The bindi ng of asp irin (ASA) and amlo dipine (AML) to hu man serum albumin (HSA ) i n aqueou s solution was investiga ted by m ultiple te chnique s such as fluor escenc e que nching, resona nce lig ht scatteri ng (RLS), thre e-dim ensiona l fluor escence spectrosc opy, FT-IR and zeta- potent ial meas urement s i n a n aqueou s s olution at pH ¼ 7.4. For the prot ein–liga nd associa tion reaction, fluoresce nce meas urement s can give importan t clue s a s t o the bindi ng of ligand s t o protein s, e.g. , the bindi ng m echanis m, bindi ng mode , binding constan ts, bindi ng sites , etc. Fluoresce nce spect roscopy show ed that ASA and AML could quench th e HSA fluor escence s pectra, and this quench ing effe ct became more significan t when both ASA and AML coexi sted . The re sults pointed at the inter action between HSA and both drugs as te rnary syst ems decreasi ng th e binding constan t and bindi ng s tability of the HSA –drug compl ex as a bina ry syst em. Ther efore, by reducin g t h e amount of drugs tran sported to their targ ets, th e free drug concen tration of the ta rget would be reduced, lowering th e effic acy of the drugs. It wa s d e monstra ted that there exists anta gonistic behav ior between the tw o drugs when it comes to bindi ng of HSA. Furth ermo re, the fluor escence res ults also showed th at the quench ing mecha nism of HSA–dr ug compl exes as bina ry and tern ary syst ems is a static procedu re. The num ber of bindi ng s ites of HSA–A SA, (HSA–AM L)ASA, HSA–A ML and (HSA –ASA) AML we re 1.31 , 0 . 92, 1 and 0. 93, res pectively. Due to the exi stence of the anta gonist ic act ion between ASA and AML, the bindi ng dis tance r wa s reduc ed. The results of synchron ous fluoresce nce and th ree-dim ensiona l fluoresce nce spect ra show ed that the anta gonistic action between ASA and AM L wou ld alter the m icro-en vironm ent around Trp and Tyr resid ues. Moreover , the s imultane ous presence of ASA and AML during bindi ng to HSA should be taken into accoun t i n multidr ug therapy, as it indu ces the nece ssity of a m onitori ng therapy owing to the possi ble increase of un controll ed toxi c effe cts. Molecul ar dynam ic stud ies show ed that the affi nity of each of the drugs to HSA wa s r e duced in the prese nce of significan t amount s o f the other. In the interact ion of HSA with both drugs, the zeta potential of the te rnary system is more nega tive than its binar y counte rpart. The zet a-potent ial re sults suggest ed induced con formati onal chan ges on HSA that confi rmed the expe rimental and theor etical results .

Keywords