Peptides should be promising drug candidates. But their small size makes delivery difficult and gives them an extremely short in vivo half-life. They are often cleared by the kidneys or reticuloendothelial system only minutes after being administered, and they are susceptible to degradation by proteolytic enzymes. These problems could be solved by linking them to polyethylene glycol (PEG). Repeating chains of ethylene oxide (CH2CH2O), PEG molecules can be long or short and straight or branched. PEG groups are linked to…