Scanning tunnelling microscopy Thin-aye rom Ultraviolet ncy disorde scattering
xxix STM Scanning tunnelling microscopy TEM Transmission electron microscopy TLC Thin-layer chromatography UV Ultraviolet UV/Vis Ultraviolet/visible VADD Vitamin A deficiency disorder WAXS Wide-angle X-ray scattering XBP Xanthophyll-binding protein
Carotenoids 2008 Birkhauser Verlag Basel Chapter 1 Special Molecules,Special Properties George Britton,Synnove Liaaen-Jensen and Hanspeter Pfander A.Introduction This is a book about functions of carotenoids.The reader may therefore be surprised to find that the first half of the book is chemistry.This should not be a surprise,however.The targe for researchers now is not simply to discover and describe functions and actions,but to understand their mechanisms.This requires understanding of the underlying fundamental principles and appreciation of the application of the advanced techniques now used to elucidate details of structure and of processes that may occurona very short timescale Carotenoids Volumes 4 and 5 take the carotenoid story into the realm of biology.dealing with the functions of carotenoids and the actions of carotenoids in nutrition and health.They can be used as stand-alone books,but they are really planned to complete the coverage of the carotenoid field begun in earlier volumes,as the final part of a coordinated series. The material presented in the earlier volumes is all relevant to studies of biological functions and actions.The common view that carotenoids are difficult to work with may be daunting to newcomers to the field,especially if they do not have a strong background in chemistry and analysis.There are difficulties;carotenoids are less stable than most natural products,but ways to overcome the difficulties and to handle these challenging compounds are well established.These are described and discussed in Volume 14,which,together with Polume /B.gives a comprehensive treatment of the isolation.analysis and spectroscopic characterization of carotenoids that is an essential foundation for all carotenoid work. Biological studies must be supported by a rigorous analytical base.Carotenoids must be identified unequivocally and the various analytical procedures described in Volumes 1A and
Carotenoids Volume 4: Natural Functions © 2008 Birkhäuser Verlag Basel Chapter 1 Special Molecules, Special Properties George Britton, Synnøve Liaaen-Jensen and Hanspeter Pfander A. Introduction This is a book about functions of carotenoids. The reader may therefore be surprised to find that the first half of the book is chemistry. This should not be a surprise, however. The target for researchers now is not simply to discover and describe functions and actions, but to understand their mechanisms. This requires understanding of the underlying fundamental principles and appreciation of the application of the advanced techniques now used to elucidate details of structure and of processes that may occur on a very short timescale. Carotenoids Volumes 4 and 5 take the carotenoid story into the realm of biology, dealing with the functions of carotenoids and the actions of carotenoids in nutrition and health. They can be used as stand-alone books, but they are really planned to complete the coverage of the carotenoid field begun in earlier volumes, as the final part of a coordinated series. The material presented in the earlier volumes is all relevant to studies of biological functions and actions. The common view that carotenoids are difficult to work with may be daunting to newcomers to the field, especially if they do not have a strong background in chemistry and analysis. There are difficulties; carotenoids are less stable than most natural products, but ways to overcome the difficulties and to handle these challenging compounds are well established. These are described and discussed in Volume 1A, which, together with Volume 1B, gives a comprehensive treatment of the isolation, analysis and spectroscopic characterization of carotenoids that is an essential foundation for all carotenoid work. Biological studies must be supported by a rigorous analytical base. Carotenoids must be identified unequivocally and the various analytical procedures described in Volumes 1A and
