Abstract-Graphene and some graphene like two dimensional materials; hexagonal boron nitride (hBN) and silicene have unique mechanical properties which severely limit the suitability of conventional theories used for common brittle and ductile materials to predict the fracture response of these materials. This study revealed the fracture response of graphene, hBN and silicene nanosheets under different tiny crack lengths by molecular dynamics (MD) simulations using LAMMPS. The useful strength of these two dimensional materials are determined by their fracture toughness. Our study shows a comparative analysis of mechanical properties among the elemental analogues of graphene and suggested that hBN can be a good substitute for graphene in terms of mechanical properties. We have also found that the pre-cracked sheets fail in brittle manner and their failure is governed by the strength of the atomic bonds at the crack tip. The MD prediction of fracture toughness shows significant difference with the fracture toughness determined by Griffth's theory of brittle failure which restricts the applicability of Griffith's criterion for these materials in case of nano-cracks. Moreover, the strengths measured in armchair and zigzag directions of nanosheets of these materials implied that the bonds in armchair direction have the stronger capability to resist crack propagation compared to zigzag direction.

We have quantitatively characterized by FT-IR spectroscopy the contents of secondary structure of beta-lactoglobulin during thermal unfolding and subsequent refolding. Our data clearly indicate that considerable amount of secondary structure, particularly beta-sheet, still remained intact even at 90 degrees C. Noticeable changes in secondary structure of beta-lactoglobulin were observed only above 70 degrees C. The refolded protein regained, within limits of experimental error, all of the secondary structure lost during thermal unfolding. The data also indicate that the refolding mechanism operating at pH 7.0 and 2.0 are the same. Identical secondary structure of native and refolded beta-lactoglobulin was also indicated by far-UV circular dichroic spectra of the two forms of protein. Near UV circular dichroic spectra of the same two forms showed considerable differences indicating less tertiary structure of refolded beta-lactoglobulin. The combined CD and FT-IR data indicated that refolded form of beta-lactoglobulin could be characterized as a molten globule state as it had native-like secondary structure and compromised tertiary structure.

Alpha-crystallin, the major eye lens protein, exists as a large oligomer of two subunits, alphaA- and alphaB-crystallin. The individual subunits assemble into the oligomer in vitro. It is generally believed that oligomerization is pre-requisite for chaperone function, although there is no hard data available on this subject. We therefore undertook a study using limited tryptic digestion as a tool for examining the relationship between oligomeric size and chaperone activity of recombinant alphaA- and alphaB-crystallin. We showed that tryptic digested fragments of both alphaA- and alphaB-crystallin much smaller than the original subunits retain considerable chaperone activity. Our results indicate that chaperone activity depends more on the sequence of the reduced peptide than on its oligomeric size. The results also suggest that the presence of the alpha-crystallin domain and hydrophobic clefts on the protein surface, which correlate poorly with oligomeric size, are important for chaperone function.

We undertook an unfolding and refolding study of alpha(L)-crystallin in presence of urea to explore the breakdown and formation of various levels of structure and to find out whether the breakdown of various levels of structure occurs simultaneously or in a hierarchal manner. We used various techniques such as circular dichroism, fluorescence spectroscopy, light scattering, polarization to determine the changes in secondary, tertiary, and quaternary structure. Unfolding and refolding occurred through a number of intermediates. The results showed that all levels of structure in alpha(L)-crystallin collapsed or reformed simultaneously. The intermediates that occurred in the 2-4 M urea concentration range during unfolding and refolding differed from each other in terms of the polarity of the tryptophan environment. The ANS binding experiments revealed that refolded alpha(L)-crystallin had higher number of hydrophobic pockets compared to native one. On the other hand, polarity of these pockets remained same as that of the native protein. Both light scattering and polarization measurements showed smaller oligomeric size of refolded alpha(L)-crystallin. Thus, although the secondary structural changes were almost reversible, the tertiary and quaternary structural changes were not. The refolded alpha(L)-crystallin had more exposed hydrophobic sites with increased binding affinity. The refolded form also showed higher chaperone activity than native one. Since the refolded form was smaller in oligomeric size, some buried hydrophobic sites were available. The higher chaperone activity of lower sized oligomer of alpha(L)-crystallin again revealed that chaperone activity was dependent on hydrophobicity and not on oligomeric size.

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