Conjugation, broken/extended

The magnetic symmetry of 1,3-butadiene [5C 1,4) 116.6 ppm 5C 2,3) 137.2 ppm] is broken by the introduction of an amino function at C(l) or C 2). The MINDO/3 calculated charge distribution for 1-dimethylamino- and 2-dimethyl amino-1,3-butadiene in the j-cis and -fran conformation appears in Table 5. The alternation of charge, typical of the enamine system, is maintained when the conjugation is extended, as is 97, the odd-numbered carbons bearing positive charge, and the nitrogen and even-numbered... 

Theoretical calculations indicate that it becomes progressively easier for a photochemical cis-trans isomerization to occur as the conjugation is extended. Furthermore, it is easiest to isomerize the central double bond and hardest to isomerize a thermal double bond. This conclusion is consistent with simple Hiickel theory since the stability of the two orthogonal radicals will be greatest when the bond broken is the central tt bond. 

In 1966 Dunathan (18) proposed that, in PLP-mediated reactions, the bond to be broken in the substrate-cofactor compound should be perpendicular to the plane of the extended conjugated system so that there would be maximum a-n overlap between the breaking bond and the ring-imine n system. Thus 3a, 3b, and 3c represent the conformations of the imine 3 best suited to achieve transamination reactions, decarboxylation reactions, and retroaldol reactions, respectively. The enzyme will be responsible for the orientation of the amino acid-PLP complex and thus dictate the nature of the resultant reaction. An example of an enzyme catalyzing two distinct reactions was found for serine hydroxymethyltransferase (EC 2.1.2.1), which normally catalyzes the retroaldol process outlined in 3c when L-serine or L-threonine are substrates. When D-alanine was used as substrate, however, a slow transamination was observed (19). Comparison of the conformations of the amino acid-PLP complexes, 3c and 17, respectively, for the retroaldol and transamination reactions shows that both the proton removed from the... 

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