Rhodopin Glucoside Lab Report
Introduction
Carotenoids are natural pigments that exist in nature among both photosynthetic and non-photosynthetic organism. They serve many significant functions some of which are light harvesting and photoprotection in plants, promoting reproduction and survival in animals and providing substrates for biosynthesis of hormones and signalling molecules. In particular this essay will explore how the structure of rhodopin glucoside determines its function in light harvesting system LH2 of a purple photosynthetic bacteria Rhodopesudomonas acidophila.
Overall structure of Rhodopin glucoside
Rhodopin glucoside (RG) is the major carotenoid in LH2 complex of Rhs acidophila revealed by high solution structure of LH2 complex at 2.5 Å (McDermott et. …show more content…
al. 1995). LH2 is integral membrane protein composed of nine identical subunits arranged in a ring shape. Basic structure of LH2 is a heterodimer of α and β apoproteins and each heterodimer binds 2 carotenoids and 2 bacteriochlorophylls. RG has a characteristic isoprenoid structure with long polyene chain comprising of 11 conjugated double bonds and a sugar residue at the end as shown in figure 1. It is a long, linear molecule with slight S-shaped distortion that runs along the entire length of the LH2 complex in an all Trans form (N.Macpherson et.al., 2001). Presence of this long hydrocarbon chain makes RG extremely hydrophobic and restricts it to hydrophobic regions of cells such as the inner thylakoid membrane. This is where LH2 is located thus hydrophobic character of RG ensures that it remains in the correct location and orientation within the cell to function efficiently.
In the polyene chain all the double bonds can exist in two conformations cis or trans. Like rhodopin glucoside, most carotenoids in nature employ trans configuration as it is the thermodynamically most stable conformer (G.Britton 1995). Here the side chains are further apart in the molecule resulting in less steric hindrance. The extended chain not only allows RG to span the complex entirely but the exposed side chains maintain interaction with neighbouring molecules in the complex. These structural features facilitate rhodopin glucoside to perform light harvesting and structural stabilising roles in LH2 complex.
Role of rhodopin glucoside in light harvesting
The primary role of RG in LH2 is to absorb light.
The polyene chain of RG, a long system of conjugated double bonds determines the light absorption properties. Once rhodopin glucoside gets photo-activated, one of the low energy bonding ∏ electrons from the conjugated system gets excited to higher energy unoccupied ∏* anti-bonding orbital. This is often referred as ∏ to ∏* transition. Due to quantum nature of an electron only the frequency that corresponds to energy gap between the two states can be absorbed. In RG the ∏ electrons are highly delocalised over the chain length so energy required to bring out this transitions is reduced. This energy corresponds to wavelength of light of 400-500nm.This visible region is spectrally not covered by the bacteriochlorophyll molecules. Therefore by utilising mixture of different bacteriochlorophyll and carotenoids absorbing photons of different wavelengths, Rhs acidophila can efficiently utilise the entire spectrum of light. Hence more energy per quanta can be fed into reaction centre where photochemistry takes place. Interestingly the characteristic strong absorption by RG at 400-500nm is a result of transition of electrons from ground state (S0) to second singlet excited state (S2). In RG this transition is very strong corresponding to large delocalisation of electrons. Due to near symmetry structure of RG the transition from ground state to first single excited state (S1) is dipole forbidden (H. A. Frank, et.al 1991). Instead the S2 state of RG has a high dipole moment therefore readily absorb
light.
Role of rhodopin glucoside in energy transfer
Rhodopin glucoside is a non fluorescent pigment and dislikes being excited. Once RG gets photo-excited, the excited S2 state almost immediately de-excites vibrationally to S1 state accompanied by heat release. This radiationles deactivation of the excited state is called internal conversion. Once energy reaches S1 state it vibrationally gets de-excited to ground state (S0). In Rhs acidophila this takes around 10 picoseconds. The lifetime of S2 and S1 state is significantly shorter in comparison to bacteriochlorophyll (Polívka,T. and Frank, H.A., 2010). Despite the rapid dissipation of heat, RG is still considered as light harvesting pigments because it transfers its excitation energy to a neighbouring chlorophyll molecule thereby initiating primary photochemical events of photosynthesis. However energy transfer pathway must be rapid for it to compete with the fast vibrational relaxation. This is achieved through optimisation of the distance and orientation of RG (donor) and bacteriochlorophyll (acceptor). Both of these pigments are hydrophobic molecules thus interact via hydrophobic interactions. The long phytyl tail of RG forms multiple ∏-∏ contacts with phytyl chains of both chlorophylls in LH2 complex. This interaction maintains a close distance between the pigments typically around 3.4 Å which not only mediates high efficiency of energy transfer but also locks the pigments in right orientation.
