{HEAD ~ R2} represents methyl, {TAIL ~ ethyl}, propyl, isopropyl, butyl, isobutyl, or sec.butyl 
represents
{ HEAD ~ R2 } represents methyl , { TAIL ~ ethyl } , propyl , isopropyl , butyl , isobutyl , or sec.butyl
HEAD HEAD HEAD HEAD HEAD REL OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH