B: Biological Pigments HL Flashcards
Reasons for colored compounds
-C=C-C=C-C=C-C=C-C=C-C=C-C=C-
In conjugated systems overlap of P2 orbitals result in electron delocalization
- Short conjugated chains like C=C absorb in the UV part of the spectrum
- Longer chains result in greater delocalization and longer wavelength
Chromophores
Groups such as C=C and C=O that absorb in the UV-vis part of the spectrum
eg Chlorophyll - green light being transmitted as red light absorbed
Porphyrin Rings
Chelates (tetradentate ligands) of metals with large nitrogen-containing macrocyclic ligands
Fe2+ in Heme
Mg2+ in Chlorophyll
The Binding of Oxygen to Hemoglobin
- Hemoglobin consists of 4 hemegroup with Fe2+ at the center
- Each Heme unit can transport 1 molecule of O2
- each unit of hemoglobin can transport 4 molecules of O2
- Myoglobin stores oxygen in the muscles; hemoglobic carries oxygen in the blood
- they bind oxygen with Fe2+ ions, which forms a weak bond with the Fe2+ in heme
- the oxydation number of Fe2+ does not change
- they bind oxygen with Fe2+ ions, which forms a weak bond with the Fe2+ in heme
Oxyhemoglobin Dissociation Curve
- Binding of O2 to hemoglobin is a cooperative process
- At lower partial pressure the hemoglobin releases some of the oxygen it is carrying
Temperature and Hemoglobin
- An increase in temp decreases the affinity of hemoglobin for O2
- At higher temps in the cells more O2 will be released
pH and Hemoglobin
- A decrease in pH reduces the affinity of hemoglobin for oxygen
- increasing the [CO2] has the same effect as CO2 dissolving to form carbonic acid H2CO3
- during increased exercise the conc of CO2 increases –> pH decreases and hemoglobin releases oxygen
Fetal Hemoglobin
- Hemoglobin exists as a different form in fetal blood
- It has a greater affinity for O2 than normal hemoglobin
- allows fetal blood in the placenta to take up oxygen from the mother’s blood
Carbon Monoxide and Hemoglobin
- CO has a much higher affinity for hemoglobin than oxygen, because it is a competitive ligand
- acts as a ligand because it will donate a pair of e- to a central metal
- Acts as a competitive inhibitor and therefore prevents oxygen binding to hemoglobin
Hb + CO —> Hb-(CO) carboxyhemoglobin
- reaction is irreversible and goes to completion
Carbon monoxide vs Oxygen graph
- the oxygen is prevents from binding –> less O2 available for body tissues
- Hemoglobin has 4 O2 binding sites, the binding of CO at one of these sites increases the O2 affinity of the remaining 3 sites –> retains oxygen that otherwise would be delivered to body tissues
Cytochromes
Myoglobin and Hemoglobin are examples of cytochromes because their heme groups contain Fe2+ ions that can be interconverted to Fe3+ ions –> redox
Color Stability of Heme
- The purplish-red color of meat is caused by myoglobin
- in myoglobin, iron is oxidized Fe2+ –> Fe3+
- The stability of color and the rate of brown metamyglobin formation can be minimized if the meat is stored in oxygen gree conditions by using plastic film with low permeability
- air is removed from the package and a storage gas (CO2) is used
Anthocyanins
- Common pigments in plants –> responsible for pink, blue and purple hues
- Many OH groups which will form H bonds with water –> soluble
- Large degree of conjugation –> Indicates delocalisation of Pz electrons and they absorb in the visible part of the spectrum
- metal ions disrupts delocalisation of electrons —> affects color
Carotenoids
- Most widespread pigment found in nature (abundance in algae)
- they generally absorb in the visible region of the spectrum and so have colors in the yellow-red region
- susceptible to oxidation, catalysed by light
- involved in light-harvesting in plants during photosynthesis
- a photon of light is absorbed to promote the electron to an excited state –> energy absorbed is transferred to chlorophyll
- involved in light-harvesting in plants during photosynthesis
- lipid soluble
Indicators
HIn <—-> H+(aq) + In-(aq)
HIn = weak acid
- at neutrality:
- [HIn] = [In-]
- KIn = [H+]
- pKIn = pH
