George W. Kenner

Peptides—XI: Synthesis of peptides derived from alpha-methylalanine

Abstract Union of α-methylalanyl residues in a peptide chain is severely sterically hindered. This hindrance can be overcome by employing either the mixed anhydride (I) of pivalic acid and benzyloxycarbonyl-α-methylalanine or, more generally, oxazolones (II), including 4,4-dimethyl-2-trifluoromethyloxazolone (II; R = CF 3 ). 4,4-Dimethyloxazolone (II; R = H) has been obtained in solution and characterized by its infra-red spectrum; it is attacked by α-methylalanine methyl ester at the methine group, in contrast to the normal reaction with cyclohexylamine at the carbonyl group.

Pyrroles and related compounds. X.

Abstract The molecular ion nearly always produces the strongest peak in mass spectra of porphyrins. The macrocyclic nucleus is remarkably stable and fragmentation gives mainly “benzylic” ions. In derivatives of porphin this involves cleavage at the bond once removed from the macrocycle, but in chlorins the entire substituent is lost from the reduced “pyrrole” ring. Methyl esters of chlorins derived from chlorophyll lose directly 147 and 159 mass units in complex processes marked by strong metastable peaks. All the spectra contain a prominent series of peaks from doubly charged ions; a novel feature is loss of ketene from propionate side-chains.Abstract The molecular ion nearly always produces the strongest peak in mass spectra of porphyrins. The macrocyclic nucleus is remarkably stable and fragmentation gives mainly “benzylic” ions. In derivatives of porphin this involves cleavage at the bond once removed from the macrocycle, but in chlorins the entire substituent is lost from the reduced “pyrrole” ring. Methyl esters of chlorins derived from chlorophyll lose directly 147 and 159 mass units in complex processes marked by strong metastable peaks. All the spectra contain a prominent series of peaks from doubly charged ions; a novel feature is loss of ketene from propionate side-chains.

Assignments of the paramagnetically shifted heme methyl nuclear magnetic resonance peaks of cyanometmyoglobin by selective deuteration

Abstract Three of the four paramagnetically shifted heme methyl nuclear magnetic resonance peaks of cyanometmyoglobin could be assigned by comparing the proton nuclear magnetic resonance spectra of myoglobins reconstituted from selectively deuterated hemes. These spectra indicate that the fourth methyl nuclear magnetic resonance peak has to be looked for outside the region −9 to −43 parts per million.

Pyrroles and related compounds—X : Mass spectrometry in structural and stereochemical problems—XC Mass spectra of linear di=, tri- and tetrapyrrolic compounds☆☆☆

Abstract Mass spectra of 55 di-, tri- and tetra-pyrroles (Table 1) are recorded (Table 2); these include prodigiosin and five bile pigments. The factors affecting the various types of cleavages observed are discussed. Fragmentations of the side-chains usually follow similar patterns to those observed with mono-pyrroles, although, with pyrromethanes, cleavage of alkyl or carboxylic ester groups neighbouring the methylene bridge may be favoured by the formation of pyrromethene-like species or tricyclic pyrromethenes respectively. Cleavages between the nuclei are profoundly affected by the nature of the linkage involved e.g. pyrromethanes and pyrroketones undergo fairly ready inter-nuclear fragmentation, whereas the pyrromethenes (as might be expected) are much more stable, and cleavage of pyrrole-pyrrole bonds (as in prodigiosin) appears to be a very unfavourable process.

Pyrroles and related compounds—XXXV: A stepwise, general synthesis of unsymmetrically substituted porphyrins☆☆☆

Abstract Using a new, general approach involving the stepwise progression through pyrrole, dipyrrole, tripyrrole, to tetrapyrrole, followed by cyclisation of the resulting a,c-biladiene by the copper salt method, syntheses of isocoproporphyrin tetramethyl ester (1b), coproporphyrin-III tetramethyl ester (24), protoporphyrin-IX dimethyl ester (26), 2,4,6,7-tetrakis (2-methoxycarbonylethyl)-5-methoxycarbonylmethyl-1,3,8-trimethylporphin (29) (the ester of the pentacarboxylic porphyrin recently associated with haem metabolism), rhodoporphyrin-XV dimethyl ester (27) and 2,4,7-triethyl-6-methoxycarbonyl-1,3,5,8-tetramethylporphin (28), are described. The route employs condensation of unsymmetrically substituted pyrromethanes with 2-formyl-5-methylpyrroles to give crystalline and fully characterised tripyrrene salts in high yield. These are then condensed with a second mole of a different 2-formyl-5-methylpyrrole to give very high yields of a,c-biladiene dihydrobromides; cyclisation with copper(II) chloride indimethylformamide gives copper(II) porphyrins which are demetallated in trifluoroacetic acid containing sulphuric acid to give high overall yields of the corresponding metal free porphyrins.

