Tender morsels for microbes
Tender morsels for microbes
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DOI：
10.1038/360535a0
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发表时间：
1992-12
期刊：
影响因子：
64.8
通讯作者：
P. Calvert
中科院分区：
文献类型：
--
作者：
P. Calvert
POLYI (R)-3-HYDROXYBUIYRATE] is a microbial storage polyester which forms in intracellular granules about 0.5 micrometres in diameter. The polymer has been commercialized as a biodegradable fibre-forming material with properties similar to those of polypropylene. Curiously, in vivo the polymer is soft and readily degradable, but after extraction it becomes a tough, stiff fibre. Why this change occurs has led to a disagreement. Sanders and co-workers, at Cambridge, suggestL2 that some clever biological plasticization occurs in vivo, but de Koning and Lemstra argue3 that the small particles found in microbes are simply too small to crystallize and remain rubbery even when 150 oc below the melting point.To put the difference in perspective it is worth considering the plant polymers cellulose and starch, which are both based on glucose. Cellulose, whose function is structural, has a regular arrangement with/)-linked glucose units, which allow the polymer chains to be densely packed and highly crystalline. The impenetrability of the structure makes chemical and enzymatic digestion very slow. The a-linkage in starch, the energy-storage version of the polymer, drives the chain into an open helix, and frequent chain branching in the amylopectin fraction prevents extensive crystallization. The resulting material is soluble in water and readily degradable. Against this background, one would hardly expect poly [(R]-3-hydroxybutyrate](PHB) to be an unbranched and readily crystallizable polymer, if it is to serve its biological function. But its behaviour when extracted shows otherwise. ln vivo, some mechanism must operate to prevent crystallization. Sanders and colleagues have shown 1 that PHB is an amorphous rubber in its native state within cells of Alcaligenes eutrophus. Carbon-13 NMR spectra show narrow lines, and measurements of spin-lattice relaxation times,(Tt), correspond to those of a rubbery polymer with a glass transition temperature of
