Molecular organization in the twist-bend nematic phase by resonant X-ray scattering at the Se K-edge and by SAXS, WAXS and GIXRD.

Using a magnetically aligned liquid crystal mixture containing a novel Se-labelled dimer and the difluoroterphenyl dimer DTC5C7, the twist-bend nematic phase (Ntb) was studied by the resonant scattering of hard X-rays and by conventional small and wide-angle X-ray scattering (SAXS, WAXS). Resonant diffraction spots indicated a helix with a 9-12 nm pitch in the Ntb phase and an unprecedentedly high helix orientation. This enabled deconvolution of global and local order parameters. These findings, combined with the simultaneously recorded resonant and non-resonant SAXS and WAXS data, allowed us to construct a locally layered molecular model of the Ntb phase, where the average twisted conformation of each molecule was idealised as a helical segment, matching the local heliconical director field. The dimers were found to be less bent in the Ntb phase than in their minimum energy conformation, and straightening further with increasing temperature. It is proposed that on further heating their low bend angle allows the transition to the normal nematic phase, where the molecules can freely move longitudinally, without the need to perform screw-like motion as in the Ntb phase. At the low-temperature end, the increasing molecular twist becomes unsustainable, leading to a transition to a smectic phase, where no twist is required.

Evidence against the existence of the purported Saccharomyces cerevisiae PKC2 gene.

BACKGROUND: The existence of a Saccharomyces cerevisiae gene encoding a novel isoform of protein kinase C was reported recently in this journal. RESULTS: We demonstrate here that, firstly, the purported PKC2 gene does not reside at the chromosomal location to which it was assigned; secondly, it does not exist as a contiguous sequence in the S. cerevisiae genome; thirdly, some of its reported sequences do exist within other yeast genes; and fourthly, some of its reported sequences, encoding regions of the predicted protein related to protein kinase C, do not exist in any context in the yeast genome. CONCLUSIONS: We conclude from these studies that the PKC2 gene is a composite construction of unrelated yeast and non-yeast sequences.

Carcinoma of the thyroglossal duct.

A review of 39 instances of excision of a cyst of the thyroglossal duct performed at St. Paul Medical Center, Dallas, Texas, revealed two patients with carcinoma of the thyroglossal duct. A review of the English literature yielded 146 instances of this uncommon tumor. Eighty-four per cent were papillary adenocarcinoma of thyroid type and 5 per cent were squamous cell carcinoma. The patients were from six to 81 years old with a 2:1 female to male ratio. Metastatic disease to lymph nodes was noted in 11 per cent and invasion of overlying strap muscles was found in 4 per cent. Carcinoma of the thyroid gland occurred in 14 per cent. Preoperative diagnosis was rare. The Sistrunk procedure is recommended for initial surgical therapy with further surgical or adjuvant therapy dependent on associated clinical findings. Prognosis for carcinoma of the thyroglossal duct of thyroid type parallels that of carcinoma of the thyroid gland.