtc), topology file (.tpr), reference structure

Output: RMSD time series (rmsd.xvg), visualization (rmsd.png)

Visualization: Use duivytools-skills skill to plot RMSD over time

Detailed workflow: See RMSD Analysis Guide

3. Root Mean Square Fluctuation (RMSF)

Calculate RMSF to evaluate per-residue flexibility and identify flexible/rigid regions.

Purpose: Identify flexible regions, assess local stability, analyze loop dynamics, compare residue flexibility

Input: Trajectory file (.xtc), topology file (.tpr), index file (.ndx)

Output: RMSF per residue (rmsf.xvg), B-factors (bfactor.pdb), visualization (rmsf.png)

Visualization: Use duivytools-skills skill to plot RMSF per residue

Detailed workflow: See RMSF Analysis Guide

4. Radius of Gyration (Gyrate)

Calculate radius of gyration to assess protein compactness and folding state.

Purpose: Monitor protein compactness, detect unfolding/folding transitions, assess overall size changes

Input: Trajectory file (.xtc), topology file (.tpr), index file (.ndx)

Output: Gyrate time series (gyrate.xvg), per-axis data (gyrate_axes.xvg), visualization (gyrate.png)

Visualization: Use duivytools-skills skill to plot gyrate over time

Detailed workflow: See Gyrate Analysis Guide

5. Solvent Accessible Surface Area (SASA)

Calculate SASA to study solvent accessibility and surface properties of the protein.

Purpose: Analyze solvent exposure, identify hydrophobic/hydrophilic surfaces, study ligand binding sites, monitor protein unfolding

Input: Trajectory file (.xtc), topology file (.tpr), index file (.ndx)

Output: Total SASA time series (sas.xvg), per-residue SASA (sas_per_residue.xvg), visualization (sas.png)

Visualization: Use duivytools-skills skill to plot SASA over time

Detailed workflow: See SASA Analysis Guide

6. Dynamics Cross-Correlation Matrix (DCCM)

Analyze correlated motions between atomic pairs to identify coordinated movements in the protein.

Purpose: Identify pairs of atoms that move together (positive correlation) or opposite (negative correlation)

Input: Trajectory file (.xtc), topology file (.tpr), index file (.ndx)

Output: Covariance matrix (covar.dat), DCCM matrix (dccm.xpm), visualization (dccm.png)

Visualization: Use duivytools-skills skill to generate heatmaps

Detailed workflow: See DCCM Analysis Guide

7. Residue Distance Contact Matrix (RDCM)

Calculate average distances between residue pairs to analyze inter-residue contacts and spatial relationships.

Purpose: Map residue-residue distances, identify long-range contacts, analyze protein structure

Input: Trajectory file (.xtc), topology file (.tpr), index file (.ndx)

Output: Distance contact matrix (rdcm.xpm), visualization (rdcm.png)

Visualization: Use duivytools-skills skill to generate heatmaps

Detailed workflow: See RDCM Analysis Guide

8. Principal Component Analysis (PCA)

Identify collective motions and dominant conformational changes by decomposing protein motion into principal components.

Purpose: Extract major collective motions, analyze conformational flexibility, reduce dimensionality

Input: Trajectory file (.xtc), topology file (.tpr), index file (.ndx)

Output: Eigenvalues (eigenvalues.xvg), eigenvectors (eigenvectors.trr), projections (pc1.xvg, pc2.xvg)

Key metrics: First few PCs' contribution percentages

Visualization: Use duivytools-skills skill to plot eigenvalues and projections

Detailed workflow: See PCA Analysis Guide

9. Free Energy Landscape (FEL)

Map free energy surfaces to understand conformational states and transitions using either RMSD/Gyrate or PCA.

Purpose: Identify stable conformations, quantify energy barriers, understand conformational landscape

Method 1 - RMSD + Gyrate: Use structural deviation and compactness as reaction coordinates

Method 2 - PCA: Use principal components as reaction coordinates

Input: RMSD data (rmsd.xvg), Gyrate data (gyrate.xvg) OR PC projections (pc1.xvg, pc2.xvg)

Output: Free energy landscape (gibbs.xpm), energy minima (gibbs.log), frame indices (bindex.ndx), visualization (fel.png)

Visualization: Use duivytools-skills skill to generate 2D/3D FEL plots

Detailed workflow: See FEL Analysis Guide

General Workflow

Before Any Analysis

1. •Check trajectory quality: Visual inspection of trajectory for artifacts
2. •PBC correction (if needed): Use PBC correction workflow
3. •Ensure consistent atom selection: Use same index groups for related analyses

