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Yoong-Kee Choe

Publications and source records attributed to Yoong-Kee Choe.

6 recordsLinked to original sources
PubMed2007

Theoretical study of the electronic spectra of oxidized and reduced states of lumiflavin and its derivative.

Time-dependent density functional theory has been applied to investigate the electronic absorption spectrum of oxidized and reduced lumiflavin and its derivative, 8-NH(2)-lumiflavin. The calculations allow the authors to explain the origin of the difference in spectral features between oxidized and reduced states of lumiflavin. For the reduced lumiflavin, a reasonable assignment of the experimental spectrum has been made for the first time. Furthermore, the results obtained reveal that the NH(2) group plays a critical role in shaping the spectral features of 8-NH(2)-lumiflavin, and offer a reasonable explanation for the spectral changes upon substituting the NH(2) group for the CH(3) group of lumiflavin.

Computational Biology↗
PubMed2005

Effect of the axial cysteine ligand on the electronic structure and reactivity of high-valent iron(IV) oxo-porphyrins (Compound I): a theoretical study.

The effect of axial ligands on the reactivity of high-valent iron(IV) oxo-porphyrins (Compound I) was investigated using the B3LYP hybrid density functional method. We studied alkane hydroxylation using four models: Compound I with thiolate, imidazole, phenolate, and chloride anions as axial ligands. The first three ligands were employed as models for cysteinate, histidine, and tyrosinate, respectively. Our calculations show that anionic ligands and neutral ligands favor different electronic states for stationary points in the reaction coordinate, and the calculated energy barrier and energy of several reaction intermediates show similar values. A remarkable effect of axial ligands was found in the final product release step. Our calculations show that the thiolate ligand weakens a bond between heme and an alcohol. In contrast, the imidazole ligand significantly increases the interaction between heme and an alcohol, which causes the catalytic cycle to be less efficient.

Computer Simulation↗
PubMed2004

Isolation of a Se-nitrososelenol: a new class of reactive nitrogen species relevant to protein Se-nitrosation.

Nitric oxide (NO) is a messenger molecule implicated in a number of physiological processes. Nitrosation of selenoproteins has been suggested as playing an important role in NO-mediated cellular functions such as the inactivation of glutathione peroxidase (GPx), but no chemical information about Se-nitrosated species has been available to date. Here a stable Se-nitrososelenol (RSeNO), a new class of NO derivative, was synthesized and fully characterized by X-ray crystallography and spectroscopic methods. This Se-nitrososelenol can be formed by direct transnitrosation from an S-nitrosothiol to a selenol, as is the case in the proposed mechanism for the NO-mediated inactivation of GPx.

Crystallography, X-Ray↗
PubMed2003

Effect of substituents on the thermal decomposition of diazirines: experimental and computational studies.

The thermal decomposition of phenylchlorodiazirine (1), phenyl-n-butyldiazirine (2), and 2-adamantane-2,3'-[3H]diazirine (3) has been studied in solution in the presence of C(60). The C(60) probe technique indicates that in the decomposition diazirine 1 yielded exclusively phenylchlorocarbene, diazirine 2 yielded mainly a diazo intermediate, and diazirine 3 yielded a mixture of carbene and diazo compound. In the case of diazirine 2, 13% of (E)-1-phenyl-1-pentene resulted from the direct thermal rearrangement of diazirine without the participation of a carbene. As well, the thermal decomposition of these diazirines has been studied theoretically with ab initio and density functional methods. The experimental results are broadly in agreement with the theoretical predictions. The calculations further indicate that the rebound reaction between carbene and molecular nitrogen leading to the formation of a diazo intermediate is an important reaction in the gas-phase decomposition of diazirine.

Journal Article↗
PubMed2002

Intruder state avoidance multireference Møller-Plesset perturbation theory.

A new perturbation approach is proposed that enhances the low-order, perturbative convergence by modifying the zeroth-order Hamiltonian in a manner that enlarges any small-energy denominators that may otherwise appear in the perturbative expansion. This intruder state avoidance (ISA) method can be used in conjunction with any perturbative approach, but is most applicable to cases where small energy denominators arise from orthogonal-space states-so-called intruder states-that should, under normal circumstances, make a negligible contribution to the target state of interests. This ISA method is used with multireference Møller-Plesset (MRMP) perturbation theory on potential energy curves that are otherwise plagued by singularities when treated with (conventional) MRMP; calculation are performed on the 1(3)Sigma(-)(u) state of O(2); and the 2(1)Delta, 3(1)Delta, 2(3)Delta, and 3(3)Delta states of AgH. This approach is also applied to other calculations where MRMP is influenced by intruder states; calculations are performed on the (3)Pi(u) state of N(2), the (3)Pi state of CO, and the 2(1)A' state of formamide. A number of calculations are also performed to illustrate that this approach has little or no effect on MRMP when intruder states are not present in perturbative calculations; vertical excitation energies are computed for the low-lying states of N(2), C(2), CO, formamide, and benzene; the adiabatic (1)A(1)-(3)B(1) energy separation in CH(2), and the spectroscopic parameters of O(2) are also calculated. Vertical excitation energies are also performed on the Q and B bands states of free-base, chlorin, and zinc-chlorin porphyrin, where somewhat larger couplings exists, and-as anticipated-a larger deviation is found between MRMP and ISA-MRMP.

Journal Article↗