18 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



irons in optically active molecules tend to move in slightly skewed paths 

 when light shines on them. It is this skewness that causes the rotation of 

 the plane of polarization of light. Unfortunately the mechanism by which 

 the skewness does this is too complicated to be discussed here, but it is not 

 necessary to know the mechanism in order to understand the discussion 

 that follows. (See ref. 16 for a detailed discussion of the mechanism.) 



When the positions of the groups in the immediate vicinity of a chromo- 

 phoric group are altered, the skewness that they cause in the currents in- 

 duced by the light is changed and the magnitude of the optical rotation 

 changes. It turns out that these effects are so large that the optical rota- 

 tion is very much more sensitive than most properties to rather subtle 

 modifications of molecular structure. For instance, in a molecule such as 

 secondary butyl bromide, there is every reason to believe that the mag- 

 nitude (and even the sign) of the optical rotation will change drastically 

 when the ethyl group in the left half of the molecule (fig. 2) is turned about 

 the central carbon-carbon bond. Other properties, such as the refractive 

 index, polarizability and infra-red and ultra-violet spectra, would hardly 

 be affected by this slight modification in the structure. Such structural 

 changes, which involve a twisting of the molecular framework without 

 making any changes in the chemical composition and without breaking any 

 chemical bonds, are called changes in conformation. (The conformation of 

 a molecule is also sometimes called its constellation. The word configura- 

 tion is also frequently used by protein and polymer chemists as a synonym 

 for conformation, but this word has long been used by organic chemists 

 with reference to the spatial arrangement of the groups attached to the 

 asymmetric carbon atom.) 



Whenever a substance undergoes a large change in optical rotation 

 without any appreciable chemical modification of its structure, w^e can 

 be sure that there has been a change in the molecular conformation. It 

 is therefore highly significant that when proteins are denatured, they 

 usually undergo large changes in optical rotation. The magnitudes of the 

 changes will be discussed in more detail below, but the mere fact that 

 the changes are as large as they are offers strong support for the concept 

 that denaturation involves changes in the manner in which the protein 

 molecule is folded. 



Unfortunately our understanding of the details of the relationship be- 

 tween molecular structure and the magnitude of the optical activity^ is 

 still in a rather primitive state. Therefore, if the optical activity is observed 



* In this paragraph the term 'optical rotation' will be replaced by the equivalent 

 but less precise designation 'optical activity'. This helps to avoid confusion between 

 the rotation of groups about single bonds and the rotation of the plane of polarization 

 of polarized light. 



