Complexation in Chemistry

Complexation and Protein Binding

• Complexation is the process of drugs interacting with proteins
• Protein binding involves weak bonds like hydrogen, hydrophobic, and van der Waals forces
• Drug binding is often reversible
• Protein binding can also be irreversible through covalent bonding, but less common. Such binding can result in drug toxicity or carcinogenicity
• Irreversible protein binding is less common than reversible protein binding

Analysis of Complexation

• Estimation of two parameters is required: stoichiometric ratio of ligand:metal or donor:acceptor and stability constant of the complex
• pH titration is a reliable method used to assess complexation in cases where complex formation causes a pH change
• Example: chelation of cupric ion by glycine, represented as: Cu²⁺ + 2NH₃CH₂CH₂COO⁻ = Cu(NH₂CH₂COO)₂ + 2H⁺; the complex formation causes a drop in pH because two protons are formed during the reaction

Distribution Method

• Distribution behavior of a substance between two immiscible liquids is expressed in terms of distribution coefficient (or partition coefficient)
• When a solute complexes with an added substance, the solute's distribution pattern changes depending on the nature of the complex
• This method allows for estimating stability constants, for example; Iodine complexes with potassium iodide. I₂ + KI = KI₃, and K = [KI₃]/ ([I₂][K⁺I⁻])

Solubility Method

• This method uses the change in solubility profile to determine complexation
• Complexation can either increase or decrease the solubility of a substance in a mixture
• Example: the estimation of complexation between para-aminobenzoic acid (PABA) and caffeine: caffeine increases the solubility of PABA

Continuous Variation Method

• Physical properties like UV absorbance, dielectric constant, refractive index, and spectrophotometric extinction coefficient can be used to determine complexation
• In cases where there is no interaction or complexation between substances A and B, the value of the property is additive, and a linear relationship is observed in a plot
• If complexation occurs, the property value yields a nonlinear relationship

Spectroscopy Method

• Donor-acceptor or charge transfer complexations are studied using absorption spectroscopy in the visible and UV regions
• The complex usually has different UV absorbance from its constituents
• Additional methods include nuclear magnetic resonance (NMR), infrared spectroscopy (IR), polarography, and X-ray diffraction

Question on Complexes

• Identifying the central metal ion, ligand, and the coordination number of a complex
• Co(NH₃)₆⁺³: cobalt is central metal ion; NH₃ is ligand & coordination number is 6
• Cr(H₂O)₆⁺³: chromium is central metal ion; H₂O is ligand, and coordination number is 6

Protein Binding

• Protein binding refers to the interaction of compounds with proteins
• Proteins in human plasma have diverse functions, including maintaining osmotic pressure, pH, immunity, transport, enzyme/hormone function, and catalyzing metabolic reactions
• Protein-bound drugs are often inactive until released from the protein binding site

Types of Protein Binding

• Reversible protein binding involves weak bonds like hydrogen bonds, hydrophobic interactions, and van der Waals forces
• Irreversible protein binding, while less common, often involves covalent bonds, and can result in drug toxicity or carcinogenicity.

Protein-Ligand Binding Kinetics

• Describes the process of protein-ligand association and dissociation
• The rate at which binding occurs depends on factors like protein-ligand affinity, and interactions/collisions between molecules
• Time-dependent association is described as P + L $\rightleftharpoons$ kon PL (where kon is the kinetic rate constant for binding) koff
• PL $\rightleftharpoons$ koff P + L (where koff is the kinetic rate constant for unbinding)

Protein-Ligand Binding Models

• Lock-and-key, induced fit, and conformational selection models explain how proteins bind to ligands
• The lock-and-key model assumes rigid protein and ligand with perfect binding interface
• In the induced fit model, the binding site is flexible and the ligand induces conformational changes in the protein
• In the conformational selection model, protein exists as a vast ensemble of conformational states that coexist in equilibrium, and the ligand binds selectively to the most suitable state

