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