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