Fernández M S, Calderón E
Departamento de Bioquímica, Centro de Investigación y de Estudios Avanzados del Instituto Politecnico Nacional, México, D.F.
J Photochem Photobiol B. 1990 Sep;7(1):75-86. doi: 10.1016/1011-1344(90)85144-l.
At neutral pH the local anaesthetic tetracaine hydrochloride quenches the fluorescence of the lipophilic dye perylene incorporated into non-ionic micelles. The process follows the Stern-Volmer equation, suggesting that quenching occurs through encounter of fluorophore and quencher. As the pH is lowered from 5 to 1, the apparent quenching constant decreases sigmoidally, the midpoint of the curve being at pH 2.3, close to the pK value characterizing the ionization of the anaesthetic aromatic butylamino group. Quenching is completely reversed below pH 1. These results show that the ability of tetracaine to quench the fluorescence of perylene incorporated into micelles depends on the absence of charge on its aromatic amine. Quenching was also studied in homogeneous dioxane-water solution. In this system the quenching constant also decreases sigmoidally as the pH is lowered. The infection point of the curve is nearly coincident with the pK of tetracaine butylamino group in the same partially non-aqueous medium. Protonation of this group induces 60% reversal of the quenching, suggesting that the main mechanism of fluorescence extinction could be the electron transfer from unprotonated tetracaine aromatic amine to perylene in the excited state. However, an additional process which remains operative even when such an amino group is positively charged must also be involved. It can be concluded that the complete reversal of tetracaine quenching of perylene fluorescence in micelles induced by low pH is due to the inability of the anaesthetic to become partitioned into micelles upon protonation of its aromatic amine. In contrast, at neutral pH the local anaesthetic is able to reach the micelle non-polar core where perylene is located. This is consistent with the models, suggesting that the membrane-bound tetracaine assumes a rod-like configuration parallel to the surface normal with the aromatic butylamino group located into a highly hydrophobic region.
在中性pH值下，局部麻醉剂盐酸丁卡因会淬灭掺入非离子胶束中的亲脂性染料苝的荧光。该过程遵循斯特恩-沃尔默方程，表明淬灭是通过荧光团与猝灭剂的相遇而发生的。随着pH值从5降至1，表观淬灭常数呈S形下降，曲线的中点在pH 2.3处，接近表征麻醉剂芳香丁氨基基团电离的pK值。在pH 1以下，淬灭完全逆转。这些结果表明，丁卡因淬灭掺入胶束中的苝荧光的能力取决于其芳香胺上电荷的缺失。还在均相二氧六环-水溶液中研究了淬灭。在该体系中，随着pH值降低，淬灭常数也呈S形下降。曲线的拐点与丁卡因丁氨基基团在同一部分非水介质中的pK值几乎一致。该基团的质子化导致淬灭反转60%，这表明荧光猝灭的主要机制可能是未质子化的丁卡因芳香胺向激发态苝的电子转移。然而，即使该氨基带正电荷时仍起作用的另一个过程也必定参与其中。可以得出结论，低pH值诱导的丁卡因对胶束中苝荧光淬灭的完全逆转是由于麻醉剂在其芳香胺质子化后无法分配到胶束中。相反，在中性pH值下，局部麻醉剂能够到达苝所在的胶束非极性核心。这与模型一致，表明膜结合的丁卡因呈平行于表面法线的棒状构型，芳香丁氨基基团位于高度疏水区域。
