Academic literature on the topic 'ADN viral – Synthèse'

After almost 30 years since the first harvesting of the saphenous vein graft with a pedicle of surrounding tissue (no-touch technique), there is no doubt that this method is superior to the conventional technique in which a denuded vein is harvested. In summary, the no-touch harvesting technique decreases risk of graft spasm and the requirement for manual dilatation, limiting endothelial cell loss and long-term damage. The preservation of the vasa vasorum allows retrograde blood flow from the graft lumen, thereby decreasing transmural ischaemic damage. This also preserves endothelial nitric oxide synthase, known to decrease intimal hyperplasia, atherosclerosis, and long-term graft failure. Furthermore, the perivascular tissue acts as a natural external stent reducing the neointimal and medial thickening of the vein graft and preventing it from kinking, which is especially vital for sequential grafts. Finally, target vessel size, quality, and degree of stenosis have little effect on the patency rates of no-touch saphenous vein grafts, in contrast to arterial grafts.

In some respects this project was the successor to the research published in 2007 as The Prehistory of Britain and Ireland, but there are significant contrasts between the books. The results of development-led archaeology have played a central role in both, but they have influenced their contents in different ways. When the earlier book was published it was among the first to draw extensively on fieldwork undertaken as part of the planning process. To some extent the course of that research was unpredictable, for it was not clear how far the results of the new excavations and surveys would diverge from what was already known. All that was certain from the outset was that a large amount of new information had been collected and that very little of it had entered the public domain. There was a disparity between the conventional archaeological literature—journal articles, monographs, and regional syntheses—and the great majority of reports, which were prepared for planning authorities and commercial clients. Those documents were difficult to trace and sometimes difficult to access. What the project showed was that such sources were vital to any understanding of the past. It also demonstrated that at least some of the orthodoxies on which public policy depended were inconsistent with the results of work that had already taken place. The same problem affected teaching and research, for they rarely took account of the new sources of information. In retrospect, the earlier project may have influenced later research in a way that had not been foreseen. It did not, and could not, offer a completely new version of British and Irish prehistory, as it was written at a time when many excavations were still in progress—the fieldwork associated with road-building in Ireland is a good example. In any case the dissemination of information in the archaeology of these islands was so inefficient that particularly in England it was difficult to find out what had been done. Tracing the results was an even harder task, and it was not completely successful.

The German chemist Friedrich Wöhler is generally credited with the first laboratory synthesis of a known organic compound (urea) from inorganic materials. He accomplished this by the simple heating of an inorganic salt, ammonium cyanate (NH4OCN). “I must tell you,” he wrote to his mentor Jöns Jakob Berzelius in 1828, “that I can prepare urea without requiring a kidney of an animal, either man or dog.” While this report may seem relatively minor given the structural simplicity of urea, its impact was revolutionary. For the first time, the preparation and isolation of an organic compound had been achieved in the absence of the elemental “vital force” of living systems previously believed to be required for the construction of all such compounds. This milestone of 19th century organic chemistry was later followed by many others, including Kolbe’s synthesis of acetic acid in 1847 and Fischer’s synthesis of glucose in 1890. With the support of evolving methods for compound separation, purification, and spectroscopic analysis, rapid advances in the sophistication of organic synthesis followed throughout the 20th century, developing in tandem with an ever-deepening understanding of the underlying organic processes associated with living systems. While it is certainly true that syntheses of many structurally complex unnatural compounds of theoretical interest are also among the most remarkable achievements in synthetic strategy, tactical execution, and perseverance, the realm of natural products remains the dominant source for the most challenging and potentially beneficial targets available for such synthetic efforts. Figure 8.1 shows a small selection of some natural (and unnatural) products which have been produced via synthesis over the years, from Wöhler’s time to the present. Note the increasing levels of structural sophistication and stereochemical complexity that have eventually been mastered by practitioners of organic synthesis. In our own time, the traditional boundaries between organic and biological chemistry are disappearing in ways that are likely to transform the design and synthesis of organic molecules, from the construction of synthetic biologicals designed to act as biomarkers, biosensors, or drug delivery agents, to the development of molecular motors, self-replicating macromolecular systems, and even synthetic life forms.