A highly efficient coupling reagent. PyOxim is more reactive than TBTU or PyBOP. It is about as reactive as COMU (Q-2735); and, as COMU, PyOxim is safer to handle and less allergenic than HOBt-based coupling reagents. As PyOxim represents a phosphonium-type coupling reagent, N-terminal guanidinylation as a side-reaction during coupling is avoided.