each of X is independently chosen from the moieties —N—, {HEAD ~ —C}—, —O—, —OC—, {TAIL ~ —C}—, —NHC—, and —NH—C— 
each of X is independently chosen from the moieties — N — , { HEAD ~ — C } — , — O — , — OC — , { TAIL ~ — C } — , — NHC — , and — NH — C —
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH