P rings

P-type manifold—veiiical configuration. The P ring dryer (see Fig. 12-100) incorporates a single-stage classifier and was developed specifically for use with heat-sensitive materials. The undried material is reintroduced into a cool part of the dryer in which it recirculates until it is dry enough to leave the circuit. 

The oxidation of carotenes results in the formation of a diverse array of xanthophylls (Fig. 13.7). Zeaxanthin is synthesised from P-carotene by the hydroxylation of C-3 and C-3 of the P-rings via the mono-hydroxylated intermediate P-cryptoxanthin, a process requiring molecular oxygen in a mixed-function oxidase reaction. The gene encoding P-carotene hydroxylase (crtZ) has been cloned from a number of non-photosynthetic prokaryotes (reviewed by Armstrong, 1994) and from Arabidopsis (Sun et al, 1996). Zeaxanthin is converted to violaxanthin by zeaxanthin epoxidase which epoxidises both P-rings of zeaxanthin at the 5,6 positions (Fig. 13.7). The... 

CRTR-e = e-ring hydroxyiase CRTR-b = p-ring hydroxylase ZEP-1 = zeaxanthin epoxidase NXS = neoxanthin synthase VDE 1 = violaxanthin de-epoxidase. 

Most carotenoids are ahphatic, bnt some carotenoids in Chlorobiaceae and Chloroflexaceae have aromatic or P-rings. 

It is assumed that in order to have vitamin A activity a molecule must have essentially one-half of its structure similar to that of (i-carotene with an added molecule of water at the end of the lateral polyene chain. Thus, P-carotene is a potent provitamin A to which 100% activity is assigned. An unsubstituted p ring with a Cii polyene chain is the minimum requirement for vitamin A activity. y-Car-otene, a-carotene, P-cryptoxanthin, a-cryptoxanthin, and P-carotene-5,6-epoxide aU have single unsubstimted rings. Recently it has been shown that astaxanthin can be converted to zeaxanthin in trout if the fish has sufficient vitamin A. Vitiated astaxanthin was converted to retinol in strips of duodenum or inverted sacks of trout intestines. Astaxanthin, canthaxanthin, and zeaxanthin can be converted to vitamin A and A2 in guppies. ... 

Different monooxygenase enzymes hydroxylate the 3 position of the P- and s-rings of a-carotene. Hydroxylation of one ring of P-carotene produces P-cryp-toxanthin and hydroxylation of both P-rings produces zeaxanthin. Hydroxlyation... 

The 3-hydroxyl P-rings of zeaxanthin are further oxygenated by the introduction of 5,6-epoxy moieties by zeaxanthin epoxidase (ZEP). A mono-epoxidated intermediate, antheraxanthin is produced, followed by the di-epoxy xanthophyU, violaxanthin, as shown in Figure 5.3.3B. 

Yamashita M (2007) Preparation, Structure, and Biological Properties of Phosphorus Heterocycles with a C - P Ring System. 8 173-222... 

A large part of the chemical reactions of 1,3,2-diazaphosphole and NHP derivatives reported to date include transformations under substitution of functional substituents at the C, N, or P ring atoms. The interest in several of these displacement processes was mainly directed by the desire to develop synthetic pathways for specifically... 

P. Ring, Progr. Part. Nucl. Phys. 37 193 (1996), and refs therein... 

Colquhoum, H. M. Lewis, D. E Ben-Haida, A. Hodge, P. Ring-chain interconversion in high-performance polymer system. 2. Ring-opening polmerization-copolyetherification in the synthesis of aromatic poly(ether sulfones). Macromolecules 2003, 36, 3775-3778. 

P. Ring and P. Schuck, The NuclearMcmy-Bocfy Problem (Springer Veriag,... 

P. Ring and Schuck, The Nuclear Many-Body Problems, (Springer-Verlag, Berlin, 1980). 

Dreyfuss, P., Dreyfuss, M.P., Ring Opening Polymerization. Frisch, K.C., Reegen, S.L., eds. (1969) Marcel Dekker, New York, Chapter 2... 

---