Studies on interaction between Vitamin B12 and human serum albumin

The binding reaction between Vitamin B12 (B12, cyanocobalamin) and human serum albumin (HSA) was investigated by fluorescence quenching, UV–vis absorption and circular dichroism (CD) spectroscopy. Under simulative physiological conditions, fluorescence quenching data revealed that the quenching constants (K_sv) are 3.99 × 10⁴, 4.33 × 10⁴, 4.76 × 10⁴ and 5.16 × 10⁴ M⁻¹ at 292, 298, 304 and 310 K, respectively. The number of binding sites, n is almost constant around 1.0. On the basis of the results of fluorescence quenching the mechanism of the interaction of B12 with HSA has been found to be a dynamic quenching procedure. Thermodynamic parameters ΔH^Θ = −13.38 kJ mol⁻¹, ΔS^Θ = 66.73 J mol⁻¹ K⁻¹ were calculated based on the binding constant. These suggested that the binding reaction was enthalpy and entropy driven, and the electrostatic interaction played major role in stabilizing the reversible complex. The binding distance r = 5.5 nm between HSA and B12 was obtained according to Förster theory of energy transfer. The effect of B12 on the conformation of HSA was analyzed by synchronous fluorescence and CD spectroscopy. Synchronous spectra indicated that the polarity around the tryptophan (Trp214) residues of HSA was decreased and its hydrophobicity was increased; however, the α-helix content of the protein was predominant in the secondary structure but the CD spectra indicated that B12 induced minor conformational changes of HSA.     

Preferential binding of insecticide phorate with sub-domain IIA of human serum albumin induces protein damage and its toxicological significance

Phorate, an organophosphorus insecticide is known for its adverse effects on acetylcholinesterase, and other neuronal and pulmonary activities. Most likely, the toxicity of drugs/agrochemicals is modulated through cellular distribution bound to plasma proteins. Therefore, the in vitro interaction of phorate with human serum albumin (HSA) has been investigated, using sensitive techniques like fluorescence spectroscopy and circular dichroism, to ascertain its binding mechanism and toxicological implications. Fluorescence studies revealed the quenching constant (Ksv) as 2.5 × 10⁴ M⁻¹ and binding affinity (Ka) as 2.96 × 10⁴ M⁻¹ (r² = 0.99), with a primary binding site of phorate at sub-domain IIA of HSA. Circular dichroism (CD) data demonstrated a noticeable reduction in secondary structure (α-helical content) of phorate treated HSA. Albumin treated with 200-1000 μM phorate released significant amounts of acid soluble amino and carbonyl groups, whereas higher concentrations resulted in protein fragmentation. It is postulated that the 1′-O and 3-O alkyl groups of phorate have a role in binding with electrophilic centers of Trp 214, and Arg 218/Lys 195, respectively. Moreover, the significant ultrastructural changes, reactive oxygen species (ROS) generation, mitochondrial damage and cell death in phorate treated cultured human amnion epithelial (WISH) cells, elucidated phorate induced cellular toxicity.     

The interaction of plant-growth regulators with serum albumin: Molecular modeling and spectroscopic methods

The affinity between two plant-growth regulators (PGRs) and human serum albumin (HSA) was investigated by molecular modeling techniques and spectroscopic methods. The results of molecular modeling simulations revealed that paclobutrazol (PAC) could bind on both site I and site II in HSA where the interaction was easier, while uniconazole (UNI) could not bind with HSA. Furthermore, the results of fluorescence spectroscopy, three-dimensional (3D) fluorescence spectroscopy and circular dichroism (CD) spectroscopy suggested that PAC had a strong ability to quench the intrinsic fluorescence of HSA. The binding affinity (K_b) and the amounts of binding sites (n) between PAC and HSA at 291 K were estimated as 2.37 × 10⁵ mol L⁻¹ and 1, respectively, which confirm that PAC mainly binds on site II of HSA. An apparent distance between the Trp214 and PAC was 4.41 nm. Additionally, the binding of PAC induced the conformational changes of disulfide bridges of HSA with the decrease of α-helix content. These studies provide more information on the potential toxicological effects and environmental risk assessment of PGRs.     

