Single Crystals of L-O-Serine Phosphate X-Irradiated at Low Temperatures: EPR, ENDOR, EIE, and DFT Studies
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- 作者：Kjell Tage Ø ; hman ; Audun Sanderud ; Eli Olaug Hole ; Einar Sagstuen
- 刊名：Journal of Physical Chemistry A
- 出版年：2006
- 出版时间：August 10, 2006
- 年：2006
- 卷：110
- 期：31
- 页码：9585 - 9596
- 全文大小：378K
- 年卷期：v.110,no.31(August 10, 2006)
- ISSN：1520-5215
文摘
Single crystals of the phosphorylated amino acid L-O-serine phosphate were X-irradiated and studied at 10K and at 77 K using EPR, ENDOR, and EIE techniques. Two radicals, R1(10 K) and R1(77 K), were detectedand characterized as two different geometrical conformations of the protonated reduction product >CH-
(OH)2. R1(10 K) is only observed after irradiation at 10 K, and upon heating to 40 K, R1(10 K) transformsrapidly and irreversibly into R1(77 K). The transition from R1(10 K) to R1(77 K) strongly increases theisotropic hyperfine coupling of the -CH
coupling (
= 32 MHz) and the major -OH coupling ( =47 MHz), in sharp contrast to the their much reduced anisotropic hyperfine couplings after the transition. Anumbrella-like inversion of the carboxylic acid center, accompanied by minor geometrical adjustments, explainsthe changes of these observed isotropic and anisotropic couplings. DFT calculations were done on the reducedand protonated L-O-serine phosphate radical at the B3LYP/6-311+G(2df,p)//B3LYP/6-31+G(d) level of theoryin order to support the experimental observations. Two different conformations of the anion radical, relatedby an inversion at the carboxylic center, could be found within the single molecule partial energy-optimizationscheme. These two conformations reproduce the experimental hyperfine couplings from radicals R1(10 K)and R1(77 K). A third radical, radical R2, was observed experimentally at both 10 and 77 K and was shownto be due to the decarboxylated L-O-serine phosphate oxidation product, a conclusion fully supported fromthe DFT calculations. Upon thermal annealing from 77 to 295 K, radicals R1(77 K) and R2 disappeared andall three previously observed room-temperature radicals could be observed. No phosphate-centered radicalscould be observed at any temperatures, indicating that the phosphate-ester bond break for one of the room-temperature radicals does not occur by dissociative electron capture at the phosphate group.
(OH)2. R1(10 K) is only observed after irradiation at 10 K, and upon heating to 40 K, R1(10 K) transformsrapidly and irreversibly into R1(77 K). The transition from R1(10 K) to R1(77 K) strongly increases theisotropic hyperfine coupling of the -CH coupling ( = 32 MHz) and the major -OH coupling ( =47 MHz), in sharp contrast to the their much reduced anisotropic hyperfine couplings after the transition. Anumbrella-like inversion of the carboxylic acid center, accompanied by minor geometrical adjustments, explainsthe changes of these observed isotropic and anisotropic couplings. DFT calculations were done on the reducedand protonated L-O-serine phosphate radical at the B3LYP/6-311+G(2df,p)//B3LYP/6-31+G(d) level of theoryin order to support the experimental observations. Two different conformations of the anion radical, relatedby an inversion at the carboxylic center, could be found within the single molecule partial energy-optimizationscheme. These two conformations reproduce the experimental hyperfine couplings from radicals R1(10 K)and R1(77 K). A third radical, radical R2, was observed experimentally at both 10 and 77 K and was shownto be due to the decarboxylated L-O-serine phosphate oxidation product, a conclusion fully supported fromthe DFT calculations. Upon thermal annealing from 77 to 295 K, radicals R1(77 K) and R2 disappeared andall three previously observed room-temperature radicals could be observed. No phosphate-centered radicalscould be observed at any temperatures, indicating that the phosphate-ester bond break for one of the room-temperature radicals does not occur by dissociative electron capture at the phosphate group.