I am trying to understand why some PS lipids are missing an improper on the headgroup carboxylate. For example, in the tarball called toppar_c36_jul20.tgz downloaded from http://mackerell.umaryland.edu/charmm_ff.shtml
, that improper is missing for SAPS and all other PS lipids in this stream file: toppar_all36_lipid_miscellaneous.str , which has statements like this:
IMPR C21 O21 C22 O22 C31 O31 C32 O32
However, PS lipids like POPS in the file called top_all36_lipid.rtf have impropers like this:
IMPR C21 O21 C22 O22 C31 O31 C32 O32 C13 C12 O13B O13A
My working assumption is that somebody simply forgot to add this PS-specific improper when adding PS lipids to toppar_all36_lipid_miscellaneous.str, but it would be great if anybody has some insight on who developed the PS lipid parameters in this toppar_all36_lipid_miscellaneous.str file, what the associated publication is, and/or any reasoning for this absent headgroup improper.
Thank you for your help,