Irradiation of the complexes [Re(R)(CO)3(dmb)] (R = CH3, CD3, Et, i Pr, or Bz; dmb = 4,4‘-dimethyl-2,2‘-bipyridine) into their visible absorption band gives rise to a homolytic cleavage of the Re−R bond with formation of the radicals [Re(CO)3(dmb)]• and R•. In the case of R = Et, i Pr, or Bz this reaction proceeds with unit efficiency. The nanosecond time-resolved absorption (TA) spectra show that the long-lived (τ = 7 μs) [Re(CO)3(dmb)]• radicals are formed within the 7 ns laser pulse. The CH3 complex photodecomposes with a quantum yield of only 0.4. The time-resolved UV−vis and IR absorption spectra reveal that this complex, after excitation into a 1MLCT state, may either pass a barrier of 1560 cm-1 to the dissociative 3σπ* state and decompose into radicals or decay to the ground state via an excited-state having predominant 3MLCT state character. Qualitative potential energy diagrams are presented for the two types of complexes. In a glass at 80 K and in a low-temperature solution (T < 195 K), the methyl complex is photostable, and this allowed us to study its excited-state properties with time-resolved absorption, emission, and IR spectroscopies. According to these spectra the lowest-excited 3MLCT state has a significant admixture of a σπ* character. Finally, nanosecond time-resolved FT-EPR spectra were recorded of the CH3 and CD3 radicals produced by irradiation of [Re(CH3/CD3)(CO)3(dmb)]. The spectra of the CH3 radical exhibit a pronounced low-field emission/high-field absorption pattern due to an ST0 radical pair mechanism (RPM) CIDEP effect. The occurrence of this ST0 RPM confirms that the radicals are formed from an excited state, having spin-triplet character, in agreement with the proposed mechanism involving a reactive 3σπ* state.
The Escherichia coli ribonucleotide reductase (RNR) catalyzes the conversion of nucleoside diphosphates to deoxynucleotides and requires a diferric-tyrosyl radical cofactor for catalysis. RNR is composed of a 1:1 complex of two homodimeric subunits: α and β. Incubation of the E441Q-α mutant RNR with substrate CDP and allosteric effector TTP results in loss of the tyrosyl radical and formation of two new radicals on the 200 ms to min time scale. The first radical was previously established by stopped flow UV/vis spectroscopy and pulsed high field EPR spectroscopy to be a disulfide radical anion. The second radical was proposed to be a 4′-radical of a 3′-keto-2′-deoxycytidine 5′-diphosphate. To identify the structure of the nucleotide radical [1′-2H], [2′-2H], [4′-2H], [5′-2H], [U−13C, 15N], [U−15N], and [5,6 -2H] CDP and [β-2H] cysteine-α were synthesized and incubated with E441Q-α2β2 and TTP. The nucleotide radical was examined by 9 GHz and 140 GHz pulsed EPR spectroscopy and 35 GHz ENDOR spectroscopy. Substitution of 2H at C4′ and C1′ altered the observed hyperfine interactions of the nucleotide radical and established that the observed structure was not that predicted. DFT calculations (B3LYP/IGLO-III/B3LYP/TZVP) were carried out in an effort to recapitulate the spectroscopic observations and lead to a new structure consistent with all of the experimental data. The results indicate, unexpectedly, that the radical is a semidione nucleotide radical of cytidine 5′-diphosphate. The relationship of this radical to the disulfide radical anion is discussed.
Please cite this article as: Bortolato SA, Thomas KE, McDonough K, Gurney RW, Martino DM, Evaluation of photo-induced crosslinking of thymine polymers using FT-IR spectroscopy and chemometric analysis, Polymer (2012), doi: 10.1016/j.polymer.2012.09.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 2 ABSTRACTThe photo-induced immobilization of 4-vinylbenzyl thymine (VBT) and 4-vinylbenzyl triethyl ammonium chloride (VBA) copolymers has been investigated with the aid of grazing-angle specular-reflectance FT-IR spectroscopy. As irradiation time increases, changes in the structure of pendant thymine groups in the copolymer due to the crosslinking process result in shifts of the copolymer network's vibrational frequencies, as well as changes in numerous IR band intensities.In this work, chemometric methods have been applied to the FTIR data obtained while
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