Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India.
J Photochem Photobiol B. 2013 Apr 5;121:46-56. doi: 10.1016/j.jphotobiol.2013.02.010. Epub 2013 Feb 22.
Spectroscopic and crystallographic studies reveal that Merocyanine 540 (MC 540), a well-known therapeutically important anionic cyanine dye, interacts with hen egg white lysozyme in ground state. The formation of the complex is validated by two isosbestic points in absorption spectra of lysozyme with varied concentration of MC 540 and appearance of an isodichroic point in induced CD spectra of MC 540 with lysozyme. The blue shift of fluorescence maximum of lysozyme in presence of MC 540 shows hydrophobic effect on Trp due to complex formation probably through cooperative binding. Above 1:3M stoichiometric ratio (lysozyme:MC 540) an additional fluorescence hump arises because of structural changes in protein, where MC 540 acts as self-denaturant, inducing non-linearity in Stern-Volmer plot. The van't Hoff isotherms with negative changes in enthalpy at lower concentration and positive changes in entropy for entire concentration range of MC 540 depict the binding forces as hydrogen bonding/van der Waal's and ionic/hydrophobic respectively. Finally X-ray crystallographic structure of the complex shows that MC 540 adopts two conformations, cis and trans, while it binds to lysozyme. Benzoxole moiety of MC 540 interacts with Trp123 through π-stacking and SO3(2-) group is stabilized by ionic interaction/H-bonding with Arg125 of lysozyme.
光谱和晶体学研究表明，Merocyanine 540（MC 540），一种众所周知的治疗上重要的阴离子花菁染料，在基态下与鸡卵清白溶菌酶相互作用。复合物的形成通过在不同浓度的 MC 540 存在下溶菌酶的吸收光谱中的两个等色点和在 MC 540 与溶菌酶的诱导圆二色性光谱中的一个等消偏点来验证。在 MC 540 存在下溶菌酶荧光最大的蓝移表明由于复合物形成，色氨酸的疏水性效应可能是通过协同结合。在 1:3M 的化学计量比（溶菌酶：MC 540）以上，由于蛋白质结构的变化，出现了额外的荧光峰，其中 MC 540 充当自变性剂，在 Stern-Volmer 图中引起非线性。范特霍夫等温线在较低浓度下焓的负变化和整个 MC 540 浓度范围内熵的正变化表明，结合力分别为氢键/范德华力和离子/疏水力。最后，复合物的 X 射线晶体结构表明，MC 540 采用顺式和反式两种构象，同时与溶菌酶结合。MC 540 的苯并恶唑部分通过π-堆积与 Trp123 相互作用，而 SO3(2-) 基团通过与溶菌酶的 Arg125 的离子相互作用/H-键合得到稳定。
