Piperine, the bioactive phytochemical from black pepper (Piper nigrum L.), is a nontoxic natural compound exhibiting many physiological and pharmacological properties. They include antioxidant, anti-inflammatory, antimutagenic, antitumor, antiapoptotic, antigenotoxic, antiarthritic, antifungal, antimicrobial, antidepressant, anti-HBV, and gastro-protective activities. It also enhances the bioavailability of phytochemicals and drugs. The molecular mechanism of action of piperine with DNA has not yet been addressed, while its pharmacological activities have been reported. In this work we report for the first time the interaction of piperine molecule with DNA duplex. We have carried out UV-vis absorption and fluorescence spectroscopy to confirm the binding of piperine with calf thymus DNA (ctDNA). The energetics of interaction of piperine with ctDNA was monitored by isothermal titration calorimetry (ITC). Differential scanning calorimetry (DSC) and melting temperature (Tm) analysis were also performed, confirming a minor groove mode of binding of piperine with ctDNA. The binding free energy ΔG values obtained from molecular dynamics simulation studies agree well with ITC values and reveal a sequence dependent minor groove binding exhibiting a specificity toward AT rich sequences.

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The atomic models of native monoclinic lysozyme obtained by refinement at Bangalore and elsewhere [Young, Dewan, Nave & Tilton (1993). J. Appl. Cryst. 26, 309-319] differed significantly in the flexible regions of the protein molecule. The two models were reconciled starting from regions where they were in reasonable agreement to produce an improved model which yielded an R value of 0.169 for 12 816 observed reflections in the 10-2 A resolution range. The reconciled model was compared with the structure of the 88% relative humidity form obtained through a water-mediated transformation [Madhusudan, Kodandapani & Vijayan (1993). Acta Cryst. D49, 234-245]. Parts of the flexible regions of the molecule register significant movements during the transformation. The changes resulting from the transformation from the native to the low-humidity forms are pronounced in many of the side chains in the active-site region, thus indicating the relationship between hydration, mobility and enzyme action. The fact that the overall changes in molecular geometry resulting from water-mediated transformation are similar to those which occur during enzyme action, further emphasizes this relationship.

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