2 research outputs found
A Second Allosteric Site in <i>Escherichia coli</i> Aspartate Transcarbamoylase
<i>Escherichia coli</i> aspartate transcarbamoylase
is
feedback inhibited by CTP and UTP in the presence of CTP. Here, we
show by X-ray crystallography that UTP binds to a unique site on each
regulatory chain of the enzyme that is near but not overlapping with
the known CTP site. These results bring into question all of the previously
proposed mechanisms of allosteric regulation in aspartate transcarbamoylase
Selectivity of the Highly Preorganized Tetradentate Ligand 2,9-Di(pyrid-2-yl)-1,10-phenanthroline for Metal Ions in Aqueous Solution, Including Lanthanide(III) Ions and the Uranyl(VI) Cation
Some metal ion complexing properties of DPP (2,9-DiĀ(pyrid-2-yl)-1,10-phenanthroline)
are reported with a variety of LnĀ(III) (LanthanideĀ(III)) ions and
alkali earth metal ions, as well as the uranylĀ(VI) cation. The intense
ĻāĻ* transitions in the absorption spectra of aqueous
solutions of 10<sup>ā5</sup> M DPP were monitored as a function
of pH and metal ion concentration to determine formation constants
of the alkali-earth metal ions and LnĀ(III) (Ln = lanthanide) ions.
It was found that log <i>K</i><sub>1</sub>(DPP) for the
LnĀ(III) ions has a peak at LnĀ(III) = SmĀ(III) in a plot of log <i>K</i><sub>1</sub> versus 1/<i>r</i><sup>+</sup> (<i>r</i><sup>+</sup> = ionic radius for 8-coordination). For LnĀ(III)
ions larger than SmĀ(III), there is a steady rise in log <i>K</i><sub>1</sub> from LaĀ(III) to SmĀ(III), while for LnĀ(III) ions smaller
than SmĀ(III), log <i>K</i><sub>1</sub> decreases slightly
to the smallest LnĀ(III) ion, LuĀ(III). This pattern of variation of
log <i>K</i><sub>1</sub> with varying size of LnĀ(III) ion
was analyzed using MM (molecular mechanics) and DFT (density functional
theory) calculations. Values of strain energy (āU) were calculated
for the [LnĀ(DPP)Ā(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> and [LnĀ(qpy)Ā(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> (qpy = quaterpyrdine) complexes
of all the LnĀ(III) ions. The ideal MāN bond lengths used for
the LnĀ(III) ions were the average of those found in the CSD (Cambridge
Structural Database) for the complexes of each of the LnĀ(III) ions
with polypyridyl ligands. Similarly, the ideal MāO bond lengths
were those for complexes of the LnĀ(III) ions with coordinated aqua
ligands in the CSD. The MM calculations suggested that in a plot of
āU versus ideal MāN length, a minimum in āU occurred
at PmĀ(III), adjacent in the series to SmĀ(III). The significance of
this result is that (1) MM calculations suggest that a similar metal
ion size preference will occur for all polypyridyl-type ligands, including
those containing triazine groups, that are being developed as solvent
extractants in the separation of AmĀ(III) and LnĀ(III) ions in the treatment
of nuclear waste, and (2) AmĀ(III) is very close in MāN bond
lengths to PmĀ(III), so that an important aspect of the selectivity
of polypyridyl type ligands for AmĀ(III) will depend on the above metal
ion size-based selectivity. The selectivity patterns of DPP with the
alkali-earth metal ions shows a similar preference for CaĀ(II), which
has the most appropriate MāN lengths. The structures of DPP
complexes of ZnĀ(II) and BiĀ(III), as representative of a small and
of a large metal ion respectively, are reported. [ZnĀ(DPP)<sub>2</sub>]Ā(ClO<sub>4</sub>)<sub>2</sub> (triclinic, <i>P</i>1, <i>R</i> = 0.0507) has a six-coordinate ZnĀ(II), with each of the
two DPP ligands having one noncoordinated pyridyl group appearing
to be Ļ-stacked on the central aromatic ring of the other DPP
ligand. [BiĀ(DPP)Ā(H<sub>2</sub>O)<sub>2</sub>(ClO<sub>4</sub>)<sub>2</sub>]Ā(ClO<sub>4</sub>) (triclinic, <i>P</i>1, <i>R</i> = 0.0709) has an eight-coordinate Bi, with the coordination
sphere composed of the four N donors of the DPP ligand, two coordinated
water molecules, and the O donors of two unidentate perchlorates.
As is usually the case with BiĀ(III), there is a gap in the coordination
sphere that appears to be the position of a lone pair of electrons
on the other side of the Bi from the DPP ligand. The Bi-L bonds become
relatively longer as one moves from the side of the Bi containg the
DPP to the side where the lone pair is thought to be situated. A DFT
analysis of [LnĀ(tpy)Ā(H<sub>2</sub>O)<sub><i>n</i></sub>]<sup>3+</sup> and [LnĀ(DPP)Ā(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> complexes
is reported. The structures predicted by DFT are shown to match very
well with the literature crystal structures for the [LnĀ(tpy)Ā(H<sub>2</sub>O)<sub><i>n</i></sub>]<sup>3+</sup> with Ln = La
and <i>n</i> = 6, and Ln = Lu with <i>n</i> =
5. This then gives one confidence that the structures for the DPP
complexes generated by DFT are accurate. The structures generated
by DFT for the [LnĀ(DPP)Ā(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> complexes
are shown to agree very well with those generated by MM, giving one
confidence in the accuracy of the latter. An analysis of the DFT and
MM structures shows the decreasing O--O nonbonded distances as one
progresses from La to Lu, with these distances being much less than
the sum of the van der Waals radii for the smaller LnĀ(III) ions. The
effect that such short O--O nonbonded distances has on thermodynamic
complex stability and coordination number is then discussed