The chemical and sensing properties of porphyrins are frequently tuned via the introduction of peripheral substituents. In the context of the exceptionally fast second protonation step in the case of 5,10,15,20-tetraphenylporphyrin (TPP), as compared to porphin and 5,10,15,20-tetramesitylporphyrin (TMesP), we investigated the macrocycle-substituent interactions of these three porphyrin derivati...

The protonation of N2 bound to the active center of nitrogenase has been investigated using state-of-the-art density-functional theory calculations. Dinitrogen in the bridging mode is activated by forming two bonds to Fe sites, which results in a reduction of the energy for the first hydrogen transfer by 123 kJ/mol. The axial binding mode with open sulfur bridge is less reactive by 30 kJ/mol an...

In this review we discuss the role of protonation states in receptor-ligand interactions, providing experimental evidences and computational predictions that complex formation may involve titratable groups with unusual pKa's and that protonation states frequently change from unbound to bound states. These protonation changes result in proton uptake/release, which in turn causes the pH-dependenc...

The proportions of axial anomers of various glucosylamines (2) and their conjugate acids were determined by 1H NMR. The change upon N-protonation is small and can be accounted for by steric effects, without any "reverse anomeric effect." To test whether N-protonation changes the steric bulk of an imidazolyl group, ring-inversion equilibria of cis-N-(4-alkylcyclohexyl)imidazoles (3c, 4c) and of ...

We build a Markov partition of the Feigenbaum dynamical plane, and a system of “external rays”, which are invariant under rescaling. Applications to geometry of the Feigenbaum map are given.

The Constant Capacitance model provided a quantitative description of o-phosphate adsorption by 44 noncalcareous soils whose pH values ranged from 4.9 to 7.6. The intrinsic surface protonationdissociation constants, capacitance density, and phosphate packing area parameters required by the model were adopted from model calculations on reference hydrous oxide minerals. The intrinsic phosphate su...

The protonation constants of DTPA (diethylenetriaminepentaacetic acid) and two derivatives of DTPA, 1-R(4,4diphenyl cyclohexyl-phosphonyl-methyl diethylenentriaminepentaacetic acid (MS-325) and (R)-hydroxymethyl-diethylenentriaminepentaacetic acid (HMDTPA) were determined by potentiometric titration in 0.1 M NaClO4. The formation of 1 : 1 complexes of Am3+, Cm3+ and Ln3+ cations with these thre...

The protonation constants of the ligand 3,6,9,16,19,22-hexaazatricyclo[22.2.2.2]triaconta-1(26),11(12),13,24,27, 29-hexaene (L, BPXD) have been determined in water at 25°C by potentiometric procedures and indicate a lower basicity than that corresponding to the analogue ligand with m-xylyl spacers between the diethylenetriamine subunits (BMXD). The crystal structure of a salt of H6L 6+ containi...

A pragmatic approach has been developed for the estimation of aqueous ionization constants (pKa) for druglike compounds. The method involves an algorithm that assigns ionization constants in a stepwise manner to the acidic and basic groups present in a compound. Predictions are made for each ionizable group using models derived from semiempirical quantum chemical properties and information-base...