Interaction of kaempferol with human serum albumin: A fluorescence and circular dichroism study

The interaction of kaempferol (kaemp), a natural flavonoid to which antioxidative, anti-inflammatory and cardio-protective biological activities have been attributed, with human serum albumin (HSA), the main in vivo transporter of exogenous substances, was investigated by steady-state, synchronous fluorescence and circular dichroism spectroscopies. The binding constant, K, and number of binding sites, n, were computed using literature models that showed satisfactory agreement and revealed a strong interaction (K ∼ 3.5 × 10⁵ M⁻¹, n ∼ 1). The binding process was investigated at temperatures in the range 298–313 K, allowing for the evaluation of the thermodynamic parameters, which indicate the occurrence of hydrogen bonding interactions. The distance between kaemp and the tryptophan residue of HSA was estimated at 2.7 nm using Förster's theory of nonradiative resonance energy transfer. Using circular dichroism we evidenced some degree of HSA defolding upon binding.     

Study of curcumin and genistein interactions with human serum albumin

Curcumin, the yellow pigment from the rhizoma of Curcuma longa, is a widely studied polyphenolic compound which has a variety of biological activity as anti-inflammatory and antioxidative agent. Genistein one of the flavonoids found in soybean and chickpeas inhibits DNA strand breaks acting as a direct scavenger of reactive oxygen species. Human serum albumin (HSA) with high affinity binding sites is a major transporter for delivering several endogenous compounds and drugs in vivo. The aim of this study was to examine the interactions of curcumin and genistein with human serum albumin at physiological conditions, using constant protein concentration and various pigment contents. FTIR, UV–Visible, CD and fluorescence spectroscopic methods were used to analyse drug binding mode, the binding constant and the effects of pigment complexation on HSA stability and conformation. Structural analysis showed that curcumin and genistein bind HSA via polypeptide polar groups with overall binding constants of K_curcumin = 5.5 (±0.8) × 10⁴ M⁻¹ and K_genistein = 2.4 (±0.40) × 10⁴ M⁻¹. The number of bound pigment (n) is 1.33 for curcumin and 1.49 for genistein. The HSA conformation was altered by pigment complexation with reduction of α-helix and increase of random coil and turn structures suggesting a partial protein unfolding.     

Probing the binding of morin to human serum albumin by optical spectroscopy

Morin [2-(2,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-1-benzopyran-4-one], a member of flavonols, is an important bioactive compound by interacting with nucleic acids, enzymes and protein. Its binding to human serum albumin was investigated by fluorescence quenching, fluorescence anisotropy, and UV–vis absorbance under the simulative physiological condition. Fluorescence quenching data show that the interaction of morin with HSA forms a non-fluorescent complex with the binding constants of 1.394 × 10⁵, 1.489 × 10⁵, 1.609 × 10⁵ and 1.717 × 10⁵ M⁻¹ at 292, 298, 303 and 310 K, respectively. The thermodynamics parameters, enthalpy change (ΔH) and entropy change (ΔS) were calculated to be 8.97 kJ mol⁻¹ and 129.15 J mol⁻¹ K⁻¹ via van’t Hoff equation. From the spectroscopic results and thermodynamics parameters, it is observed that van der Waals and hydrogen bonds are predominant intermolecular forces when forming the complex. The distance r = 4.25 nm between donor (Trp214) and accepter (morin) was estimated based on the Förster theory of non-radiative energy transfer. The red shift of UV–vis absorbance shows that morin is bound to several amino acids on the hydrophobic pocket of HSA. Moreover, the competitive probes, such as warfarin and ibuprofen (site I and II probes, respectively), reveal that the binding location of morin to HSA in the site I of the hydrophobic pocket, which corresponds to the results of UV–vis absorbance, while morin also binds other lower affinity binding sites on HSA from the fluorescence anisotropy spectroscopy.     

Multiple spectroscopic studies on the interaction between olaquindox,a feed additive,and bovine serum albumin

The interaction between olaquindox (OLA) and bovine serum albumin (BSA) was investigated using fluorescence, UV–vis absorption and circular dichroism (CD) spectroscopy. The results showed that the fluorescence quenching of BSA by OLA was a static quenching process induced by the formation of OLA–BSA complex. The binding constant of OLA–BSA complex was calculated to be 1.299 × 10⁴ L mol⁻¹ (293 K). The negative values of ΔH⁰ and ΔS⁰ indicated that hydrogen bond and van der Waals interactions played major roles in stabilizing the complex. Site probe competition experiments and number of binding sites (n) revealed that OLA could bind to site I in subdomain IIA of BSA, and the binding distance (r) was evaluated to be 3.643 nm according to Förster’s non-radiative energy transfer theory. The results of CD and three-dimensional fluorescence spectra suggested some conformational changes of BSA after OLA binding.     

