N-Hydroxyproline

As shown in Scheme 54, proline was transformed into the N-cyano-ethyl derivative (270) and oxidised with m-chloroperbenzoic acid to the ra 5 -N-oxide (271). Cope elimination in boiling acetone gave N-hydroxyproline (44). N-Hydroxypipecolic acid (45) and N-hydroxysar-cosine (43) were obtained analogously 101). 

The tendencies of the amino acids to stabilize or destabilize a-helices are different in typical proteins than in polyamino acids. The occurrence of the common amino acids in helices is summarized in Table 6.1. Notably, proline (and hydroxyproline) act as helix breakers due to their unique structure, which fixes the value of the —N—C bond angle. Helices can be formed from either... 

In a study of the deprotection of the MEM ethers of hydroxyproline and serine derivatives, it was found that the MEM group was stable to conditions that normally cleave the t-butyl and BOC groups [CF3COOH, CH2CI2, 1 1 (v v)]. The MEM group was also stable to 0.2 N HCl, but not stable to 2.0 N HCl or HBr-AcOH. ... 

Early examples of enantioselective extractions are the resolution of a-aminoalco-hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion) [184-186] by partition between aqueous and lipophilic phases containing esters of tartaric acid [184-188]. Alkyl derivatives of proline and hydroxyproline with cupric ions showed chiral discrimination abilities for the resolution of neutral amino acid enantiomers in n-butanol/water systems [121, 178, 189-192]. On the other hand, chiral crown ethers are classical selectors utilized for enantioseparations, due to their interesting recognition abilities [171, 178]. However, the large number of steps often required for their synthesis [182] and, consequently, their cost as well as their limited loadability makes them not very suitable for preparative purposes. Examples of ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino acid derivatives have also been described. 

For the separation of D,L-leucine, Ding et al. [62] used poly(vinyl alcohol) gel-coated microporous polypropylene hollow fibers (Fig. 5-11). An octanol phase containing the chiral selector (A-n-dodecyl-L-hydroxyproline) is flowing countercur-rently with an aqueous phase. The gel in the pores of the membrane permits diffusion of the leucine molecules, but prevents convection of the aqueous and octanol phase. At a proper selection of the flow ratios it is possible to achieve almost complete resolution of the D,L-leucine (Fig. 5-12). 

The submitters obtained N-Boc-4-hydroxyproline methyl ester from Synthetech, Inc. [Albany, OR, (503) 967-6575] or Bachem California [Torrance, CA, (310) 530-1571] and dried THF over 4A molecular sieves for two days prior to use (Karl-Fischer titration gave 145 pg/mL water). The checkers purchased N-Boc hydroxyproline methyl ester (97%) and fert-butyl acetate (99+%) from Aldrich Chemical Co. and obtained THF (HPLC grade) from Fisher Scientific. 

HydTOX5 proline-derived polyesters are usually readily soluble in a variety of organic solvents (benzene, toluene, chloroform, dichloro-methane, carbon tetrachloride, tetrahydrofuran, dimethylformamide, etc.) As expected, the solubility in hydrophobic solvents increased with increasing chain length of the N protecting group, while the solubility in polar solvents decreased. For example, poly(N-hexanoyl-hydroxyproline ester) is slightly soluble in ether but easily soluble in acetonitrile, while poly(N-palmitoylhydroxyproline ester) is readily soluble in ether but virtually insoluble in acetonitrile. 

The easy processibility of hydroxyproline-derived polyesters is in marked contrast to the unfavorable material properties of most conventional poly (amino acids) that cannot usually be processed into shaped objects by conventional polymer-processing techniques (7). Furthermore, since the synthesis of poly(N-acylhydroxyproline esters) does not require the expensive N-carboxyanhydrides as monomeric starting materials, poly(N-acylhydroxyproline esters) should be significantly less expensive than derivatives of conventional poly(hy-droxyproline). 

Compression-molded devices of poly(N-palmitoyl hydroxyproline ester) (side chain length 16 carbons), poly(N-decanoylhydroxy-proline ester) (side chain length 10 carbons), and poly(JN-hexanoyl-hydrox roline ester) (side chain length 6 carbons) were prepared with dye contents of 1,5,10, and 20% of either -nitroaniline or acid orange. Release curves were obtained by placing the loaded devices into phospate buffer (pH 7.4) at 37 C. The amount of released dye was followed spectrophotometrically in the usual fashion. 

In contrast, implantation of four small pieces of poly(N-palmitoyl-hydroxyproline ester) into four rabbit corneas elicited no pathological response in three corneas and a very mild inflammatory response in one cornea. Histological examination of the corneas 4 weeks postimplantation showed no invading blood vessels or migrating inflammatory cells in the area around the implants. 

ENANTIOSELECTIVE SYNTHESIS OF (Z)-N-CARBOBENZYLOXY-3-HYDROXYPROLINE ETHYL ESTER... 

Plaquet et al. (PI) found in the urine of rachitic children peptides consisting of proline, hydroxyproline, and glycine, which they believed to be the products of collagen degradation. Two similar peptides containing considerable amounts of proline and hydroxyproline were isolated from the urine of a patient with rheumatoid arthritis by Mechanic et al. (Ml). One of these peptides consisted of three proline, two hydroxyproline, and nine glutamic acid residues, the second one consisted of four proline, four hydroxyproline, and one glutamic acid residues. The N-terminal amino acid in the first peptide was demonstrated to be hydroxyproline. 

H Kuroda, S Kubo, N Chino, T Kimura, S Sakakibara. Unexpected racemization of pro line and hydroxyproline phenacyl ester during coupling reactions with Boc-amino acids. Int J Pept Prot Res 40, 114, 1992. 

Nitrosated amino acids during cooking yield corresponding volatile nitrosamines N-nitrosoproline (NPRO), A-nitroso-4-hydroxyproline (NHPRO) and N-nitrososarcosine (NSAR), respectively. 

Collagen is the most abundant extracellular matrix protein family in vertebrates. Proteins in the collagen superfamily all have three polypeptide chains with the required -Gly-Xaa-Yaa- repeated sequence, where Xaa and Yaa are frequently proline and 4-hydroxyproline, respectively. At present, more than 30 molecular species of vertebrate proteins called collagen are classified into 28 types as type I, II, III,..., XXVIII. They are typically called type N collagen , or collagen N . In addition, there are many more collagen-like proteins that... 

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