2 George Britton,Synnove Liaaen-Jensen and Hanspeter Pfander 1B must be understood and applied correctly,whether they are being used for quantitative analysis.identification or in complex studies of carotenoids in situ. Volume 2 describes methods for the chemical synthesis of carotenoids on a laboratory and a commercial scale.Synthetic compounds are needed as analytical standards and on a larger scale for biological trials.Functions and actions are inextricably linked with biosynthesis and metabolism,as covered in Volme 3. Biologists recognize the value of Field Guides for identifying species they encounter.The Carofenoids Handbook is the Field Guide'for the identification of carotenoids. B.Structure,Properties and Function The carotenoids have special properties that no other group of substances possesses.The functions and actions of carotenoids depend on these special properties.These properties are determined by the structural features of the Understanding the intricacies of the relationships between structure,properties and function is therefore essential for understanding the importance of carotenoids in a biological context,as well as for devising and optimizing commercial applications. The Carotenoids Handbook lists more than 700 carotenoids that have been isolated from natural sources.The structures of about 500 of these have been fully elucidated.So why is it necessary to address structure again?In Voltmne 4.the structural features that are identified as being most important for determining the biological roles of carotenoids are emphasized.The overall molecular geometry (size,three-dimensional shape,presence of functional groups)is vital for ensuring that the carotenoid fits into cellular,sub-cellular and molecular structures in the correct location and orientation to allow it to function efficiently.Then the conjugated double-bond system determines the light-absorption properties and chemical reactivity that form the basis of most functions.Specific interactions with other molecules in the immediate vicinity strongly influence the properties of a carotenoid and are also crucial to functioning. 1.Three-dimensional shape Carotenoids are not simply flat two-dimensional structures,like formulae drawn on a page. They have a precise three-dimensional shape which is an important determinant of their functions.Several different stereochemical factors contribute to the shape of the molecule and must be considered when describing and defining the three-dimensional structure. a)Configuration:geometrical isomers Any carotenoid can exist in a number of geometrical (cis/trans or E/Z)isomeric forms.There is currently much interest in cis isomers and whether their different shape,solubility and
2 George Britton, Synnøve Liaaen-Jensen and Hanspeter Pfander 1B must be understood and applied correctly, whether they are being used for quantitative analysis, identification or in complex studies of carotenoids in situ. Volume 2 describes methods for the chemical synthesis of carotenoids on a laboratory and a commercial scale. Synthetic compounds are needed as analytical standards and on a larger scale for biological trials. Functions and actions are inextricably linked with biosynthesis and metabolism, as covered in Volume 3. Biologists recognize the value of Field Guides for identifying species they encounter. The Carotenoids Handbook is the ‘Field Guide’ for the identification of carotenoids. B. Structure, Properties and Function The carotenoids have special properties that no other group of substances possesses. The functions and actions of carotenoids depend on these special properties. These properties are determined by the structural features of the molecules. Understanding the intricacies of the relationships between structure, properties and function is therefore essential for understanding the importance of carotenoids in a biological context, as well as for devising and optimizing commercial applications. The Carotenoids Handbook lists more than 700 carotenoids that have been isolated from natural sources. The structures of about 500 of these have been fully elucidated. So why is it necessary to address structure again? In Volume 4, the structural features that are identified as being most important for determining the biological roles of carotenoids are emphasized. The overall molecular geometry (size, three-dimensional shape, presence of functional groups) is vital for ensuring that the carotenoid fits into cellular, sub-cellular and molecular structures in the correct location and orientation to allow it to function efficiently. Then the conjugated double-bond system determines the light-absorption properties and chemical reactivity that form the basis of most functions. Specific interactions with other molecules in the immediate vicinity strongly influence the properties of a carotenoid and are also crucial to functioning. 1. Three-dimensional shape Carotenoids are not simply flat two-dimensional structures, like formulae drawn on a page. They have a precise three-dimensional shape which is an important determinant of their functions. Several different stereochemical factors contribute to the shape of the molecule and must be considered when describing and defining the three-dimensional structure. a) Configuration: geometrical isomers Any carotenoid can exist in a number of geometrical (cis/trans or E/Z) isomeric forms. There is currently much interest in cis isomers and whether their different shape, solubility and
