Unusual Catalyst Concentration Effects in the Hydrolysis of Phenyl Phosphate Esters and of DNA: A Systematic Investigation of the Lanthanide Series1

Saturation kinetics are measured with all lanthanides and bis(nitrophenyl) phosphate BNPP as substrate. They show rather constant K M values; the k cat values, however, increase by up to 66 times for La 3+ to Er 3+ and decrease again for Yb 3+ and Lu 3+ . With all lanthanides, hydrolysis of the intermediate mononitrophenyl phosphate NPP is 2−30 times faster than that of BNPP. The k cat values measured with BNPP correlate with the ion diameter of the lanthanides, in line with accepted mechanisms but with the notable exception of the higher lanthanides. A similar correlation holds for the cleavage rates with plasmid DNA, with striking differences again observed with the higher lanthanides, however. Thus, a concentration increase from 5 × 10 - 5 to 1 × 10 - 2 M leads to 64% and 84% more DNA cleavage with La 3+ and Pr 3+, respectively, but to up to 68% less DNA cleavage, respectively, but with Yb 3+, Tm 3+ or Lu 3+ . In contrast to the BNPP cleavage, saturation kinetics derived k cat values with DNA change little with the used cation, which on the other hand led to larger variations in the K M parameters. Preliminary UV and CD studies with plasmid DNA indicate lanthanide-induced conformational changes with pseudo-first-order rate constants 10−100 times higher than the cleavage rate under the same conditions. Again, Yb 3+ shows different effects than Eu 3+ . The unusual behavior of the higher lanthanides is discussed on the basis of cation clustering, which, in contrast to earlier assumptions by Bamann et al., leads to diminished activities. Addition of salts such as of NaCl or MgCl 2 leads to distinct decrease of catalytic effects of for instance Eu 3+ . The corresponding rates correlate well with Debye−Hückel ionic strength parameters. These as well as the effects of added amines are in line with a simple competition mechanism of the added cations for the anionic substrates.