Yu. A. Borisov

New development in the tritium labelling of peptides and proteins using solid catalytic isotopic exchange with spillover-tritium.

Summary.u2002The mechanism of the reaction of high temperature solid state catalytic isotope exchange (HSCIE) of hydrogen in peptides with spillover-tritium at 140–180°C was analyzed. This reaction was used for preparing [3H]enkephalins such as [3H]DALG with specific activity of 138u2009Ci/mmol and [3H]LENK with specific activity of 120u2009Ci/mmol at 180°C. The analogues of [3H]ACTG4–10 with specific activity of 80u2009Ci/mmol, [3H]zervamicin IIB with specific activity of 70u2009Ci/mmol and [3H]conotoxin G1 with specific activity 35u2009Ci/mmol were produced. The obtained preparations completely retained their biological activity. [3H]Peptide analysis using 3H NMR spectroscopy on a Varian UNITY-600 spectrometer at 640u2009MHz was carried out. The reaction ability of amino fragments in HSCIE was shown to depend both of their structures and on the availability and the mobility of the peptide chain. The reaction of HSCIE with the β-galactosidase from Termoanaerobacter ethanolicus was studied. The selected HSCIE conditions allow to prepare [3H] β-galactosidase with specific activity of 1440u2009Ci/mmol and completely retained its the enzymatic activity.

Protonation of ferrocene, ruthenocene, and osmocene: a density functional study

Protonated forms of the ferrocene, ruthenocene, and osmocene molecules in the gas phase were calculated using the density functional approach with the Becke—Lee—Young—Parr functional. The proton affinity energies of ferrocene, ruthenocene, and osmocene were estimated at 214.2, 220.3, and 229.7 kcal mol–1, respectively. The addition of a proton to carbon atoms of the cyclopentadienyl ring in the ferrocene molecule and to the metal atom in the ruthenocene and osmocene molecules is more energetically favorable. No minimum corresponding to ring protonation was located on the potential energy surface of protonated osmocene. The C—Hendo bond in the ring-protonated [C10H11M]+ (M = Fe, Ru) cations is involved in agostic interaction with the metal atom. Transition states of interconversions between the ring-protonated and metal-protonated forms were identified. A specific group of protonated forms of the ferrocene and ruthenocene molecules includes four types of structures, viz., ring-protonated (1a,b) and metal-protonated (2a,b) structures, transition states of the 1 ⇌ 2 interconversion (3a,b), as well as ring-protonated structures with the cyclopentadiene ring folded along the C(2)—C(5) line so that the M—Hendo interaction is virtually negligible. The latter structures are required for [1,5]-sigmatropic shift of the exo-hydrogen atom in the Cp ring to occur. The results obtained were used for the interpretation of the available schemes of electrophilic substitution reactions in metallocenes and of the sigmatropic shift mechanisms.

Structure and vibrational spectra of dicyclopentadienylzinc. A DFT study

The quantum-chemical DFT calculations of the Cp2Zn structure confirm the conclusion made earlier from the vibrational spectra that the sandwich structure (η5-C5H5)2Zn (A) is not energetically favorable and more favorable are the close in energy πσ-structure (η5-C5H5)(η1-C5H5)Zn (B) and σ-structure (η1-C5H5)2Zn (C). The vibrational spectra of structures B and C with the DFT-derived force fields were calculated. A comparison of the calculated spectra of the isolated Cp2Zn molecules with the experimental data gives no way of deciding between the B and C structures. It is most likely that the molecule is nonrigid and experiences a strong influence from the nearest environment in solution or in the crystalline state.

Study of the solid-state hydrogen isotope exchange ofl-alanine

The solid-state reaction of isotope exchange ofl-alanine (l-Ala) with spillover-hydrogen activated on a Rh(Pd)-supported catalyst was studied. The reactivity of the carbon atoms and the activation energies of isotope exchange of the hydrogen at the C(2) and C(3) atoms of thel-Ala molecule were determined using tritium NMR. Theab initio calculations of the activation energy of a model reaction between the alanine molecule and a hydroxonium cation were carried out. The mechanism and plausible structures of the transition states of this reaction were proposed.

AB INITIO CALCULATIONS OF CHLORIDE COMPLEXES OF AU, HG, TL, PB, AND BI IN ANOMALOUS OXIDATION STATES (2S1/2 ELECTRON STATE)

Ab initio calculations of chloride complexes of Au, Hg, Tl, Pb, and Bi in anomalous oxidation states (2S1/2 electron state) were carried out by the Becke-Lee-Yang-Parr density functional method using the Dunning-Hay LanL2DZ basis set. Optimum geometric parameters and electronic characteristics of MCln(H2O)mn (n=1–4 andm=0,4,5) complexes were determined. In each of the considered series the spin, population on the central metal atom decreases as its atomic number increases. The energy of transition of the unpaired electron to the lowest unoccupied MO decreases in the same order. The unpaired electron occupies an orbital that is mostly a linear combination of the s-orbital of the metal atom and the p-orbital of the Cl atom (the antibonding σ-orbital of the M−Cl bond). Distinctions in the changes in spectral properties of aquacomplexes and chloride complexes in isoelectronic series, observed as the degree of oxidation of the metal atom increases, were explained. The results of calculations are in agreement with the experimental data obtained by ESR and optical spectroscopy.

