Fig. 4. MALDI-TOF mass spectrum (positive-ion mode) of Ac-His-6-MBHA-1d obtained by reaction of Ac-His-6 with a ten-fold excess of 1d. In each assignment, the value in parentheses is the number of naphthalene substituents for each Ac-His molecule. M9 denotes the peaks assigned to the highly substituted species bearing compound 9 as a counterion of the imidazolium salt moieties.