Molecular spectroscopic studies on the interaction between Ractopamine and bovine serum albumin

To investigate the interaction between Ractopamine (RAC), an animal growth promoter, and bovine serum albumin (BSA), three spectroscopic approaches (fluorescence, UV–vis and FT-IR) and three different experiments (two mole-ratio and a Job's methods) were used to monitor the biological kinetic interaction procedure. The Stern–Volmer quenching constants K_SV, the binding constants K_a, and the number of binding sites n at 298, 301 and 304 K were evaluated by molecular spectroscopic approaches. The values of enthalpy (−13.47 kJ mol⁻¹) and entropy (78.39 J mol⁻¹ K⁻¹) in the reaction indicated that RAC bound to BSA mainly by electrostatic and hydrophobic interaction. The site markers competitive experiments indicated that the binding of RAC to BSA primarily took place in site I. The spectra data matrix was further investigated with multivariate curve resolution-alternating least squares (MCR-ALS), and the concentration profiles and the pure spectra for three species (BSA, RAC and RAC-BSA) existed in the kinetic interaction procedure, as well as the apparent equilibrium constants, were obtained.     

Reviewing the binding of a series of parabens to human serum albumin

To better understand the factors that contribute to the accumulation of unmetabolized parabens (p‐hydroxybenzoic acid esters) in breast cancer tissue, the binding of a series of parabens (methyl‐, ethyl‐, butyl‐, benzyl‐paraben) to human serum albumin (HSA) was investigated by fluorescence spectroscopy and also their ability to modify the binding parameters of albumin site markers. Emission spectra of HSA upon fluorescence excitation of Trp 214 residue at 295 nm were recorded at different molar ratios of PB/HSA and data were corrected for the inner‐filter effect. A significant inner‐filter effect was obtained for molar ratios of 2.0 and above. For lower molar ratios, a slight increase in fluorescence of HSA was detected. p‐Hydroxybenzoic acid, the main metabolite of parabens, did not modify the fluorescence of HSA whatever the molar ratio used. Binding parameters for compounds that are markers of site I, bilirubin and warfarin, were determined in the absence and presence of methyl, butyl and benzyl paraben at molar ratios of PB/HSA of 0, 1 and 2. No variation of the binding constants of these markers was observed. The results indicate that parabens weakly interact with HSA thus suggesting that they are in a free form in blood and therefore more available to reach tissues. Copyright © 2013 John Wiley & Sons, Ltd.     

Interaction of sanguinarine and its dihydroderivative with the Na/K-ATPase. Complex view on the old problem

The effects of sanguinarine (SG) and its metabolite dihydrosanguinarine (DHSG) on Na⁺/K⁺-ATPase were investigated using fluorescence spectroscopy. The results showed that the enzyme in E1 conformation can bind both charged and neutral (pseudobase) forms of SG with a K_D = 7.2 ± 2.0 μM or 11.7 ± 0.9 μM, while the enzyme in E2 conformation binds only the charged form of SG with a K_D = 4.7 ± 1.1 μM. Fluorescence quenching experiments suggest that the binding site in E1 conformation is located on the surface of the enzyme for both forms but the binding site in E2 conformation is protected from the solvent. We found no evidence for interaction of Na⁺/K⁺-ATPase and DHSG. This implies that any in vivo effect of SG attributable to inhibition of Na⁺/K⁺-ATPase can be considered only prior to SG → DHSG transformation in the gastro-intestinal tract and/or blood. Hence, Na⁺/K⁺-ATPase inhibition will be effective in SG topical application but its duration will be very limited in SG oral or parenteral administration.     

On-line molecular imprinted solid-phase extraction flow-injection fluorescence sensor for determination of florfenicol in animal tissues

A simple and convenient on-line molecular imprinted solid-phase extraction flow-injection fluorescence sensor was developed in this study. Florfenicol (FF)-imprinted polymer was prepared by self-assembly with acrylamide (AM) as functional monomer and ethylene glycol dimethacrylate (EGDMA) as cross-linker. The binding characteristics of the imprinted polymer to florfenicol were evaluated by equilibrium binding experiments, and the morphology of the imprinted polymer was studied by scanning electron microscope (SEM). Fluorescence intensity of 3-p-nitrylphenyl-5-(2′-sulfonophenylazo) rhodanine (4NRASP) and bovine serum albumin (BSA) was inhibited by FF. Under optimum conditions the intensity of fluorescence shows a linear relationship with the concentration of FF over the range of 1.2 × 10⁻⁶ to 2.6 × 10⁻⁵ g mL⁻¹ with a regression equation of ΔI = 23.54 + 17.86c (c 10⁻⁶ g mL⁻¹) (r = 0.9965, n = 6). The low detection limit of FF was found to be 3.4 × 10⁻⁷ g mL⁻¹ according to 11 parallel determinations for the blank solution. The relative standard deviation for the determination of 2.0 × 10⁻⁶ g mL⁻¹ FF solution was 3.5% (n = 11). This proposed sensor could be satisfactorily applied to the determination of FF in liver and meat samples.     