Analysis Independence

Most analyses are independent and can be performed in any order:

Independent analyses (no dependencies):

• •RMSD, RMSF, Gyrate, SASA - Basic stability and property analysis
• •DCCM, RDCM - Contact and correlation analysis
• •PCA - Collective motion analysis

Dependent analyses (require other analyses):

• •FEL - Requires RMSD/Gyrate OR PCA results as reaction coordinates

Recommended Pre-Analysis Step

PBC Correction (optional but recommended):

• •Consider PBC correction if RMSD shows abrupt jumps or molecules cross box boundaries
• •PBC correction is not always necessary - only apply when indicated by trajectory quality
• •Analysis issues may have other causes beyond PBC artifacts

After Each Analysis

• •Verify output files: Check that all expected files are generated
• •Visual inspection: Use appropriate visualization to assess results
• •Documentation: Record analysis parameters and observations

Key Considerations

Atom Selection

• •Backbone: For overall protein motion and RMSD analysis
• •C-alpha: For PCA and DCCM (reduces computational cost)
• •Protein: For full protein analysis
• •Protein_Lig: For protein-ligand complexes

Time Selection

• •Equilibration phase: Exclude initial equilibration period (typically first 10-20% of simulation)
• •Production phase: Use production phase for analysis
• •Consistency: Use same time range for related analyses

Index Groups

• •Create appropriate index groups using gmx make_ndx
• •Ensure index groups match analysis requirements
• •Document index group compositions

When to Call DuIvyTools-Skills

Call the duivytools-skills skill for visualization tasks:

• •XVG files: Plot RMSD, RMSF, energies, hydrogen bonds, gyrate
• •XPM files: Visualize DCCM, RDCM, FEL matrices
• •Projections: Plot PC1, PC2 projections
• •Statistical analysis: Calculate averages, distributions

Troubleshooting

Common Issues

RMSD shows abrupt jumps: PBC artifacts - apply PBC correction

DCCM values all near zero: Check atom selection, ensure sufficient dynamics

PCA shows uniform eigenvalues: May indicate no dominant collective motion or excessive noise

FEL shows unrealistic barriers: Check time range selection, ensure sufficient sampling

Files don't match: Verify tpr and xtc have same atom count, use gmx convert-tpr if needed

Reference Documentation

For detailed step-by-step workflows, consult these references:

Basic Analysis

• •PBC Correction Guide - Complete PBC correction workflow
• •RMSD Analysis Guide - Root mean square deviation for stability assessment
• •RMSF Analysis Guide - Root mean square fluctuation for flexibility analysis
• •Gyrate Analysis Guide - Radius of gyration for compactness analysis
• •SASA Analysis Guide - Solvent accessible surface area for surface properties

Advanced Analysis

• •DCCM Analysis Guide - DCCM calculation and interpretation
• •RDCM Analysis Guide - Distance contact matrix analysis
• •PCA Analysis Guide - Principal component analysis workflow
• •FEL Analysis Guide - Free energy landscape mapping

Quick Reference

Required Input Files

• •Trajectory: .xtc or .trr file from GROMACS simulation
• •Topology: .tpr file (must match trajectory atom count)
• •Index: .ndx file with custom atom groups

Common Output Files

• •XVG: Time-series data (RMSD, eigenvalues, projections)
• •XPM: Matrix data (DCCM, RDCM, FEL)
• •TRR: Vector data (eigenvectors)
• •PDB: Structure files (average, extreme conformations)

Analysis Order Recommendation

Analyses can be performed flexibly based on your research questions:

For basic stability assessment: Start with RMSD, RMSF, Gyrate, SASA (any order)

For contact and correlation analysis: Perform DCCM and RDCM (independent)

For collective motion analysis: Perform PCA (independent)

For conformational landscape: Generate FEL after obtaining RMSD/Gyrate or PCA data

Note: Consider PBC correction only if RMSD shows abrupt jumps or trajectory quality issues or User asks. Many analyses work fine without PBC correction.

Best Practices

• •Never overwrite existing files - use unique output filenames
• •Use consistent time ranges for all related analyses
• •Document all parameters and index group selections
• •Visualize intermediate results to catch issues early
• •Verify atom count consistency between tpr and xtc files
• •Check statistical convergence before interpreting results