Two possible pathways for energy transfer have been postulated. One route involves direct transfer from S2 state while the other firstly requires internal conversion to S1 state and then subsequent transfer from S1 state. The length of conjugated chain governs the pathway taken. RG has 11 conjugated double bonds so the energy gap between S2 and S1 state is reasonably large. This large gap reduces the probability of internal conversion from S2 to S1state thereby facilitating the direct pathway (Frank,H,a et al.,1991). Surprisingly when S1 state lifetime was measured in methanol and its native LH2 complex no change was detected. Typically environment affects the rate of internal conversion however in both conditions the measured S1 lifetime remained the same which confirmed that LH2 utilises S2 route for energy transfer (Polívka, T. et al., 2002). Also bacteriochlorophyll excited state formation occurred within the same time (-220fs) as RG S2 excited state decay. This further supported the idea of S2 pathway being the high efficient energy transfer pathway.
Role of rhodopin glucoside in structural stability
As shown in 2D crystal structure of LH2 complex, RG starts at the periplasmic side of membrane where the glucosyl (sugar) ring interacts with the polar residues Lys 5 and Thr 8 α-apoprotein (McDermott et. al. 1995). This glucose head is more distorted compared to the rigid polyene chain. This is due to presence of single C-0 bond which connects ring to the chain. Generally single bonds allow rotation of molecules so they exist in more than one conformation, here in RG this single bond optimises the interaction of glucose ring to its residue. RG then extends across the complex making several close Van der Waal contacts with the hydrophobic residues of both apoproteins and bacteriochlorophylls (McDermott et. al. 1995). In total RG makes contact with at least 15 amino acids residues of LH2 subunit. Both chlorophyll and RG are intertwined around α and β apoproteins of LH2. These interactions help RG to hold and lock the subunits in right orientation thus maintaining internal structural stability of LH2 complex. Furthermore they also fill up any space between each subunit further enhancing this stabilising effect. This stabilising nature of RG is confirmed through mutagenesis studies on LH2 complex (Zurdo et al. 1995) where mutants lacking carotenoid were generated. These mutants failed to assemble LH2 complex and the structure collapsed. As a consequence, this destabilised LH2 complex cannot absorb photons and failed to funnel this excitation energy energetically downhill towards reaction centre thereby reducing its photosynthetic function.
Conclusion
In conclusion Rhodopin glucoside is a primary carotenoid in LH2 complex of Rhodopesudomonas acidophila. Similar to other carotenoids in nature, the structure of RG governs its physical and chemical properties which in turn determine its function. It ensures RG remains in its designated cellular locations in correct orientation and interact with right molecule with correct bonds to function properly. The long conjugated chain and glucosyl head allows RG to serve its role as a light harvester by absorbing at visible region of spectrum and also as structural stabiliser by maintaining interactions with the molecules of LH2 complex.
Carotenoids are natural pigments that exist in nature among both photosynthetic and non-photosynthetic organism. They serve many significant functions some of which are light harvesting and photoprotection in plants, promoting reproduction and survival in animals and providing substrates for biosynthesis of hormones and signalling molecules. In particular this essay will explore how the structure of rhodopin glucoside determines its function in light harvesting system LH2 of a purple photosynthetic bacteria Rhodopesudomonas acidophila.
Overall structure of Rhodopin glucoside
Rhodopin glucoside (RG) is the major carotenoid in LH2 complex of Rhs acidophila revealed by high solution structure of LH2 complex at 2.5 Å (McDermott et. …show more content…
al. 1995). LH2 is integral membrane protein composed of nine identical subunits arranged in a ring shape. Basic structure of LH2 is a heterodimer of α and β apoproteins and each heterodimer binds 2 carotenoids and 2 bacteriochlorophylls. RG has a characteristic isoprenoid structure with long polyene chain comprising of 11 conjugated double bonds and a sugar residue at the end as shown in figure 1. It is a long, linear molecule with slight S-shaped distortion that runs along the entire length of the LH2 complex in an all Trans form (N.Macpherson et.al., 2001). Presence of this long hydrocarbon chain makes RG extremely hydrophobic and restricts it to hydrophobic regions of cells such as the inner thylakoid membrane. This is where LH2 is located thus hydrophobic character of RG ensures that it remains in the correct location and orientation within the cell to function efficiently.
In the polyene chain all the double bonds can exist in two conformations cis or trans. Like rhodopin glucoside, most carotenoids in nature employ trans configuration as it is the thermodynamically most stable conformer (G.Britton 1995). Here the side chains are further apart in the molecule resulting in less steric hindrance. The extended chain not only allows RG to span the complex entirely but the exposed side chains maintain interaction with neighbouring molecules in the complex. These structural features facilitate rhodopin glucoside to perform light harvesting and structural stabilising roles in LH2 complex.
Role of rhodopin glucoside in light harvesting
The primary role of RG in LH2 is to absorb light.
