A new molecular mechanics force field for the design of oxotechnetium(V) and oxorhenium(V) radiopharmaceuticals

Dinuclear ZnII and mixed CuII-ZnII complexes of artificial patellamides as phosphatase models

The patellamides (cyclic pseudo-octapeptides) are produced by Prochloron, a symbiont of the ascidians, marine invertebrate filter feeders. These pseudo-octapeptides are present in the cytoplasm and a possible natural function of putative metal complexes of these compounds is hydrolase activity, however the true biological role is still unknown. The dinuclear Cu^II complexes of synthetic patellamide derivatives have been shown in in vitro experiments to be efficient hydrolase model catalysts. Many hydrolase enzymes, specifically phosphatases and carboanhydrases, are Zn^II-based enzymes and therefore, we have studied the Zn^II and mixed Zn^II/Cu^II solution chemistry of a series of synthetic patellamide derivatives, including solution structural and computational work, with the special focus on model phosphatase chemistry with bis-(2,4-dinitrophenyl)phosphate (BDNPP) as the substrate. The Zn^II complexes of a series of ligands are shown to form complexes of similar structure and stability compared to the well-studied Cu^II analogues and the phosphatase reactivities are also similar. Since the complex stabilities and phosphatase activities are generally a little lower compared to those of Cu^II and since the concentration of Zn^II in Prochloron cells is slightly smaller, we conclude that the Cu^II complexes of the patellamides are more likely to be of biological importance.

Synthesis, Characterization, and In Vitro Evaluation of New (99m)Tc/Re(V)-Cyclized Octreotide Analogues: An Experimental and Computational Approach

Radiolabeled proteolytic degradation-resistant somatostatin analogues have been of long-standing interest as cancer imaging and radiotherapy agents for targeting somatostatin receptor-positive tumors. Our interest in developing (186)Re- and (188)Re-based therapeutic radiopharmaceuticals led to investigation of a new Re(V)-cyclized octreotide analogue, Re(V)-cyclized, thiolated-DPhe(1)-Cys(2)-Tyr(3)-DTrp(4)-Lys(5)-Thr(6)-Cys(7)-Thr(OH)(8) (Re-SDPhe-TATE) using both experimental and quantum chemical methods. The metal is directly coordinated to SDPhe-TATE through cyclization of the peptide around the [ReO](3+) core. Upon complexation, four isomers were observed; the isolated/semi-isolated isomers exhibited different somatostatin receptor (sstr) binding affinities, 0.13 to 1.5 μM, in rat pancreatic tumor cells. Two-dimensional NMR experiments and electronic structure calculations were employed to elucidate the structural differences among the different isomers. According to NMR studies, the metal is coordinated to three thiolates and the backbone amide of Cys(2) in isomers 1 and 4, whereas the metal is coordinated to three thiolates and the backbone amide of Tyr(3) in isomer 2. Quantum chemical methods clarified the stereochemistry of Re-SDPhe-TATE and the possible peptide arrangements around the [ReO](3+) core. The re-cyclization reaction was translated to the (99m)Tc radiotracer level with four isomers observed on complexation with comparable HPLC retention times as the Re-SDPhe-TATE isomers. About 85% total (99m)Tc labeling yield was achieved by ligand exchange from (99m)Tc-glucoheptonate at 60 °C for an hour. About 100% and 51% of (99m)Tc(V)-cyclized SDPhe-TATE remained intact in phosphate buffered saline and 1 mM cysteine solution under physiological conditions at 6 h, respectively.

Re(I) and Tc(I) complexes for targeting nitric oxide synthase: influence of the chelator in the affinity for the enzyme

Aiming to design (99m) Tc complexes for probing nitric oxide synthase (NOS) by SPECT, we synthesized conjugates (L4-L6) comprising a NOS-recognizing moiety connected to a diamino-propionic acid (dap) chelating unit. The conjugates led to complexes of the type fac-[M(CO)3 (ĸ(3) -L)] (M = Re/(99m) Tc; Re4/Tc4: L = L4; Re5/Tc5: L = L5; Re6/Tc6: L = L6). Enzymatic studies showed that L4 and L5, but not L6, gave complexes (Re4 and Re5) that are less potent than the conjugates. To rationalize these results, we performed docking and molecular dynamics simulations. The high affinity of L4 and L5 is due to the strong interactions between the dap chelator and polar residues of the binding cavity. These interactions are hampered by metallation resulting in complexes with lower affinity. The higher potency of Re5 compared to Re4 was assigned to the increased bulkiness of Re5 and the presence of additional anchoring groups that better fit the active site and provide more extensive contacts. In turn, Re6 is too bulky and its organometallic tail is oriented toward the peripheral pocket of iNOS, leading to loss of contacts and a lower affinity. These results were compared with our previous results obtained with analogue complexes stabilized by a pyrazolyl-diamine chelating unit.

Optimization of a molecular mechanics force field for type-II polyoxometalates focussing on electrostatic interactions: a case study

In this study, we have focussed on type-II polyanions such as [M(7)O(24)](6-), and we have developed and validated optimized force fields that include electrostatic and van der Waals interactions. These contributions to the total steric energy are described by the nonbonded term, which encompasses all interactions between atoms that are not transmitted through the bonds. A first validation of a stochastic technique based on genetic algorithms was previously made for the optimization of force fields dedicated to type-I polyoxometalates. To describe the new nonbonded term added in the functional, a fixed-charged model was chosen. Therefore, one of the main issues was to analyze that which partial atomic charges could be reliably used to describe these interactions in such inorganic compounds. Based on several computational strategies, molecular mechanics (MM) force field parameters were optimized using different types of atomic charges. Moreover, the influence of the electrostatic and van der Waals buffering constants and 1,4-interactions scaling factors used in the force field were also tested, either being optimized as well or fixed with respect to the values of CHARMM force field. Results show that some atomic charges are not well adapted to CHARMM parameters and lead to unrealistic MM-optimized structures or a MM divergence. As a result, a new scaling factor has been optimized for Quantum Theory of Atoms in Molecules charges and charges derived from the electrostatic potential such as ChelpG. The force fields optimized can be mixed with the CHARMM force field, without changing it, to study for the first time hepta-anions interacting with organic molecules.