Special Molecules,Special Properties stability compared with the linear all-trans isomer give rise to different biological properties The topic of geometrical isomers and isomerization is treated in detail in Chapter 3. b)Absolute configuration:chirality Most of the known carotenoids have structures that contain at least one chiral centre or axis, and can therefore exist as different optical isomers,including enantiomers.With carotenoids as with other compounds,biological actions maybe specific for one enantiomer Structural aspects of chirality are treated in Chapter 2. e)Conformation In principle,rotation is possible about any C-C single bond,so a carotenoid can, theoretically,adopt an infinite number of shapes or conformations.In practice,however,the X-ray crystallography methods to define the linear extended conformation of the rigid polyene chain,ring shape,and the preferred angle of twisting about the C(6)-C(7)single 2.The conjugated double-bond system The extended delocalized x-electron system that characterizes the central part of the structure is the key to many important properties of carotenoids a)Light absorption and photochemical properties In carotenoids,the -electrons of the conjugated double-bond system are highly delocalized, so the energy required to bring about the transition to the comparatively low-energy excited state is relatively small and corresponds to light in the visible region in the wavelength rang 400-500 nm.This gives rise to the yellow,orange and red colours generally associated with carotenoids.The relationship between electronic structure and light-absorption properties is developed more fully in Chapter9and in Volume IB.Chapter 1.The relationship between the polyene chromophore and absorption spectrum,used in the analysis of carotenoids,is discussed in Volume IB.Chapter 2. The energy levels of carotenoid singlet and triplet states are ideally positioned for the carotenoids to participate in energy-transfer processes.This singlet-singlet and triplet-triple energy transfer is the basis for both the accessory light-harvesting role and the photoprotective roles of carotenoids,respectively.The fundamental photochemistry and photophysics of carotenoids and their roles in energy transfer and related processes ar explained in Chapter 9.The important functioning of carotenoids in light-harvesting and
Special Molecules, Special Properties 3 stability compared with the linear all-trans isomer give rise to different biological properties. The topic of geometrical isomers and isomerization is treated in detail in Chapter 3. b) Absolute configuration: chirality Most of the known carotenoids have structures that contain at least one chiral centre or axis, and can therefore exist as different optical isomers, including enantiomers. With carotenoids as with other compounds, biological actions may be specific for one enantiomer. Structural aspects of chirality are treated in Chapter 2. c) Conformation In principle, rotation is possible about any C-C single bond, so a carotenoid can, theoretically, adopt an infinite number of shapes or conformations. In practice, however, the carotenoid will exist in a particular preferred, low-energy conformation. The application of X-ray crystallography methods to define the linear extended conformation of the rigid polyene chain, ring shape, and the preferred angle of twisting about the C(6)-C(7) single bond in carotenoids with cyclic end groups is described in Chapter 4. 2. The conjugated double-bond system The extended delocalized -electron system that characterizes the central part of the structure is the key to many important properties of carotenoids. a) Light absorption and photochemical properties In carotenoids, the -electrons of the conjugated double-bond system are highly delocalized, so the energy required to bring about the transition to the comparatively low-energy excited state is relatively small and corresponds to light in the visible region in the wavelength range 400-500 nm. This gives rise to the yellow, orange and red colours generally associated with carotenoids. The relationship between electronic structure and light-absorption properties is developed more fully in Chapter 9 and in Volume 1B, Chapter 1. The relationship between the polyene chromophore and absorption spectrum, used in the analysis of carotenoids, is discussed in Volume 1B, Chapter 2. The energy levels of carotenoid singlet and triplet states are ideally positioned for the carotenoids to participate in energy-transfer processes. This singlet-singlet and triplet-triplet energy transfer is the basis for both the accessory light-harvesting role and the photoprotective roles of carotenoids, respectively. The fundamental photochemistry and photophysics of carotenoids and their roles in energy transfer and related processes are explained in Chapter 9. The important functioning of carotenoids in light-harvesting and