The polyene chain of RG, a long system of conjugated double bonds determines the light absorption properties. Once rhodopin glucoside gets photo-activated, one of the low energy bonding ∏ electrons from the conjugated system gets excited to higher energy unoccupied ∏* anti-bonding orbital. This is often referred as ∏ to ∏* transition. Due to quantum nature of an electron only the frequency that corresponds to energy gap between the two states can be absorbed. In RG the ∏ electrons are highly delocalised over the chain length so energy required to bring out this transitions is reduced. This energy corresponds to wavelength of light of 400-500nm.This visible region is spectrally not covered by the bacteriochlorophyll molecules. Therefore by utilising mixture of different bacteriochlorophyll and carotenoids absorbing photons of different wavelengths, Rhs acidophila can efficiently utilise the entire spectrum of light. Hence more energy per quanta can be fed into reaction centre where photochemistry takes place. Interestingly the characteristic strong absorption by RG at 400-500nm is a result of transition of electrons from ground state (S0) to second singlet excited state (S2). In RG this transition is very strong corresponding to large delocalisation of electrons. Due to near symmetry structure of RG the transition from ground state to first single excited state (S1) is dipole forbidden (H. A. Frank, et.al 1991). Instead the S2 state of RG has a high dipole moment therefore readily absorb
light.
Role of rhodopin glucoside in energy transfer
Rhodopin glucoside is a non fluorescent pigment and dislikes being excited. Once RG gets photo-excited, the excited S2 state almost immediately de-excites vibrationally to S1 state accompanied by heat release. This radiationles deactivation of the excited state is called internal conversion. Once energy reaches S1 state it vibrationally gets de-excited to ground state (S0). In Rhs acidophila this takes around 10 picoseconds. The lifetime of S2 and S1 state is significantly shorter in comparison to bacteriochlorophyll (Polívka,T. and Frank, H.A., 2010). Despite the rapid dissipation of heat, RG is still considered as light harvesting pigments because it transfers its excitation energy to a neighbouring chlorophyll molecule thereby initiating primary photochemical events of photosynthesis. However energy transfer pathway must be rapid for it to compete with the fast vibrational relaxation. This is achieved through optimisation of the distance and orientation of RG (donor) and bacteriochlorophyll (acceptor). Both of these pigments are hydrophobic molecules thus interact via hydrophobic interactions. The long phytyl tail of RG forms multiple ∏-∏ contacts with phytyl chains of both chlorophylls in LH2 complex. This interaction maintains a close distance between the pigments typically around 3.4 Å which not only mediates high efficiency of energy transfer but also locks the pigments in right orientation.
Two possible pathways for energy transfer have been postulated. One route involves direct transfer from S2 state while the other firstly requires internal conversion to S1 state and then subsequent transfer from S1 state. The length of conjugated chain governs the pathway taken. RG has 11 conjugated double bonds so the energy gap between S2 and S1 state is reasonably large. This large gap reduces the probability of internal conversion from S2 to S1state thereby facilitating the direct pathway (Frank,H,a et al.,1991). Surprisingly when S1 state lifetime was measured in methanol and its native LH2 complex no change was detected. Typically environment affects the rate of internal conversion however in both conditions the measured S1 lifetime remained the same which confirmed that LH2 utilises S2 route for energy transfer (Polívka, T. et al., 2002). Also bacteriochlorophyll excited state formation occurred within the same time (-220fs) as RG S2 excited state decay. This further supported the idea of S2 pathway being the high efficient energy transfer pathway.
Role of rhodopin glucoside in structural stability
As shown in 2D crystal structure of LH2 complex, RG starts at the periplasmic side of membrane where the glucosyl (sugar) ring interacts with the polar residues Lys 5 and Thr 8 α-apoprotein (McDermott et. al. 1995). This glucose head is more distorted compared to the rigid polyene chain. This is due to presence of single C-0 bond which connects ring to the chain. Generally single bonds allow rotation of molecules so they exist in more than one conformation, here in RG this single bond optimises the interaction of glucose ring to its residue. RG then extends across the complex making several close Van der Waal contacts with the hydrophobic residues of both apoproteins and bacteriochlorophylls (McDermott et. al. 1995). In total RG makes contact with at least 15 amino acids residues of LH2 subunit. Both chlorophyll and RG are intertwined around α and β apoproteins of LH2. These interactions help RG to hold and lock the subunits in right orientation thus maintaining internal structural stability of LH2 complex. Furthermore they also fill up any space between each subunit further enhancing this stabilising effect. This stabilising nature of RG is confirmed through mutagenesis studies on LH2 complex (Zurdo et al. 1995) where mutants lacking carotenoid were generated. These mutants failed to assemble LH2 complex and the structure collapsed. As a consequence, this destabilised LH2 complex cannot absorb photons and failed to funnel this excitation energy energetically downhill towards reaction centre thereby reducing its photosynthetic function.
Conclusion
In conclusion Rhodopin glucoside is a primary carotenoid in LH2 complex of Rhodopesudomonas acidophila. Similar to other carotenoids in nature, the structure of RG governs its physical and chemical properties which in turn determine its function. It ensures RG remains in its designated cellular locations in correct orientation and interact with right molecule with correct bonds to function properly. The long conjugated chain and glucosyl head allows RG to serve its role as a light harvester by absorbing at visible region of spectrum and also as structural stabiliser by maintaining interactions with the molecules of LH2 complex.