Polyene chains, hydrogen bonds

Hydrogen abstraction — The abstraction of a hydrogen atom H from a saturated carbon atom in a position allylic to the polyene chain can generate a resonance-stabilized neutral radical by homolytic cleavage of a C-H bond CAR = X - H. Then X - H -H R- X + RH. 

The one-dimensional chain of hydrogen atoms is merely a model. Flowever, compounds do exist to which the same kind of considerations are applicable and have been confirmed experimentally. These include polyene chains such as poly acetylene. The p orbitals of the C atoms take the place of the lx functions of the H atoms they form one bonding and one antibonding n band. Due to the Peierls distortion the polyacetylene chain is only stable with alternate short and long C-C bonds, that is, in the sense of the valence bond formula with alternate single and double bonds ... 

It appears, therefore, that the reaction of carbethoxynitrene with the polyenes opens the way to two new kinds of unsaturated macromolecules, one comprising the urethane groups which are the source of hydrogen bonds between the chains, the other free amine functional groups, capable of reacting like weak bases. An important aspect of these reactions lies in the degree of modification which is determined by the amount of reagent used and may therefore vary within considerable proportions. 

The method has been applied to polyene, polyacetylene and polydiacetylene chains, to formamide chains (both hydrogen-bonded and stacked). Applications have been done also to TCNQ and TTF stacks, to (SN) and to periodic DNA models (the four homopolynucleotides), to the sugar phosphate chain of DNA and to different periodic protein models (homopolypeptides). All these systems have relatively broad valence and conduction bands (bandwidths around or larger than 0.5eV) according to our results. 

Woodall et al. suggested that the different reactivities of the carotenoids against free radicals can be partly attributed to the differences in electron distribution along the polyene chain of different chromophores that would alter the susceptibility of free radical addition to the conjugated double bond system. However, other factors must be also considered. Stearic hindrance, hydrogen abstraction from the allylic position to the polyene chain (C-4 of P-carotene and its derivatives, end of lycopene), would reduce radical scavenging activity. In addition, the stability of the polyene radical is important in determining the rate of the loss of carotenoids and hence it affects their antioxidant activity. 

In contrast to what happens when only a single covalent bond connects two carbons, a double bond restricts the rotation around the carbon-to-carbon bond. This gives rise to geometric isomers. Consider a compound much more simple than the polyene chain of vitamin A, namely, dichlorethylene. The two carbons, the two hydrogens, and the two chlorides are located in the same plane, so there are two possible relationships for the two chlorides they can both be in the upper part of the plane cis form) or one chloride can be above and the other below (the median plane, or trans, form). 

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