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1 - SOD > Chelation and Stability > Flashcards

Chelation and Stability Flashcards

1
Q

What is the HSAB theory?

hard and soft acids and bases

A

theory is used to predict which donor groups are likely to show an affinity for a particular metal ion
- predict which metal ions and ligands will interact most favourably.

can be used to predict the thermodynamic stability of a complex = depends on properties of ligand, metal Iona and type of bonding.

  • hard acids and hard bases forms a strong bond = greatest affinity
  • soft acid and soft bases form a strong bond = greatest affinity
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2
Q

What is hard acid?

A

acceptor of electrons pairs

  • non polarisable cations = positively charged
  • high electron/charge density

forms bonds that are more ionic in nature

most metals in their normal oxidation state
H+ = proton

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3
Q

What is soft acid?

A

softness refers to polarisability

acceptor of electrons pairs
- more polarisable cation = more polarisable than hard acids

Cu (I), Rh (I), Ag (I), Au (I), Pd (II), Pt (II)
- transition metals mainly

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4
Q

What are borderline acids?

A

acceptor of electrons pairs

  • intermediate character
  • Fe (II), Pb (II), Ni (II), Cu (II)wh
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5
Q

How does the nature transition metal changes across the periodic table?

A

metals become more soft in character/nature as you go across the periodic table from left tor right

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6
Q

What are metals and ligands?

A

metal
- lewis acid = electron acceptor
ligand
- lewis base = electron donor

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7
Q

Wha are soft bases?

A

donors of electron pairs

  • big
  • more polarisable than hard bases
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8
Q

What are hard bases?

A

donors of electron pairs

  • small
  • non-polarisable
  • electronegative
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9
Q

What are the favoured combinations for biologically important metals?

A

bulk metal ions are generally found in combination with oxygen donors (O or O-)
bulk metal ion
- Na, Ca, Mg

as the periodic table is crossed, transition metal go from preferring O donors to N and O donors to N and S donors to S donors

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10
Q

What is the law of mass action?

A

The mass action law states that if the system is at equilibrium at a given temperature, then the following ratio is a constant: products / reactant (to the power of their stoichiometry)

Law of mass action, law stating that the rate of any chemical reaction is proportional to the product of the masses of the reacting substances, with each mass raised to a power equal to the coefficient that occurs in the chemical equation.

  • the stronger the new bond made, the more the reaction favours the RHS (product side)
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11
Q

What is the formula for gibbs free energy?

A

standard change in gibbs free energy = -RTlnK

standard change in gibbs free energy = standard change in enthalpy - (temperature x standard change in entropy)

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12
Q

How can you tell if a reaction is spontaneous? What affects it?

A

a reaction is spontaneous if change in gibbs free energy is negative (large negative values)

  • achieved when K (equilibrium constant) is large
  • do not require external energy input

a reaction is not spontaneous if change in gibbs free energy is positive
- requires external energy input

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13
Q

What is the difference between stepwise and overall stability/equilibrium constant?

A

stepwise stability constant
- individual stability constants for each reaction = each addition of ligand

overall stability constant - beta
- the product of the total reaction (multiplied together) = after every ligand addition

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14
Q

What does overall stability tell you about the reaction?

A

a large overall stability constant tell you that

  • the reaction favours the RHS = concentration of product complex is greater than the reactant
  • the reaction is more stable

large values of overall stability constant are written as log

if log beta is greater than one 
- gibbs free energy is negative 
- reaction is spontaneous 
- no further substitution 
if log beta is less than one 
- gibbs free energy is positive
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15
Q

What is the trend in overall stability constantoverall stability constant? What causes a change in the trend?

A

overall stability constant should decrease with each reaction (addition of ligand)

  • increased number of ligands, causes a decrease in the number of sites available for substitution
  • makes the reaction less likely to happen
  • formed ligand can break as ligands disassociate

change in geometry can result in a change in the trend
example - K3 > K2
- octahedral geometry is preferred over tetrahedral

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16
Q

What are chelate complexes?

A

chelate complexes

- can form several bonds to a single metal ion (comes from one molecule)

17
Q

Why are chelate complexes more stable than complexes involving monodentate ligands?

A

chelate complexes

  • higher stepwise stability constants
  • higher overall stability constant
  • have a higher affinity for metal ions than monodentate ligands = have more binding sites/bonds
18
Q

What happens when a chelate ligand replaces monodentate ligands in a complex?

