1 70 Discussion 



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DISCUSSION 



Interpretations of Absorption Spectra of Haemoproteins 



Perrin: Would Drabkin please indicate the theoretical significance he attaches to his 

 serial band analysis? In, say. Fig. 9 of his paper (p. 155), how much latitude does his 

 assigning of seven bands, with the resulting sixteen adjustable constants, allow in 

 analysing the spectrum? 



Drabkin: I have frankly admitted in my paper that my analytical approach was an 

 empirical one. My proposal was originally made a good many years before high- 

 and low-spin complexes were recognized and before it was fashionable to speak of 

 d- and 7T-electrons. It seemed to me very worthwhile, as a first step, to make the most 

 simplifying assumptions. I used Gaussian curves because I hked their shape, and 

 perhaps naively believed that the components (bands) in the complex absorption 

 curves might indeed have such a form. I may say that I anticipated Perrin's searching 

 questions, and posed them to myself, without a wholly satisfactory answer. 



I recognize that from a rigid physical viewpoint the bands are spaced too far apart 

 to be regarded as electronic in the usual sense, and probably represent a special case. 

 As to the latitude of the analytical procedure, it is probably rather broad. For the 

 pronounced maxima in the absorption spectra they would appear to be unequivocal. 

 For regions of masked absorption the solution may not be unique. However, reference 

 to my Table 2 (p. 153) will disclose that assembling the various derivatives, actual 

 representatives with definite maxima in the postulated locations have been found. It 

 should be stressed that this is the important experimental finding, independent of and, 

 indeed, guiding the analysis. Yet, the proposal of the equally spaced, frequency- 

 distributed series was made before all the data were available. The possible existence 

 of certain maxima was prophesied, a prophecy fulfilled by later finding them in the 

 spectra of oxidized and reduced cytochrome c. 



Williams: My views and those of Drabkin as to the nature of iron porphyrin spectra are 

 rather different. Drabkin has given us a detailed survey, empirically based, of a large 

 number of absorption bands. This will be most valuable. I have attempted to use 

 Piatt's theory {Radiation Biology, 3, 1956) of the porphyrin spectra to interpret the 

 spectra of metal porphyrin complexes. The analysis led to the conclusion that some 

 bonds in Fe+++ porphyrins were due to the iron and had little to do with the porphyrin, 

 notably at 650 m/f and about 500 m/f. In Fe++ porphyrins there is a band at about 

 500 m/< due to the iron alone. There is no requirement for a frequency series in 

 Piatt's theory. 



