An Intriguing Subtlety of Porphyrin Bio-Chemistry—Part II


The intriguing chemistry of porphyrins comes, in part, from the seemingly ease of synthesis with simple starting materials. Many University-level chemistry majors perform the lab work in their elementary sophomore/junior classes. The lab work is an adaptation of Paul Rothemund’s synthesis from 1935. A simplified (?) reaction scheme is illustrated in Fig. 1: (The beauty of the scheme is the “set-up” takes place in one reaction “vessel.” The major drawback is the low amount of desired product at the end of the lab.) The low “yield” at the other end (of the equation) seems to characterize a majority of attempts to emulate nature (?) in one reaction vessel. The reason for the statement is—try to imagine the length of time in which “life” itself seemed to utilize porphyrin-like structures—early bacteria utilizing chlorophyll or porphyrin-like structures. According to most estimates—it is in the ballpark of 500,000,000 years (1/2 of a billion years). The illustrated reaction takes approximately 30 minutes to complete—or one lab session.

Fig. 1 Source Wikipedia—One “pot” synthesis of a porphyrin

The attempts to understand (or bio-mimic) nature hinge upon certain aspects of classical chemistry (e.g. thermodynamics—the study of heat, temperature, energy and “randomness or dis-order.”) The important aspect of “randomness” is better termed—entropy. In a nutshell, entropy is a (major) controlling factor which might be stated: as time goes by . . . disorder increases. Entropy is a difficult concept (in part) because most of us think of the world as being readily determined—without complication. (Aside: entropy is just one reason why an internal combustion engine may get only 35 miles to a gallon of gasoline while it should, hypothetically, attain 45 miles to the gallon.)

The subtle bio-chemistry that utilizes porphyrins for respiration or sensory processes evolved a long, long time ago—it may not easily surrender the mechanistic detail of its origins. However, piecing the puzzle of our origins is a magnificent journey that appreciates with time.

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References and Links:

Porphyrin lab synthesis

  • P. Rothemund (1935). “Formation of Porphyrins from Pyrrole and Aldehydes”. J. Am. Chem. Soc.
    57 (10): 2010–2011. doi:10.1021/ja01313a510.

     

Thermodynamics for all

  • P. W. Atkins (2010). The Laws of Thermodynamics: A Very Short Introduction. Oxford University Press.

    (Available from Amazon or Barnes & Noble)

     

An Approach to Physical Law

  • R. Feynman (1964). The Character of Physical Law (The Messenger Lectures). MIT Press.

    (Available from Amazon or Barnes & Noble)


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