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

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~00、ΔS~00(静电力)在高盐浓度下转变为ΔH00、ΔS00(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。

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

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 the ΔG0 was almost invariant, namely enthalpy-entropy compensation. Finally, the thermodynamic parameters changed from ΔH0 < 0, ΔS0 > 0 (electrostatic force) to ΔH0 < 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.