Spin dynamics of hydrothermally synthesized δ-MnO2 nanowhiskers.

We have reported novel 2D monoclinic, P63/mnm, δ-MnO2 nanowhiskers synthesized through a simple and facile hydrothermal route under optimized conditions without using any template. The X-ray diffraction pattern shows the formation of the δ phase of MnO2, which is further confirmed by Fourier transform infrared (FT-IR) spectroscopy and Raman spectroscopy. The transmission electron micrograph revealed nanowhiskers having a diameter of ∼7 nm and the high-resolution TEM and SAED patterns demonstrated the interplanar spacing and distinguished diffraction rings corresponding to the monoclinic phase of δ-MnO2. Fitting the temperature-dependent susceptibility with the Curie-Weiss law confirms the strong antiferromagnetic ordering and high effective magnetic moment of Mn4+ present in δ-MnO2. The large effective magnetic moment is attributed to the presence of both Mn3+ and Mn4+, as confirmed by XPS. The reduced valency of Mn from 4 to 3 is accompanied with oxygen vacancies, affording the exact composition of MnO1.58. The dynamic magnetic properties of the δ-MnO2 nanowhiskers were investigated using the frequency-dependent AC susceptibility fitted with various phenomenological models like the Vogel-Fulcher law and power law, indicating the existence of interacting spin clusters, which could freeze at ∼11.2 K. The time dependence of thermoremanent magnetization fitted well with a stretched exponential function, supporting the existence of relaxing spin clusters. Thus, the spin glass relaxation in the δ-MnO2 nanowhiskers is attributed to the interaction between Mn4+ and Mn3+, which results in intrinsic magnetic frustration and weak ferromagnetism with finite coercivity below Tf.

A systematic classification of Plasmodium falciparum P-loop NTPases: structural and functional correlation.

BACKGROUND: The P-loop NTPases constitute one of the largest groups of globular protein domains that play highly diverse functional roles in most of the organisms. Even with the availability of nearly 300 different Hidden Markov Models representing the P-loop NTPase superfamily, not many P-loop NTPases are known in Plasmodium falciparum. A number of characteristic attributes of the genome have resulted into the lack of knowledge about this functionally diverse, but important class of proteins. METHOD: In the study, protein sequences with characteristic motifs of NTPase domain (Walker A and Walker B) are computationally extracted from the P. falciparum database. A detailed secondary structure analysis, functional classification, phylogenetic and orthology studies of the NTPase domain of repertoire of 97 P. falciparum P-loop NTPases is carried out. RESULTS: Based upon distinct sequence features and secondary structure profile of the P-loop domain of obtained sequences, a cladistic classification is also conceded: nucleotide kinases and GTPases, ABC and SMC family, SF1/2 helicases, AAA+ and AAA protein families. Attempts are made to identify any ortholog(s) for each of these proteins in other Plasmodium sp. as well as its vertebrate host, Homo sapiens. A number of P. falciparum P-loop NTPases that have no homologue in the host, as well as those annotated as hypothetical proteins and lack any characteristic functional domain are identified. CONCLUSION: The study suggests a strong correlation between sequence and secondary structure profile of P-loop domains and functional roles of these proteins and thus provides an opportunity to speculate the role of many hypothetical proteins. The study provides a methodical framework for the characterization of biologically diverse NTPases in the P. falciparum genome.The efforts made in the analysis are first of its kind; and the results augment to explore the functional role of many of these proteins from the parasite that could provide leads to identify novel drug targets against malaria.