Chlorin Explained

Chlorin should not be confused with chlorine.

In organic chemistry, chlorins are tetrapyrrole pigments that are partially hydrogenated porphyrins.^[1] The parent chlorin is an unstable compound which undergoes air oxidation to porphine.^[2] The name chlorin derives from chlorophyll. Chlorophylls are magnesium-containing chlorins and occur as photosynthetic pigments in chloroplasts. The term "chlorin" strictly speaking refers to only compounds with the same ring oxidation state as chlorophyll.

Chlorins are excellent photosensitizing agents. Various synthetic chlorins analogues such as m-tetrahydroxyphenylchlorin (mTHPC) and mono-L-aspartyl chlorin e6 are effectively employed in experimental photodynamic therapy as photosensitizer.^[3]

Chlorophylls

The most abundant chlorin is the photosynthetic pigment chlorophyll. Chlorophylls have a fifth, ketone-containing ring unlike the chlorins. Diverse chlorophylls exists, such as chlorophyll a, chlorophyll b, chlorophyll d, chlorophyll e, chlorophyll f, and chlorophyll g. Chlorophylls usually feature magnesium as a central metal atom, replacing the two NH centers in the parent.^[4]

Variation

Microbes produce two reduced variants of chlorin, bacteriochlorins and isobacteriochlorins. Bacteriochlorins are found in some bacteriochlorophylls; the ring structure is produced by Chlorophyllide a reductase (COR) reducing a chlorin ring at the C7-8 double boud.^[5] Isobacteriochlorins are found in nature mostly as sirohydrochlorin, a biosynthetic intermediate of vitamin B₁₂, produced without going through a chlorin. In living organisms, both are ultimately derived from uroporphyrinogen III, a near-universal intermediate in tetrapyrrole biosynthesis.^[6]

Synthetic chlorins

Numerous synthetic chlorins with different functional groups and/or ring modifications have been examined.^[7]

Contracted chlorins can be synthesised by reduction of B(III)subporphyrin or by oxidation of corresponding B(III)subbacteriochlorin.^[8] The B(III)subchlorins were directly synthesized as meso-ester B(III)subchlorin from meso-diester tripyrromethane, these class of compound showed very good fluorescence quantum yield and singlet oxygen producing efficiency^{[9] [10]}

See also

• Corrin
• Photodynamic therapy

Further reading

• Juse´lius. Jonas. Sundholm. Dage. The aromatic pathways of porphins, chlorins and bacteriochlorins. Physical Chemistry Chemical Physics. 2000. 2. 10. 2145–2151. 10.1039/b000260g. 2000PCCP....2.2145J.

Notes and References

1. European Journal of Biochemistry. 1988. 178. 2. 277–328. Nomenclature of Tetrapyrroles. Recommendations 1986. Gerard P. Moss. 10.1111/j.1432-1033.1988.tb14453.x. 3208761. free.
2. 10.1039/b002635m. Tetrapyrroles: The pigments of life. 2000. Battersby. Alan R.. Natural Product Reports. 17. 6. 507–526. 11152419.
3. Spikes. John D.. New trends in photobiology. Journal of Photochemistry and Photobiology B: Biology. July 1990. 6. 3. 259–274. 10.1016/1011-1344(90)85096-F. 2120404.
4. Book: K. Eszter . Borbas . Handbook of Porphyrin Science: 181: Chlorins . worldscientific . 10.1142/9789813149564_0001 . 9814322326 .
5. 10.1146/annurev.micro.61.080706.093242 . Chlorophyll Biosynthesis in Bacteria: The Origins of Structural and Functional Diversity . 2007 . Chew . Aline Gomez Maqueo . Bryant . Donald A. . Annual Review of Microbiology . 61 . 113–129 . 17506685 .
6. 10.1039/B002635M . Tetrapyrroles: The pigments of life: A Millennium review . 2000 . Battersby . Alan R. . Natural Product Reports . 17 . 6 . 507–526 . 11152419 .
7. 10.1021/acs.chemrev.5b00696. Synthetic Chlorins, Possible Surrogates for Chlorophylls, Prepared by Derivatization of Porphyrins. 2017. Taniguchi. Masahiko. Lindsey. Jonathan S.. Chemical Reviews. 117. 2. 344–535. 27498781. 1534468.
8. Osuka . Atsuhiro . Kim . Dongho . Synthesis and Characterization of meso-Aryl-Substituted Subchlorins . Journal of the American Chemical Society . 2008 . 130 . 2 . 438–439 . 10.1021/ja078042b . 18095693 .
9. Meso-Free Boron(III)subchlorin and Its μ-Oxo Dimer with Interacting Chromophores . Organic Letters . 3 Dec 2020 . 22 . 24 . 10.1021/acs.orglett.0c03813 . Chandra . Brijesh . Soman . Rahul . Sathish Kumar . B. . Jose . K. V. Jovan . Panda . Pradeepta K. . 9735–9739 . 33270460 . 227282229 .
10. A2B- and A3-Type Boron(III)Subchlorins Derived from meso-Diethoxycarbonyltripyrrane: Synthesis and Photophysical Exploration . The Journal of Organic Chemistry . 15 Jul 2021 . 10.1021/acs.joc.1c01001 . Soman . Rahul . Chandra . Brijesh . Bhat . Ishfaq A. . Kumar . B. Sathish . Hossain . Sk Saddam . Nandy . Sridatri . Jose . K. V. Jovan . Panda . Pradeepta K. . 86 . 15 . 10280–10287 . 34264670 . 235959639 .