Thermodynamics of the interaction between oxytocin and its myometrial receptor in sheep: A stepwise binding mechanism

Entropy (ΔS), enthalpy (ΔH) and heat capacity (ΔC_p) changes attending the oxytocin interaction with its two binding sites on myometrial cell membranes in sheep were derived from the temperature dependence of K_d values. The high affinity oxytocin site (K_d on the order of 10⁻⁹ mol l⁻¹, 25 °C), ascribed to the oxytocin receptor (OXTR), is entropy-driven in the temperature range 0–37 °C. Enthalpy component prevails as a driving force in the binding to the low affinity site (K_d ≈ 10⁻⁷) within the higher temperature range. ΔC_p values in both cases do not differ significantly from zero but become highly relevant in the presence of a GTP analog (10⁻⁴ M GTP-γS). Under these conditions, ΔC_p in the low site interaction becomes negative and ΔS is shifted toward negative values (enthalpy drift); ΔC_p of the high affinity site rises to a high positive value and the interaction is even more strongly entropy driven. Atosiban, a competitive antagonist of oxytocin at OXTR displays a single significant binding site on myometrial cells (K_d about 10⁻⁷ mol l⁻¹). Thermodynamic profiles of atosiban and the low affinity oxytocin site show conspicuous similarities, indicating that the inhibitor is bound to the low affinity site, and not, with a lower affinity, to the putative receptor protein. It is suggested that the interaction of oxytocin with its responding system on myometrial membranes follows in two distinct steps that are likely to be associated with several independent binding domains in the GPCR receptor.     

Alpha1-acid glycoprotein concentration and serum protein binding of carbamazepine and carbamazepine-10,11 epoxide in children with epilepsy

Summary The relationship between the serum protein binding of carbamazepine (CBZ) and carbamazepine-10,11 epoxide (CBZ-E) and the concentration of ₁-acid glycoprotein (AAG) and albumin (HSA) was examined in 39 CBZ-treated epileptic children aged 4 months to 12 years.A significant inverse correlation was found between the free fraction of both compounds and serum AAG, even though changes in AAG concentration explained only part of the variation in binding. No correlation was found between the free fraction of CBZ and CBZ-E and HSA, probably due to the small intersubject variation in HSA concentration. In vitro experiments showed that both CBZ and CBZ-E were bound to HSA and to a lesser extent to AAG. At equivalent HSA concentrations, the binding of CBZ and its metabolite increased proportionately with increasing AAG concentration within the range occurring clinically.     

Interaction of anthracycline disaccharide with human serum albumin: investigation by fluorescence spectroscopic technique and modeling studies

Anthracyclines are considered to be some of the most effective anticancer drugs for cancer therapy. However, drug resistance and cardiotoxicity of anthracyclines limit their clinical application. An 3′-azido disaccharide analogue of daunorubicin, 7-[4-O-(2,6-dideoxy-3-O-methyl-α-l-arabino-hexopyranosyl)-3-azido-2,3,6-trideoxy-α-l-lyxo-hexopyranosyl]daunorubicinone (ADNR-3), was shown to exhibit 10-fold better activity than parent compound daunorubicin against the drug-resistant cells and completely overcomes the drug resistance with same IC₅₀ in both drug-resistant and drug-sensitive cells. In this paper, the interactions between ADNR-3 and human serum albumin (HSA) have been studied by spectroscopic techniques. By the analysis of fluorescence spectrum and fluorescence intensity, it was observed that the ADNR-3 has a strong ability to quench the intrinsic fluorescence of HSA through a static quenching procedure. The association constants of ADNR-3 with HSA were determined at different temperatures based on fluorescence quenching results. The negative ΔH and positive ΔS values in case of ADNR-3–HSA complexes showed that both hydrogen bonds and hydrophobic interactions play a role in the binding of ADNR-3 to HSA. Furthermore, synchronous fluorescence spectroscopy data and UV–vis absorbance spectra have suggested that the association between ADNR-3 and HSA changed the molecular conformation of HSA and the hydrophobic interactions play a major role in ADNR-3–HSA association. Moreover, the study of computational modeling indicated that ADNR-3 could bind to the site I of HSA and hydrophobic interaction was the major acting force for the second type of binding site, which was in agreement with the thermodynamic analysis. The distance, r, between donor (HSA) and acceptor (ADNR-3) was obtained according to the Förster's theory of non-radiation energy transfer. In addition, the effects of common ions on the binding constants of ADNR-3–HSA complexes were also investigated.     

