A per-O-methylated beta-cyclodextrin dimer, Py2CD, was conveniently prepared via two steps: the Williamson reaction of 3,5-bis(bromomethyl)pyridine and beta-cyclodextrin (beta-CD) yielding 2A,2'A-O-[3,5-pyridinediylbis(methylene) bis-beta-cyclodextrin (bisCD) followed by the O-methylation of all the hydroxy groups of the bisCD. Py2CD formed a very stable 1:1 complex (Fe(III)PCD) with [5,10,15,20-tetrakis(psulfonatophenyl)porphinato]iron(III) ((FeTPPS)-T-II) in aqueous solution. Fe(III)PCD was reduced with Na2S2O4 to afford the (FeTPPS)-T-II/Py2CD complex (Fe(II)PCD). Dioxygen was bound to Fe(II)PCD, the P-1/2(O2) values being 42.4 +/- 1.6 and 176 +/- 3 Torr at 3 and 25 degrees C, respectively. The k(on)(O2) and k(off) (O2) values for the dioxygen binding were determined to be 1.3 x 10(7) M(-1)s(-1) and 3.8 x 10(3) s(-1), respectively, at 25 degrees C. Although the dioxygen adduct was not very stable (K-O2 = k(on)(O2) /k(off)(O2) = 3.4 x 10(3) M-1), no autoxidation of the dioxygen adduct of Fe(II)PCD to Fe(III)PCD was observed. These results suggest that the encapsulation of (FeTPPS)-T-II by Py2CD strictly inhibits not only the extrusion of dioxygen from the cyclodextrin cage but also the penetration of a water molecule into the cage. The carbon monoxide affinity of Fe(II)PCD was much higher than the dioxygen affinity; the P-1/2(CO), k(on)(CO), k(off)(CO), and K-CO values being (1.6 +/- 0.2) x 10(-2) Torr, 2.4 x 106 M(-1)s(-1), 4.8 x 10(-2) s(-1), and 5.0 x 10(7) M-1, respectively, at 25 degrees C. Fe(II)PCD also bound nitric oxide. The rate of the dissociation of NO from (NO)Fe(II)PCD ((5.58 +/- 0.42) x 10(-1) s(-1)) was in good agreement with the maximum rate ((5.12 +/-0.18) X 10(-5) S-1) of the oxidation of (NO)Fe(II)PCD to Fe(III)PCD and NO3-, suggesting that the autoxidation of (NO)Fe(II)PCD proceeds through the ligand exchange between NO and O-2 followed by the rapid reaction of (O-2)Fe(II)PCD with released NO, affording Fe(II)PCD and the NO3- anion inside the cyclodextrin cage.