Calcium in the oxygen-evolving complex: structural and mechanistic role determined by X-ray spectroscopy

This review describes the results from X-ray Absorption Spectroscopy studies that have contributed to an understanding of the role of Ca in the photosynthetic water-oxidation reaction. The results include the first Mn, Ca and Sr X-ray spectroscopy studies using Ca or Sr-substituted PS II samples that established the presence of a MnCa heteronuclear structure and its orientation, and the most recent Sr X-ray spectroscopy study using biosynthetically prepared Sr-containing PS II in the various S-states that provide important insights into the requirement for Ca in the mechanism of the Mn(4)Ca catalytic center.     

Prominent signal intensity of T1/T2 prolongation in subcortical white matter of the anterior temporal region on conventional screening MRI of late preterm infants with normal development

Purpose

This study discusses prominent signal intensity of T₁/T₂ prolongation of subcortical white matter within the anterior temporal region in premature infant brains that radiologists may encounter when interpreting conventional screening MRIs.

Materials and Methods

T₁- and T₂-weighted images of 69 preterm and term infants with no neurological abnormalities or developmental delays were evaluated retrospectively for areas of prominent signal intensity of T₁/T₂ prolongation in white matter. We measured signal intensities of anterior temporal white matter, deep temporal white matter, frontopolar white matter and subcortical white matter of the precentral gyrus. We accessed chronological changes in signal intensity in the anterior and deep temporal white matter. We also analyzed variance tests among the signal intensity ratios to the ipsilateral thalamus of white matter areas by gestational age.

Results

There was high frequency of prominent signal intensity of T₁/T₂ prolongation in the temporal tip, particularly at a gestational age of 36–38 weeks. Signal intensity ratio of the anterior temporal white matter was lower on T₁-weighted images and higher on T₂-weighted images, and the finding became less prominent with increasing gestational age. The signal intensity ratios of anterior temporal white matter at a gestational age of 36–37 weeks and 38–39 weeks were significantly different from other regions.

Conclusion

Prominent signal intensity of T₁/T₂ prolongation of subcortical white matter of the anterior temporal region is seen in normal premature infants, especially those at 36–39 gestational weeks. Although it is a prominent finding, radiologists should understand that these findings do not represent a pathological condition.     

Intramolecular ring opening of a 2,3-epoxy alcohol by a xanthate anionic center; stereospecific preparation of 2-mercapto-1,3-diol units

Stereospecific ring openings of optically active 2,3-epoxy alcohols were performed by the reaction of 1 , 3 , 5 , and 7 with carbon disulfide and sodium hydride to give the five-membered xanthates 2 , 4 , 6 , and 8 . Both enantiomers of 2-mercapto-1,3-diol triacetates, 11 and 14 , were derived from 4 and 6 , respectively. The ring opening reaction proceeded at −78°C to −30°C, and the yields were around 80%. However, at a higher temperature between 0°C to room temperature, a complicated reaction took place and led to the formation of two isomers of the cyclic thiol carbonates 15 and 16 from 1 or 5 . These processes were also stereospecific, and mechanisms have been proposed. In the case of the 3,4-epoxy alcohol 20 , the epoxide ring opening gave the six-membered xanthate 21 stereospecifically.