Noncovalent Inhibitors of Sickle Hemoglobin Gelation: Effects of Aryl-Substituted Alanines

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The effects of 42 β-aryl-substituted alanines on the inhibition of gelation of sickle hemoglobin (Hb S) were evaluated. These included 18 derivatives of phenylalanine, 16 derivatives of tryptophan, and 4 analogues of each of these aromatic amino acids. Among the para-halogen-substituted phenylalanine derivatives, the order of molar effectiveness in increasing the solubility of deoxy-Hb S was I ∼ Br > CI > F; substituents at the equivalent 5 position of the indole ring of tryptophan gave the order Br > 0CH3 ∼ OH > CH3 > F. For three phenylalanine derivatives substituted on the alanyl side chain by a methyl group, the order was N-CH3 > α-CH3 ≫ β-CH3. A similar situation prevailed for the N-CH3 and α-CH3 derivatives of tryptophan. For two other para-substituted phenylalanines the nitro derivative was more effective than the amino derivative by nearly 3-fold. For the analogues of phenylalanine, β-(1-naphthyl)alanine was 2.5 times more effective than β-(2-naphthyl)alanine, while β-(2-thienyl)alanine and β-(3-thienyl)alanine were equipotent and somewhat less effective than phenylalanine. All three analogues of tryptophan were more potent than the parent compound and showed the following order of effectiveness: 7-azaindol-3-yl > 3-benzothienyl > 3-benzofuranyl. From the overall results obtained, the following generalizations can be made regarding the efficacy of β-aryl-substituted alanines as noncovalent inhibitors of gelation: (1) bicyclic aromatic nuclei are considerably more potent than monocyclic ones and (2) the nature of the substituent at a fixed position on a particular aromatic ring exerts a profound effect on the expression of antigelling activity. The best gelation inhibitor to emerge from this comprehensive survey was 5-bromotryptophan, which was 5 times more effective than phenylalanine. By suitable manipulation of either the aromatic nucleus or the alanyl side chain, it should be possible to increase antigelling activity to a level potent enough to be useful in a therapy for sickle cell anemia. © 1982, American Chemical Society. All rights reserved.

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