4 George Britton,Synnove Liaaen-Jensen and Hanspeter Pfander photoprotection in photosynthesis are described in detail in Chapter /4.Photoprotection in eye and in skin can be found in Volume 5.Chapters 15 and 16. b)Reactivity The susceptibility of the electron-rich polyene chain to attack by electrophilic reagents and oxidizing free radicals is the basis for the behaviour of carotenoids as antioxidants or pro- biological significance for health in Volume 5,Chapter 12. This susceptibility to oxidation also has important practical implications.Pure.even erystalline,carotenoids may be broken down rapidly if stored in the presence of traces of oxygen.Even if there are no obvious signs of bleaching,stored samples may contain strongly pro-oxidant carotenoid peroxides.Standards should be monitored for purity This instability can have serious consequences for large-scale trials of carotenoids for biological activity.Extreme care should be taken to ensure that samples used in such investigations are free from peroxides and other degradation products,otherwise misleading results may be obtained. c)Carotenoid radicals Carotenoid radicals and radical ions are stabilized by delocalization of the unpaired along the polyene chain and have distinctive properties that are relevant to the functioning of the carotenoids.for example.in photosynthesis and as antioxidants/pro-oxidants.Under standing the structure,properties and reactivity of these radicals,as presented in Chapter7is therefore essential.The remarkable light-absorption properties of radical cations are treated in Chapter 8. 3.Molecular interactions Following decades of extensive investigation by chemists much is known about the properties of carotenoids in simple organic solution,but this only tells part of the story about carotenoid properties in vivo.In sifu in living organisms,and in formulations for commercial applications,carotenoids are part of a much more complex system,and are frequently found in organized and ordered sub-cellular structures.The physical and chemical properties of a carotenoid are inevitably influenced by interactions with other molecules in its immediate vicinity.such as proteins and lipids.Complementary to this.the carotenoid may influence these neighbouring molecules and the structure and properties of the matrix in which it is located.Understanding these interactions and their effects is thus a major part of under- standing how carotenoids function and act in vivo,and is addressed in Chapters 5 and 6
4 George Britton, Synnøve Liaaen-Jensen and Hanspeter Pfander photoprotection in photosynthesis are described in detail in Chapter 14. Photoprotection in other systems is outlined in Chapter 10. Examples of photoprotection by carotenoids in the eye and in skin can be found in Volume 5, Chapters 15 and 16. b) Reactivity The susceptibility of the electron-rich polyene chain to attack by electrophilic reagents and oxidizing free radicals is the basis for the behaviour of carotenoids as antioxidants or prooxidants (see Chapter 10). The chemical principles of this are treated in Chapter 7, and the biological significance for health in Volume 5, Chapter 12. This susceptibility to oxidation also has important practical implications. Pure, even crystalline, carotenoids may be broken down rapidly if stored in the presence of traces of oxygen. Even if there are no obvious signs of bleaching, stored samples may contain strongly pro-oxidant carotenoid peroxides. Standards should be monitored for purity. This instability can have serious consequences for large-scale trials of carotenoids for biological activity. Extreme care should be taken to ensure that samples used in such investigations are free from peroxides and other degradation products, otherwise misleading results may be obtained. c) Carotenoid radicals Carotenoid radicals and radical ions are stabilized by delocalization of the unpaired electron along the polyene chain and have distinctive properties that are relevant to the functioning of the carotenoids, for example, in photosynthesis and as antioxidants/pro-oxidants. Understanding the structure, properties and reactivity of these radicals, as presented in Chapter 7 is therefore essential. The remarkable light-absorption properties of radical cations are treated in Chapter 8. 3. Molecular interactions Following decades of extensive investigation by chemists, much is known about the properties of carotenoids in simple organic solution, but this only tells part of the story about carotenoid properties in vivo. In situ in living organisms, and in formulations for commercial applications, carotenoids are part of a much more complex system, and are frequently found in organized and ordered sub-cellular structures. The physical and chemical properties of a carotenoid are inevitably influenced by interactions with other molecules in its immediate vicinity, such as proteins and lipids. Complementary to this, the carotenoid may influence these neighbouring molecules and the structure and properties of the matrix in which it is located. Understanding these interactions and their effects is thus a major part of understanding how carotenoids function and act in vivo, and is addressed in Chapters 5 and 6