Altered serum protein binding of carbamazepine in disease states associated with an increasedα 1-acid glycoprotein concentration

Summary The protein binding of carbamazepine (CBZ) in vitro was assessed in sera from 47 patients with various diseases known to alter ₁-acid glycoprotein (AAG) concentration and from 20 drug-free normal control subjects. In the patient group, AAG and albumin (HSA) concentrations ranged from 6 to 74 µmol/l and from 377 to 652 µmol/l, respectively; in the controls, protein concentrations were less variable, ranging from 11 to 26 µmol/l for AAG and from 623 to 754 µmol/l for HSA. In both the patient and the combined patient and control groups, free CBZ fractions were inversely correlated with the serum AAG concentration (r=–0.62). No significant relationship could be found between the free CBZ fraction and the serum HSA concentration. The free CBZ fraction was moderately but significantly decreased in patients with AAG levels above 26 µmol/l (the highest value found in controls) as compared either to patients with a normal AAG concentration or to control subjects (19±5% vs 23±4% and 23±2%), despite the finding of a higher HSA concentration in the control group. The data confirm AAG as an important determinant of interindividual variability in serum CBZ binding.     

Interaction of Benzothiadiazides with Human Serum Albumin Studied by Dialysis and Spectroscopic Methods

The interaction of a series of benzothiadiazides with human serum albumin (HSA) was investigated by equilibrium dialysis (ED) and spectroscopic methods including circular dichroism (CD). The primary binding site of benzothiadiazides was designated site II, the diazepam site on the HSA molecule, as indicated by displacement experiments using different site-selective probes. Tyrosine and lysine amino acid residues were probably involved in the binding site of these compounds to HSA. Both electrostatic and hydrophobic interactions were found to play a role in the binding of these compounds to HSA. Among the compounds tested, chlorothiazide had the highest affinity (K₁ = 5.5 × 10⁴M^–1, K₂ = 5.8 × 10³ M^–1).The primary binding affinity of the compounds for HSA was of the order: chlorothiazide > cyclopenthiazide > polythiazide > ethiazide > trichlormethiazide = methyclothiazde > hydrochlorothiazide. Binding was insensitive to the N-B transition of HSA. The binding site is proposed to consist of a cationic site on the surface of the HSA molecule with a hydrophobic crevice to accommodate the aromatic ring of the compounds. Positions 3 and 7 of the benzothiadiazide molecule is thought to affect the binding affinity to HSA.     

Development of a selective competitive receptor binding assay for the determination of the affinity to NR2B containing NMDA receptors

A selective, rapid and efficient competitive binding assay for the determination of the affinity of compounds towards the ifenprodil binding site of NR2B subunit containing NMDA receptors has been developed. In the assay system, [³H]ifenprodil was used as radioligand and membrane homogenates from L(tk-) cells stably expressing recombinant human NR1a/NR2B receptors served as the receptor material. Sonication of the cells during work-up, performing all steps with 96-well multiplates and using a solid scintillator represent particular features of this assay. The binding kinetics was investigated by saturation and association/dissociation experiments. [³H]ifenprodil bound to a single, saturable site on human recombinant NR1a/NR2B receptors, resulting in a B_max-value of 25.8 pmol/μg protein and K_d-value of 7.6 ± 2.3 nM (SEM). The dissociation rate constant (k_off) was 0.03861 min⁻¹ and the association rate constant k_on resulted in 0.00625 nM⁻¹ min⁻¹. The specificity of the assay was proved with cells not treated with dexamethasone, which has to be added to induce NMDA receptor synthesis of the cells. Additionally, the absence of α₁, σ₁ and σ₂ receptors was shown. The K_i-values of the NR2B ligands ifenprodil and eliprodil determined with the new assay are in good accordance with reported data.     

