Adenine as an organocatalyst for the ring-opening polymerization of lactide: scope, mechanism and access to adenine-functionalized polylactide

Nucleobase-functionalized polymers are widely used in the fields of supramolecular chemistry and self-assembly, and their development for biomedical applications is also an area of interest. They are usually synthesized by tedious multistep procedures. In this study, we assess adenine as an organoinitiator/ organocatalyst for the ring-opening polymerization of lactide. L-Lactide can be quantitatively polymerized in the presence of adenine. Reaction conditions involving short reaction times and relatively low temperatures enable the access to adenine end-capped polylactide in a simple one-step procedure, in bulk, without additional catalyst. DFT calculations show that the polymerization occurs via hydrogen bond catalysis. The mechanism involves (i) a hydrogen bond between the NH9 of adenine and the carbonyl moiety of lactide, leading to an electron deficient carbon atom, and (ii) a second hydrogen bond between the N3 of adenine and the NH2 of a second adenine molecule, followed by a nucleophilic attack of the latter activated amine on the former electron deficient carbon on the monomer. For longer reaction times and higher temperatures, macrocyclic species are formed, and a mechanism involving the imidazole ring of adenine is proposed based on literature studies. Depending on the reaction conditions, adenine can thus be considered as an organoinitiator or an organocatalyst for the ring-opening polymerization of lactide.

Adenine as an organocatalyst for the ring-opening polymerization of lactide: scope, mechanism and access to adenine-functionalized polylactide

Nucleobase-functionalized polymers are widely used in the fields of supramolecular chemistry and self-assembly, and their development for biomedical applications is also an area of interest. They are usually synthesized by tedious multistep procedures. In this study, we assess adenine as an organoinitiator/organocatalyst for the ring-opening polymerization of lactide. L-Lactide can be quantitatively polymerized in the presence of adenine. Reaction conditions involving short reaction times and relatively low temperatures enable the access to adenine end-capped polylactide in a simple one-step procedure, in bulk, without additional catalyst. DFT calculations show that the polymerization occurs via hydrogen bond catalysis. The mechanism involves (i) a hydrogen bond between the NH9 of adenine and the carbonyl moiety of lactide, leading to an electron deficient carbon atom, and (ii) a second hydrogen bond between the N3 of adenine and the NH2 of a second adenine molecule, followed by a nucleophilic attack of the latter activated amine on the former electron deficient carbon on the monomer. For longer reaction times and higher temperatures, macrocyclic species are formed, and a mechanism involving the imidazole ring of adenine is proposed based on literature studies. Depending on the reaction conditions, adenine can thus be considered as an organoinitiator or an organocatalyst for the ring-opening polymerization of lactide.

DFT STUDIES ON THE MECHANISM OF RING-OPENING POLYMERIZATION OF LACTIDE WITH ADENINE

The use of organic molecules as catalysts for the ring-opening polymerization (ROP) of cyclic esters has gained much interest last years.[1] The use of a molecule of biological interest, able to initiate ROP of cyclic esters without any cocatalyst is even more interesting, as the resulting material will not contain any catalytic residue. Nucleobase-polymer conjugates development is thus an emerging area envisaging biomedical applications.[2] However, they are usually synthesized by tedious multistep procedures. Recently, adenine was used as organoinitiator for the ROP of L-lactide.[3] Reaction conditions involving short reaction times and relatively low temperatures enable the access to adenine-polylactide(Adn-PLA)conjugates in a simple one-step procedure, without additional catalyst and in the absence of solvent. In this study, computational investigations with density functional theory (DFT) were performed in order to clarify the reaction mechanism leading to the desired Adn-PLA. The results show that a hydrogen bond catalytic mechanism, involving a nucleophilic attack of the activated amine group of adenine onto the carbonyl group of lactide, seem to be plausible.

Global transcriptional response to salt shock of the plant microsymbiont Mesorhizobium loti MAFF303099

Soil salinity affects rhizobia both as free-living bacteria and in symbiosis with the host. The aim of this study was to examine the transcriptional response of the Lotus microsymbiont Mesorhizobium loti MAFF303099 to salt shock. Changes in the transcriptome of bacterial cells subjected to a salt shock of 10% NaCl for 30 min were analyzed. From a total of 7231 protein-coding genes, 385 were found to be differentially expressed upon salt shock, among which 272 were overexpressed. Although a large number of overexpressed genes encode hypothetical proteins, the two most frequently represented COG categories are "defense mechanisms" and "nucleotide transport and metabolism". A significant number of transcriptional regulators and ABC transporters genes were upregulated. Chemotaxis and motility genes were not differentially expressed. Moreover, most genes previously reported to be involved in salt tolerance were not differentially expressed. The transcriptional response to salt shock of a rhizobium with low ability to grow under salinity conditions, but enduring a salinity shock, may enlighten us concerning salinity stress response mechanisms.