Hello Mrinalini,

When reconstructing a 3D volume from 2D projections in cryoEM image processing, there is a 50% probability of getting either chirality. It is relatively easy to visualize why if you cast shadows of your hands: you can easily make the shadow look the same with both your right and left hand as the object casting it, despite your two hands not being the same in 3D due to chirality.

But natural proteins only have L-amino acids, their alpha helices are right-handed, etc. So the correct solution in your case is not to build a model into a map of the wrong chirality, because the only way it would work in terms of stereo-chemistry would be with D-amino acids. You simply need to mirror the map.

In ChimeraX, assuming your map is loaded as model #1, this command will mirror it: volume flip #1

(The resulting mirrored map will only live in your ChimeraX session, so if you want to keep it, you need to save it as an MRC file.)

There is also a job to mirror a map in CryoSPARC, if this is the program you are using to process your data: https://guide.cryosparc.com/processing-data/all-job-types-in-cryosparc/utilities/job-volume-tools (this is the option "flip handedness" in this job).

To tell chirality only from a map, in general, you need sufficient resolution to trace the backbone, and a protein containing alpha helices: then you can determine whether they are right-handed (the correct chirality) and invert the map if they are not. Some particles have chiral features that can be identified at much lower resolution (for example the super-helical turn of DNA around nucleosomes). But it is not always easy to tell depending on particle and resolution. If auto-building programs like ModelAngelo fail or produce many short disconnected fragments when applied to a good-looking map, this is often a sign of wrong chirality.

I hope this helps,

Guillaume