Release of proteolytic activity following reduction in therapeutic human serum albumin containing products: Detection with a new neoepitope endopeptidase immunoassay

Botulinum type A toxin (BoNT/A) is defined by its specific endopeptidase cleavage of SNAP25 between Gln¹⁹⁷ and Arg¹⁹⁸ under reducing conditions. The neurotoxin is widely used for therapeutic or cosmetic purposes, but should not contain other toxin serotypes or unwanted protease activities. Using a neoepitope endopeptidase immunoassay, additional cleavage between Arg¹⁹⁸ and Ala¹⁹⁹ was detected with a range of therapeutic BoNT/A products confirming an earlier report of an unidentified proteolytic component. By developing the assay and making it insensitive to BoNT/C1, any activity due to the type C1 toxin was excluded. Therapeutic preparations consist of ng quantities of toxin protein which are typically stabilised by 0.125–30 mg of HSA. An excellent correlation (R² = 0.993) between HSA content per vial and measured activity was obtained within the therapeutic BoNT/A products tested. No activity was detected in any of the non-albumin formulated preparations, thereby identifying HSA as the source of the unknown protease for the first time. To investigate the cause of this activity, either as an intrinsic molecular activity of albumin or due to an albumin-associated purification contaminant, further studies on a variety of commercial plasma-derived HSA products or recombinant HSA materials free from potential plasma contaminants were carried out. The measured proteolytic levels were highly consistent amongst preparations, and could all be partially inhibited by the presence of zinc and blocked by PKSI-527 and aprotinin. By contrast, the data did not support the role of plasmin, kallikrein, trypsin, α₂-antiplasmin–plasmin complexes or HSA purification contaminants, PKA (prekallikrein activator) or kallikrein-like activity. Taken together, these findings indicate a new intrinsic proteolytic activity of the albumin molecule revealed under reducing conditions as the source of the unexpected Arg–Ala cleaving activity.     

Fluorescence study on site-specific binding of perfluoroalkyl acids to human serum albumin

Binding of five perfluoroalkyl acids with human serum albumin (HSA) was investigated by site-specific fluorescence. Intrinsic fluorescence of tryptophan-214 in HSA was monitored upon addition of the chemicals. Although perfluorobutyl acid (PFBA) and perfluorobutane sulfonate (PFBS) did not cause fluorescence change, perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), and perfluorododecanoic acid (PFDoA) induced fluorescence quenching, from which binding constant of 2.7 × 10⁵ M⁻¹ for PFOA and 2.2 × 10⁴ M⁻¹ for PFOS was calculated. Two fluorescent probes, dansylamide (DA) and dansyl-l-proline (DP), were employed in fluorescence displacement measurements to study the interaction at two Sudlow’s binding sites. At Site I, both PFBA and PFBS displaced DA with binding constants of 1.0 × 10⁶ M⁻¹ and 2.2 × 10⁶ M⁻¹. At Site II, PFBS and PFDoA displaced DP with binding constants of 6.5 × 10⁶ M⁻¹ and 1.2 × 10⁶ M⁻¹, whereas PFBA did not bind. The data were compared with fatty acids to evaluate the potential toxicological effect of these environmental chemicals. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Study of the interaction of an anticancer drug with human and bovine serum albumin: spectroscopic approach

The interactions between gemcitabine hydrochloride (GEM) and bovine serum albumin (BSA) or human serum albumin (HSA) have been studied by spectroscopic techniques. By the analysis of fluorescence spectrum and fluorescence intensity, it was observed that the GEM has a strong ability to quench the intrinsic fluorescence of both BSA and HSA through a static quenching procedure. The association constants of GEM with BSA and HSA were determined at different temperatures based on fluorescence quenching results. The negative ΔH° and positive ΔS° values in case of GEM–BSA and GEM–HSA complexes showed that both hydrogen bonds and hydrophobic interactions play a role in the binding of GEM to BSA or HSA. Experimental results showed that the binding of GEM to BSA or HSA induced conformational changes in BSA and HSA. From the quantitative analysis data of CD spectra, the α-helix of 57.58% and 34.82% in free BSA and free HSA decreased to 40.82% and 29.84% in BSA–GEM and HSA–GEM complexes, respectively, and hence confirmed that the secondary structure of protein was altered by GEM. The interactions of BSA and HSA with GEM were also confirmed by UV absorption spectra. The distance, r, between donor (BSA or HSA) and acceptor (GEM) was obtained according to the Förster's theory of non-radiation energy transfer. The effects of common ions on the binding constants of both BSA–GEM and HSA–GEM complexes were also investigated.