A

there is a large increase in entropy
- increased number of free molecules in the system = one chelate ligand replaces several monodentate ligands

  • the higher the entropy change (more positive), the greater the stepwise stability constant and the more stable the product formed is
19
Q

Why is EDTA an optimal ligand?

A

it is the best ligand for making octahedral complexes

  • has 6 binding sites
  • overall stability constant is high = product is stable
20
Q

What are the requirements for a suitable agent for chelation therapy?

A

drugs and its metabolites must be non-toxic

drug must be orally active

drug must form thermodynamically and kinetically stable complexes with the target metal ion

drug must be selective to the target metal ion

  • must contain suitable functional group = have greater affinity
  • must not deplete essential metal ions/elements

metal-drug complexes must be excreted and not redistributed around the body

  • especially not to the brain
  • must contain hydrophilic groups to facilitate renal

excretion

must be economic to produce

21
Q

What is the treatment for acute metal poisoning?

A

if treatment is given immediately, drug is confined to extracellular space = blood plasma

drug must bind more strongly to the metal than naturally occurring ligands
example - amino acids

drug must be hydrophilic to be excreted in the urine and not redistributed to the tissues

22
Q

What is the treatment for chronic metal poisoning?

A

metal will be mostly confined to the intracellular space = tissues
- drug must be lipophilic enough to reach the metal ions in the tissue - pass the plasma membrane

  • drug must be able to form a complex with the metal which is lipophilic enough to pass back into the plasma
  • drug must be hydrophilic enough to be excreted in the urine and not redistributed to the tissues
23
Q

What is synergistic therapy?

A

uses a combination of 2 drugs
first drug
- must be a lipophilic agent that mobilises the metal from the tissues into blood plasma
second drug
- must be a hydrophilic agent that forms a complex with the metal to excrete it into the urine
- must have a greater affinity for the toxic metal than the first drug = must compete for it

can be used to treat chronic metal poisoning

24
Q

What are the different therapeutic chelating agents?

A

dithiols
- have SH groups
- BAL = British Anti-Lewisite = 2,3-Dimercaptopropanol
water soluble derivatives of BAL
- DMSA = 2,3-Dimercaptosuccinic acid
- DMPS = 2,3-Dimercaptopropane sulphonic acid
aminopolycarboxylic acids
- EDTA = Ethylenediaminetetraacetic acid
- DTPA = Diethylenetriaminepentaacetic acid
D-Penicillamine
Triethylenetetramine

25
Q

What are the advantages and disadvantages of dithiols as chelating agents?
BAL

A

advantages

  • lipid soluble = can mobilise tissue bound metals (intracellular)
  • complexes are sufficiently water soluble to be excreted in the urine

disadvantages

  • susceptible to oxidation
  • side effects = nausea, headaches
26
Q

What are the advantages and disadvantages of the water derivatives of BAL as chelating agents?

A

DMSA
DMPS

advantages

  • less toxic = same functional groups but water soluble
  • available as oral administration as tablets
  • are stable for a long time

disadvantages

  • can only mobilise extracellular drugs (blood plasma) as they are water soluble
  • side effects = GI discomfort, skin reactions
27
Q

What are the advantages and disadvantages of aminopolycarboxylic acids as chelating agents?
EDTA
DTPA

A

advantages
- undergo little, if any, metabolic change in vivo
- modest toxicity
- rapidly excreted
disadvantages
- lack of specificity = chelate a wide range of metal ions
- must be given as intravenous injections = painful

EDTA
- hydrophilic so only removes extracellular (serum). tissue bound may be eventually mobilised as equilibrium is established = results in prolonged treatment

28
Q

How should EDTA and DPTA be given?

A

EDTA
- must be given as Na2CaEDTA to prevent calcium depletion = causes tetany if depleted
DPTA
- must be given as Na3ZnDTPA = it has a high affinity for zinc and may be depleted otherwise

29
Q

What are the advantages and disadvantages of D-Penicillamine as chelating agents?

A

has 3 different donor groups - NH2, SH, COOH

  • general chelating agent
  • orally active but intestinal absorption is only about 50%
  • has low toxicity

clinical use is limited by side effects

30
Q

What are the advantages and disadvantages of Triethylenetetramine as chelating agents?

A

has 4 amine group donors = tetradentate
- high affinity for copper so is used to treat Wilson’s disease

side effects

  • skin rash
  • anemia

1 - SOD (27 decks)

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