Substituents with a specified function, for example those that impart water solubility to the compound, may be included at {HEAD ~ any} of {TAIL ~ R1 R6} 
of
Substituents with a specified function , for example those that impart water solubility to the compound , may be included at { HEAD ~ any } of { TAIL ~ R1 R6 }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL