Ions in the region of 1707?1400 cm21 [41,42]. The band at 1707.3 cm21 (G, T) related to mainly guanine shifted to 1715, 1700, 1701 and 1700.5 in Mg2+DNA, Mg2+-DNA-theophylline, Mg2+-DNA-Autophagy theobromine and Mg2+-DNA-caffeine complexes respectively. The changes observed in the band at 1658 cm21 (T, G, C) 
mainly for thymine [41,42], cytosine band at 1484.2 cm21 (C, G) and for adenine at 1600 cm21 upon drug complexation, indicating binding of methylxanthines were greatly enhanced in the presence of Mg2+. Especially theobromine binding was improved when compared to its non-metal complexes, where a minor change alone was noticed in the C = O frequency of drug (Fig. 3 and 4). Together with the changes observed in the PO22 band of DNA during complexation with metal and drugs, changes were also observed in the main IR marker bands at 890 cm21 (sugarphosphate stretch) and 836 (phosphodiester mode). These IR marker bands showed some variations in complexes at 897, 825 cm21 (Mg2+-DNA); 898 cm21 (Mg2+-DNA-theophylline); 895, 830 cm21 (Mg2+-DNA-theobromine) and 898, 832 cm21 (Mg2+-DNA-caffeine). Hence the DNA structure was shifted from B family to A- family in the above complexes. Other than the structural alteration, the changes in the PO22 band of DNA can also be attributed to the metal interaction with N7 adenine/ guanine, thymine O2 and N3 cytosine. Here the study encompassing the drug interaction in the presence of lower metal ion concentration (1, 5 and 10 mM) did not show any major shifting as explained above 1662274 and resembled as that of DNA-drug complexes in the absence of metal ions.Methylxanthines Binding with DNAFurthermore, the helix-coil transition observed using pH variation method in the presence and absence of drugs as well revealed an increased binding affinity of methylxanthines with DNA. This is supported by the fact that the percentage of hyperchromicity of the free DNA was increased with respect to its state of helix-coil transition (due to slow increase in pH), and intriguingly the percentage of hyperchromicity of free DNA 23727046 was still more increased to 30?5 upon addition of methylxanthines (P/D: 3 and 6), supporting the enhanced binding activity of methylxanthines during helix-coil transition of DNA (Figs. 8A ). The above findings (Tm/pH Autophagy melting profiles) suggest the preferential binding of methylxanthines to single stranded DNA rather than to a native double helical.Binding affinity of methylxanthinesInterestingly there is a binding affinity difference with DNA is noticed for these three methylxanthines in the presence of divalent metal ions and with heat or pH melted DNA as compared to that of the native double helical DNA. A prominent increase in the binding efficacy is noticed for theophylline and theobromine than caffeine (Figs. 5, 7 and 8) in the above set up. This suggests that caffeine interacts with double helical DNA (Fig. 2D) by establishing H-bonding interaction from outside to DNA helix and forming aggregation along the sides of DNA polymer [2,3]. However caffeine interaction with the denatured form of DNA (Tm/pH-melted) (DNA structure closer to single strand) or in the presence of divalent metal ions reveals lesser binding activity for caffeine (Figs. 5, 7 and 8). This could be substantiated by the fact that the binding affinity 
of these xanthine derivatives were enhanced with respect to the degree of exposure of DNA bases to give rise more binding sites for drugs. This in turn renders the binding efficacy to increase for e.Ions in the region of 1707?1400 cm21 [41,42]. The band at 1707.3 cm21 (G, T) related to mainly guanine shifted to 1715, 1700, 1701 and 1700.5 in Mg2+DNA, Mg2+-DNA-theophylline, Mg2+-DNA-theobromine and Mg2+-DNA-caffeine complexes respectively. The changes observed in the band at 1658 cm21 (T, G, C) mainly for thymine [41,42], cytosine band at 1484.2 cm21 (C, G) and for adenine at 1600 cm21 upon drug complexation, indicating binding of methylxanthines were greatly enhanced in the presence of Mg2+. Especially theobromine binding was improved when compared to its non-metal complexes, where a minor change alone was noticed in the C = O frequency of drug (Fig. 3 and 4). Together with the changes observed in the PO22 band of DNA during complexation with metal and drugs, changes were also observed in the main IR marker bands at 890 cm21 (sugarphosphate stretch) and 836 (phosphodiester mode). These IR marker bands showed some variations in complexes at 897, 825 cm21 (Mg2+-DNA); 898 cm21 (Mg2+-DNA-theophylline); 895, 830 cm21 (Mg2+-DNA-theobromine) and 898, 832 cm21 (Mg2+-DNA-caffeine). Hence the DNA structure was shifted from B family to A- family in the above complexes. Other than the structural alteration, the changes in the PO22 band of DNA can also be attributed to the metal interaction with N7 adenine/ guanine, thymine O2 and N3 cytosine. Here the study encompassing the drug interaction in the presence of lower metal ion concentration (1, 5 and 10 mM) did not show any major shifting as explained above 1662274 and resembled as that of DNA-drug complexes in the absence of metal ions.Methylxanthines Binding with DNAFurthermore, the helix-coil transition observed using pH variation method in the presence and absence of drugs as well revealed an increased binding affinity of methylxanthines with DNA. This is supported by the fact that the percentage of hyperchromicity of the free DNA was increased with respect to its state of helix-coil transition (due to slow increase in pH), and intriguingly the percentage of hyperchromicity of free DNA 23727046 was still more increased to 30?5 upon addition of methylxanthines (P/D: 3 and 6), supporting the enhanced binding activity of methylxanthines during helix-coil transition of DNA (Figs. 8A ). The above findings (Tm/pH melting profiles) suggest the preferential binding of methylxanthines to single stranded DNA rather than to a native double helical.Binding affinity of methylxanthinesInterestingly there is a binding affinity difference with DNA is noticed for these three methylxanthines in the presence of divalent metal ions and with heat or pH melted DNA as compared to that of the native double helical DNA. A prominent increase in the binding efficacy is noticed for theophylline and theobromine than caffeine (Figs. 5, 7 and 8) in the above set up. This suggests that caffeine interacts with double helical DNA (Fig. 2D) by establishing H-bonding interaction from outside to DNA helix and forming aggregation along the sides of DNA polymer [2,3]. However caffeine interaction with the denatured form of DNA (Tm/pH-melted) (DNA structure closer to single strand) or in the presence of divalent metal ions reveals lesser binding activity for caffeine (Figs. 5, 7 and 8). This could be substantiated by the fact that the binding affinity of these xanthine derivatives were enhanced with respect to the degree of exposure of DNA bases to give rise more binding sites for drugs. This in turn renders the binding efficacy to increase for e.
