146 BIG MOLECULES 



The salient point is the following: If the extra energy in the molecule is large 

 enough, quite by chance it may collect at a critical bond and loosen it sufficiently so 

 that a rearrangement of groups within the molecule can occur, and thus produce a dif- 

 ferent isomer. When this occurs in the DNA molecule of the gene, a mutation is the 

 result. 



There are many other biological processes which seem to involve excited 

 electronic states of molecules: oxidations seem to be in a class by themselves 

 because of the number of reactions of molecule + 2 + hght which have 

 been demonstrated. In some reactions light is absorbed, and then im- 

 mediately (within 10~ 12 sec) re-emitted, at least in part (fluorescence); in 

 others the absorbed energy is retained for some appreciable time, perhaps a 

 few seconds (phosphorescence). However, the extra energy to excite elec- 

 trons in a molecule may also be derived from chemical reactions in the 

 metabolism, for there is plenty of it there! This obviously occurs in some 

 bacteria (pseudomonas, vibrio, etc.), some crustaceans, the elaterid beetle, 

 and the firefly, for these animals are chemiluminescent. 



That human beings are not luminescent may be a subtle reminder of two 

 important facts: (a) in man the energy-producing metabolic reactions are 

 more carefully delineated by enzymes, constrained to occur in many small 

 steps, each one linked intimately with an energy-consuming metabolic 

 process; and (b) there are electron and proton transfer reactions along large 

 molecules, transfer mechanisms which can conduct the "energy" to where 

 it can be used. In other words, in humans, because of the extra complexity 

 of the system, the extra energy of excitation of molecules need not be radi- 

 ated and lost; there is a mechanism provided by which it can be used. 



This can be illustrated further. Although most proteins in vitro have no 

 phosphorescence at room temperature where molecular mechanical motion 

 is relatively large, at low temperature (77° K) all the following proteins, plus 

 at least 18 amino acids, show phosphorescence: fibrinogen, y 2 globulin, 

 keratin, gelatin, zein, and bovine serum albumin, as well as egg albumin and 

 silk fibroin. Aromatic rings with it (Pi) bonds in the molecules are a neces- 

 sary condition for the phosphorescence. 



In some simple organic molecules (certain ketones, for example) the extra 

 energy has been found to excite one of the unshared pair or nonbonding (n) 

 electrons on the oxygen atom. Its excited position is one of the so-called 

 7r positions or orbitals of the molecule. The transition is called an "/? — ir" 

 transition (Figure 6-10). The energy absorbed during an n - w transition is 

 about 80 kcal/mole, and can be produced by ultraviolet light of wave length 

 about 3000 A. 



The unshared pair of electrons form no bond, but they are paired in the 

 sense that they have opposite "spins." The molecule which contains only 

 paired electrons is said to be in a "singlet" 1 state (S = In + 1,' where n is 