Aquacomplexes of ions of group IB−IVB metals in anomalous oxidation states (2S1/2 electron state): a theoretical study

Ab initio calculations of aquacomplexes of the ions of Group IB−IVB metals in anomalous oxidation states (2S1/2 electron state) were carried out by the Becke-Le-Yang-Parr (BLYP) density functional method using the Dunning-Hay DZ basis set. Optimum geometric parameters and electronic characteristics were determined of aquacomplexes M(H2O)nz+, where Mz+=Au0 (n=2, 4); Ag0, Cu0 (n=2, 4, 6); Zn1+, Cd1+, Hg1+ (n=4, 6); Ga2+, In2+, Tl2+ (n=6); and Ge3+, Sn3+, Pb3+ (n=6). The spin corresponding periods. The energy of transition of the unpaired electron to the first vacant level increases in the same order. The results of calculations are in agreement with experimental data obtained by ESR and optical spectroscopy.

Isotopic effects in the electronic spectra of tryptophan

Summary.No influence of isotopic substitution in deuterium-substituted tryptophan on the florescence excitation spectrum has previously been found out. Here, the isotopic effects of electronic excitation of deuterium-substituted tryptophan were experimentally and theoretically analyzed for first time. It was shown a short-wave shift of the UV-absorption maximum at 220u2009nm corresponding to the 360u2009cal/mol and short-wave shift for fluorescence spectrum corresponding to the 210u2009cal/mol. To account for this effect, the quantum chemical calculations of the geometric and electron structure, frequencies of normal vibrations and transition energies have been performed. The isotopic effects originate from the zero-point energies of ground and excited states. It was found that isotopic shifts depend on the position of isotope in the molecule and kind of transition. So, it can be utilized in the analysis of proteins structure and complexation.

Quantum-chemical calculation of a spillover model on a graphite support

The simplest quantum-chemical models of hydrogen spillover over a graphite-like surface as a proton or radical have been considered. The condensed planar C24H12 molecule was used as a model surface. Theab initio calculations of the interaction of hydrogen with the model surface were carried out by the restricted Hartree-Fock (HF) method in the STO-3G and 6-31 G* basis sets. The radical hydrogen can not bind to such a surface, whereas the proton binds to it with an energy release of 186 kcal mol−1. The activation energy of the transfer of the proton between two neighboring carbon atoms (10 kcal mol−1) has been determined. The simplest model of the hydrogen migration as a proton over the model surface can be used for describing the spillover of hydrogen over the graphite surface.

Effect of substituents on electronegativities of Ar3Si and Ar3Sn groups: comparison of the results ofab initio quantum-chemical calculation and19F NMR data for Ar3MQC6H4F-4 compounds

Ab initio calculations of group electronegativities (χ) of Ar3Si and Ar3Sn groups containing 13 types ofmeta-, para- and polysubstituted phenyl groups have been performed. Calculated values of χ(Ar3Si) and χ(Ar3Sn) correlate better with the σ0 Taft constants than with the σ Hammett constants, which is indicative of the inductive nature of the effect of aryl groups on electronegativities of Ar3M groups. Good correlations have been found between19F chemical shifts and the corresponding values of χ(Ar3Si) and χ(Ar3Sn) for the Ar3SiC6H4F-4, Ar3SnC6H4F-4, Ar3SnCH2C6H4F-4, and Ar3SnSC6H4F-4 compounds.

Nature of Fe—CH2 bonds in ferrocenylmethyl and ferrocenylenedimethyl cations

Quantum-chemical calculations of ferrocenylmethyl ([C5H5FeC5H4CH2]+) and ferrocenylenedimethyl ([C5H5FeC5H3(CH2)2]2+) cations with full geometry optimization were carried out using the Hartree—Fock (HF) approximation, density functional theory (DFT), and at the second-order Møller—Plesset (MP2) level of perturbation theory in the 6-311G* basis set. The methods with inclusion of electron correlation in explicit form indicate that the CH2groups deviate from the cyclopentadienyl ring planes toward the Fe atom due to formation of the Fe—CH2bonds. According to Hartree—Fock calculations, ligands in these ions are virtually planar. The metallonium character of the ions studied was demonstrated based on the results of analysis of the electron density distribution and frontier orbitals.