Mechanisms of Interactions

• H-bonding: Proteins can interact with small molecules as a result of H-bonding between the donor and acceptor functional groups in amino acid sequences
• Electrostatic interactions: Occur between charged amino acids on proteins and ligand molecules with opposite charges
• Van der Waals forces: Include dipole-dipole, dipole-induced dipole, and dispersion forces
• Hydrophobic interactions: Interfacial phenomenon that results from the attraction between nonpolar (hydrophobic) groups in a solution containing water molecules

Implications of Protein Binding

• A bound drug is inactive, while a free drug shows its activity
• The free drug distributes from the vasculature to the organs, tissues or it can be metabolized/eliminated before reaching the target site.
• A bound drug can act as a reservoir
• If the medication is extensively bound, a minor change can have a considerable effect

Factors Influencing Protein Drug Binding

• Drug-related factors; Physicochemical characteristics and lipophilicity are directly related to the extent of protein binding. A greater lipophilicity leads to a greater extent of protein binding
• Concentration of drug/protein in the body: Protein binding process can be altered by changes in the concentration of drug or protein
• Affinity of a drug for specific binding components: Depends on the attraction/affinity of protein molecules or tissues towards drug moieties.
• Protein/tissue-related factors: Physicochemical characteristics of protein such as amino acid sequence/structure that affects the degree of attraction to the drug
• Drug interactions: Competition between drugs for binding sites, or interactions with normal body constituents, can affect protein binding.

Patient-Related Factors

• Age: Neonates and elderly patients may have altered albumin content and require adjustments to drug dosages accordingly
• Inter-subject variability: Differences in genetics and environmental factors influence drug responses
• Disease states: Certain diseases may affect plasma protein concentration and binding properties, thus changing the amount of free drug

Binding to Blood Components

• Protein binding can occur in extracellular spaces and intracellular spaces.
• Specific proteins can bind to drugs, including human serum albumin (HSA), α1-acid glycoprotein, Lipoproteins, and blood cells
• The extent of drug binding varies among different proteins: Albumin > α1-acid glycoprotein > Lipoproteins > Globulins.

Binding of Drugs

• Binding of drugs to Human Serum Albumin (HSA): HSA is an abundant plasma protein characterized by a large drug binding capacity
• Binding of drugs to α1-acid glycoprotein (orosomucoid): Drug binding is primarily due to hydrophobic interactions. Basic drugs bind to the acidic alpha-1 acid glycoprotein
• Binding of drugs to Lipoproteins: Drugs bind to these components by hydrophobic interactions, and the lipid core serves as a solvent

Binding of Drugs to Globulins

• a1 Globulin (Corticosteroid binding globulin), plays a role in binding of steroidal drugs, Thyroxin, and Cyanocobalamine (Vitamin B12)
• a2 Globulin (Ceruloplasmin) binds Vitamins A, D, E, K
• b1 Globulin (transferrin) binds Ferrous ions
• b2 Globulin binds to Carotinoids
• y Globulin binds antigens

Binding of Drugs to Blood Cells

• Drugs can bind to blood components, including 40% blood cells (primarily red blood cells)
• Hemoglobin is a protein within RBCs and it can bind to certain drugs like phenytoin and pentobarbital

Significance of Protein Binding

• Absorption: Protein binding affects the absorption of drugs as it changes the amount of free drug. It changes absorption equilibrium
• Distribution: Protein binding affects the drug's distribution across membranes and tissues
• Metabolism: Protein binding influences drug metabolism, only unbound drugs can be metabolized (by liver).
• Elimination: Protein binding prevents drug entry to the metabolizing organ (liver) and glomerular filtration, influencing drug elimination. Only unbound drugs can be eliminated

Other Factors of Protein Binding

• Diagnosis; Diseases/disorders can change the protein status to be used as biomarkers. Proteins are also used to complex radioactive substances for diagnosis
• Solubility; Lipoproteins act as a vehicle for hydrophobic drugs such as steroids, heparin, and oil-soluble vitamins.
• Drug Action; Protein binding inactivates the drugs as it reduces free drug concentration, so it cannot create a sufficient drug concentration needed in the receptor site to have an effect
• Sustain release; Drug-protein complex acts as a reservoir and continuously delivers the free drug