{HEAD ~ R3} represents methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec.butyl, hydroxymethyl, hydroxyethyl, mercaptomethyl, or {TAIL ~ methylthioethyl} or 
represents
{ HEAD ~ R3 } represents methyl , ethyl , propyl , isopropyl , butyl , isobutyl , sec.butyl , hydroxymethyl , hydroxyethyl , mercaptomethyl , or { TAIL ~ methylthioethyl } or
HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH