Polyhydroxyalkanoates (PHA) are storage polymers accumulated by numerous prokaryotes in form of intracellular granules. Native PHA granules are formed by amorphous polymer which reveals considerably higher elasticity and flexibility as compared to crystalline pure PHA polymers. The fact that bacteria store PHA in amorphous state has great biological consequences. It is not clear which mechanisms protect amorphous polymer in native granules from transition into thermodynamically favorable crystalline state. Here, we demonstrate that exposition of bacterial cells to particular stressors induces granules aggregation, which is the first but not sufficient condition for PHA crystallization. Crystallization of the polymer occurs only when the stressed bacterial cells are subsequently dried. The fact that both granules aggregation and cell drying must occur to induce crystallization of PHA indicates that both previously suggested hypotheses about mechanisms of stabilization of amorphous state of native PHA are valid and, in fact, both effects participate synergistically. It seems that the amorphous state of the polymer is stabilized kinetically by the low rate of crystallization in limited volume in small PHA granules and, moreover, water present in PHA granules seems to function as plasticizer protecting the polymer from crystallization, as confirmed experimentally for the first time by the present work.

ARENA Arbeitsgemeinschaft für Ressourcenschonende and Nachhaltige Technologien Inffeldgasse 21b 8010 Graz Austria

Biology Centre The Czech Academy of Sciences v v i Branisovska 31 370 05 Ceske Budejovice Czech Republic

Faculty of Chemistry Brno University of Technology Purkynova 118 612 00 Brno Czech Republic

Faculty of Science University of South Bohemia Branisovska 31 370 05 Ceske Budejovice Czech Republic

Institute of Chemistry NAWI Graz University of Graz Heinrichstrasse 28 3 8010 Graz Austria

Institute of Scientific Instruments The Czech Academy of Sciences v v i Kralovopolska 147 612 64 Brno Czech Republic

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