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DNA-surfactant interactions: Coupled cooperativity in ligand binding leads to duplex stabilization
No full textThe cooperative nature of interaction of cationic surfactants with short oligonucleotides leading to eventual stabilization of DNA duplexes is demonstrated. At submicellar concentrations and DNA:surfactant charge ratios of 0.2 to 0.8, the association of single chain (CTAB) and double chain (DOTAP) surfactants to oligonucleotides is initiated by electrostatic interaction of cationic ligands with polyanionic DNA that aligns the surfactant molecules on the DNA template. This is followed by binding of new surfactant ligands to the initial complex, driven cooperatively by the hydrophobic forces, leading to in situ formation of surfactant-bound and bare duplexes as separate species. These exhibit independent melting behaviour characterised by double transition in thermal UV profiles, with a higher T(m) for surfactant-DNA complexes. Understanding the cooperative binding of the cationic surfactants to the DNA described here may have implications for rational design of DNA binding drugs and DNA delivery systems
DNA–Surfactant Interactions: Coupled Cooperativity in Ligand Binding Leads to Duplex Stabilization
No full textThe cooperative nature of interaction of cationic surfactants with short oligonucleotides leading to eventual stabilization of DNA duplexes is demonstrated. At submicellar concentrations and DNA:surfactant charge ratios of 0.2 to 0.8, the association of single chain (CTAB) and double chain (DOTAP) surfactants to oligonucleotides is initiated by electrostatic interaction of cationic ligands with polyanionic DNA that aligns the surfactant molecules on the DNA template. This is followed by binding of new surfactant ligands to the initial complex, driven cooperatively by the hydrophobic forces, leading to in situ formation of surfactant-bound and bare duplexes as separate species. These exhibit independent melting behaviour characterised by double transition in thermal UV profiles, with a higher Tm for surfactant–DNA complexes. Understanding the cooperative binding of the cationic surfactants to the DNA described here may have implications for rational design of DNA binding drugs and DNA delivery systems
ANION INDUCED BLUE TO PURPLE TRANSITION IN BACTERIORHODOPSIN
Full text linkPurple membrane (PM, λ role= presentation \u3eλmax role= presentation \u3emax 570 nm) of H. halobium on treatment with sulphuric acid changes its colour to blue (λ role= presentation \u3eλmax role= presentation \u3emax 608 nm). The purple chromophore can be regenerated from the blue chromophore by exogeneous addition of anions such as CI− role= presentation \u3e− and HPO42− role= presentation \u3e2−4. Chloride ion is found to be more effective than the dibasic phosphate ion in regenerating the purple chromophore. Nevertheless, one thing common to the anion regeneration is that both CI− role= presentation \u3e− and HPO42− role= presentation \u3e2−4 show marked pH effect. At pH 1.0 the efficiency of regeneration of the purple chromophore is greater than at pH 2.0, for the same anion concentration. Fluorescence and circular dichroic studies indicate that the proteins do not undergo drastic changes at the secondary\u27 or tertiary structure level and the native structure is preserved during this transition. However, chromophoric-site interactions between retinal and the apoprotein are affected during this colour transition. A molecular mechanism is advanced for this transition
Retinylidene Schiff bases in phosphatidylcholine reverse micelles: formation, protonation and stability
No full textAll-trans-N-retinylidene-n-butylamine 3 has been formed in inverted micelles of phosphatidylcholine (PC)–hexane containing varying amounts of water ([H2O]/[PC]= 0–3) and the formation, protonation and stability have been studied. The micelles have been found to catalyse the Schiff-base formation. The Schiff-base was found to be stable in the presence of structured water molecules bonded to the polar head groups of the micelles. A larger water-pool causes the decomposition of the Schiff-base. Schiff-base 3 intercalated in the inverted micelle was found to undergo protonation in the presence of 3-chloropropionic acid, the extent of which depended on the water-pool size. The results are discussed in terms of the formation, protonation and stability of retinylidene Schiff-base chromophores in rhodopsins
Bioorganic chemistry of the purple membrane of Halobacterium halobium — Chromophore and apoprotein modified bacteriorhodopsins
No full textIodophenyl and anthryl retinal analogues have been synthesized. Thetrans-isomers have been isolated and purified by high pressure liquid chromatography. The purified isomers have been further characterized by nuclear magnetic resonance and ultraviolet-visible spectroscopy. Incubation of these retinal analogues with apoprotein (bacterioopsin), isolated from the purple membrane ofHalobacterium halobium gave new bacteriorhodopsin analogues. These analogues have been investigated for their absorption properties and stability. The iodophenyl analogue has been found to bind to bacterioopsin rapidly. The pigment obtained from this analogue showed a dramatically altered opsin shift of 1343 cm-1. The anthryl analogue based bacteriorhodopsin, however, showed an opsin shift of 3849 cm-1. It has been found that bacteriorhodopsin is quite unrestrictive in the ionone ring site. The apoprotein seems to prefer chromophores that have the ring portion co-planar with the polyene side chain.
The purple membrane has also been modified by treatment with fluorescamine, a surface active reagent specific for amino groups. Reaction under controlled stoichiometric conditions resulted in the formation of a modified pigment. The new pigment showed a band at 390 nm—indicative of fluorescamine reaction with amino group (s) of apoprotein-besides retaining its original absorption band at 560 nm. Analysis of the fluorescamine modified bacteriorhodopsin resulted in the identification of lysine 129 as the modified amino acid residue. Fluorescamine-modified-bacteriorhodopsin suspension did not release protons under photolytic conditions. However, proteoliposomes of fluorescamine-modified-bacteriorhodopsin were found to show proton uptake, though at a reduced rate