AG9/AB9与牛血清白蛋白相互作用的德拜休克尔极限理论分析及其焓熵补偿

Interaction of AG9/AB9 with BSA: Analysis by Debye-Hückel Limiting Law and Observation of Enthalpy-Entropy Compensation

本研究通过多光谱技术深入探讨盐浓度(离子强度)对酸性绿9(acid green 9,AG9)和酸性蓝9(acid blue 9,AB9)与牛血清白蛋白(bovine serum albumin,BSA)之间相互作用的影响。运用德拜休克尔极限理论定量分析,计算得到真实的吉布斯自由能变化(ΔG^0_I→0)和BSA的阴离子受体腔内有效电荷(Z_B)。配体AG9/AB9所带电荷Z_C的值为负,Z_B的值为正,意味着相反电荷之间离子对的形成,即AG9/AB9的负电荷与BSA的净正电荷结合腔(位点-Ⅰ)相结合,表明BSA的局部电荷主导BSA-AG9/AB9体系的相互作用,而不是它的整体或表面电荷。随着盐浓度的增加,这两个体系的熵变逐渐由正值变为负值,且焓变逐渐增大,使ΔG^0几乎不变而发生焓熵补偿。最终其热力学参数由ΔH^0<0、ΔS^0>0(静电力)在高盐浓度下转变为ΔH0<0、ΔS0<0(非静电力),表明AG9/AB9与BSA相互作用时,它们的移动或局部运动由于主导力的转变而受阻。

Multispectral techniques were employed to detect salt concentration(i.e.ionic strength)-modulated interaction of acid green 9(AG9)and acid blue 9(AB9)with bovine serum albumin(BSA).The Debye-Hückel limiting law was applied to quantitative analysis to calculate the true Gibbs free energy change(ΔG0I→0)and effective charge(ZB)in the anion receptor pocket of BSA.The sign of ZC value for AG9/AB9 was negative and the ZB values were positive.These results indicated the formation of ion-pair between opposite charges;for example,the negatively charged AG9/AB9 bound to the net positively charged binding pocket(site-I)of BSA,suggesting that the local charge rather than the overall or surface charge of BSA played a key role in dominating the interaction strength of BSA-AG9/AB9 system.Moreover,with the increase of salt concentration,the entropy gradually varied from positive to negative and the exothermic enthalpy term increased so that theΔG0 was almost invariant,namely enthalpy-entropy compensation.Finally,the thermodynamic parameters changed fromΔH0<0,ΔS0>0(electrostatic force)toΔH0<0,ΔS0<0(non-electrostatic force)at high salt concentration.These results showed that the mobility/local motion of AG9/AB9 complexed with protein was retarded due to the transformation of dominant force.