Stability of porphyrin-calix[4]arene complexes analyzed by electrospray ionization mass spectrometry

The stability of some porphyrin-calix[4]arene sodium-ion complexes were determined by a collision-activated decomposition (CAD) method utilizing electrospray ionization mass spectrometry (ESI-MS). Comparing the values of E_1/2, the collision energy at which the relative intensity of the complex ion is 0.5, we found that the porphyrin-calix[4]arene complex with the higher value of E_1/2 corresponded to that with the larger association constant (K_ass), as measured by ¹H-NMR in CDCl ₃. Both our ESI-MS and NMR studies proved that the number of hydrogen bonds and the rigidity of the calix[4]arene stabilized the complex. The ESI-MS technique could be successful in screening the binding affinity in host-guest systems with a small amount of sample.