Peptides—XIII : Effects of configuration on dielectric increments and cyclization of some simple peptides

Abstract Cyclization of the diastereoisomeric glycylleucylleucylglycylglycines, either through their p-nitrophenyl thiolesters or by means of dicyclohexyl carbodiimide, yields 39% of the D-L and 12% of the l - l cyclic pentapeptide. Correspondingly the dielectric increment of the open-chain d - l pentapeptide is only 64% that of the l - l , and similar differences have been found with several pairs of diastereoisomeric di- and-tri-peptides. A preliminary discussion of these results in terms of preferred conformations of peptide chains is given.

Peptides—XXXII: The use of phenyl esters in peptide synthesis

Abstract Phenyl esters of N-terminal amino-protected peptides are valuable intermediates in synthesis of polypeptides. The phenyl ester function is stable during the customary manipulations of chain extension, and it can be removed selectively by treatment with one equivalent of hydrogen peroxide at pH 10.5 in a variety of solvents such as 80% acetone, dimethylfonnamide, hexamethylphosphoramide or trifluoroethanol. The method has always been satisfactory providing that dimethylsulphide is used as a scavenger.

Pyrroles and related compounds. Part XXXII. Biosynthesis of protoporphyrin-IX from coproporphyrinogen-III

New syntheses of 4,6,7-tris-(2-methoxycarbonylethyl)-1,3,5,8-tetramethyl-2-vinylporphyrin (2c) and its 4-vinyl isomer (2d), from pyrromethanes, are reported. These porphyrins and coproporphyrin-III tetramethyl ester (2b) were labelled at the meso-positions with tritium, and incubated in the form of the porphyrinogen carboxylic acids, with a cell-free system from Euglena gracilis. Coproporphyrinogen-III (3b), the standard for the system, was incorporated into protoporphyrin-IX (1a) to the extent of 2·0%, whereas the 2- and 4-vinylporphyrinogens gave incorporations of 3·0 and 0·5%, respectively. These results show conclusively that, in the biosynthesis of protoporphyrin-IX from corproporphyrinogen-III in Euglena gracilis, the 2-propionic acid group is converted into vinyl before the 4-substituent is modified. Further confirmation was obtained by detection of radiochemically labelled 2-vinylporphyrinogen (3c)[by isolation of the corresponding porphyrin (1c) using dilution analysis] as a product of the coproporphyrinogen-III feeding; the isomeric identity of the porphyrin (1c) was established using high-pressure liquid chromatography.

Pyrroles and related compounds. Part XXIV. Separation and oxidative degradation of chlorophyll derivatives

A convenient, large-scale method for separation of the aand b series metal-free chlorophyll derivatives is described, utilising the Girard ‘T’ reagent with subsequent chromatography. Efficient procedures for conversion of phaeophytin-a(3) into rhodoporphyrin-XV dimethyl ester (1) and the corresponding 2-vinyl compound (2), involving the classical oxidation to purpurin derivatives, followed by further degradation, are described: n.m.r. data for several important chlorophyll degradation products are reported. Novel oxidative reactions of the isocyclic ring in methyl mesopyrophaeophorbide-a(19) are also described.

SYNTHETIC STUDIES ON PORPHYRIN SYSTEMS

Considerable advances in porphyrin synthesis have been reported’V2 during the past decade. The classical syntheses of Hans Fischer and his contemporaries allow acceptable yields of porphyrins to be obtained only when the target molecules include certain symmetry characteristics; when applied to the synthesis of biologically significant porphyrins, which in the main lack symmetry, these routes usually furnish poor yields of porphyrin mixture^.^ I t should not be forgotten, however that one of the major contributions of the Munich School was the development of expertise and methods which facilitated the separation of these mixtures and the characterisation of even the smallest component. The MacDonald route4 to porphyrins from pyrromethanes, which was reported around 1960, and which will figure prominently in this paper, was a timely addition to existing methodology; however, like Fischer’s methods, it also suffered certain symmetry limitations in that one of the two pyrromethanes must be symmetrical about the interpyrrolic carbon atom, or else self-condensation of two molecules of a monoformylpyrromethane must be employed. There can be little doubt that the most aesthetically pleasing routes to porphyrins are those making use of open-chain tetrapyrrolic intermediates which can be cyclized unambiguously to the required macrocycles. In addition, such routes in which the tetrapyrrolic intermediates can be constructed in a stepwise sense bearing an unsymmetrically disposed array of 8-side-chains can be classed as truly general routes to porphyrins. In recent years, Johnson and his coworkers at Nottingham have reported5 such a synthesis through a,c-biladienes, whereas at Liverpool we have both developed and exploited2 no less than three such routes, through a-oxobilanes,6 b-oxobilanes,’ and b-bilenes? About two and a half years ago an opportunity arose for us to employ our syntheses in the solution of a challenging problem in hemoprotein chemistry. Shulman and his collaborators at Bell Laboratories have measured the high resolution paramagnetic NMR spectra of certain low-spin iron(ll1) hemes and hemoproteins? Certain temperature-dependent proton shifts can be related to electronspin density distributions in the n-orbitals of the porphyrin ring, and on the basis of these shifts, a quantum mechanical model for the interpretation of the experimental results have been derived.’” FIGURE 1 shows the spectrum of the iron(II1) cyanide chelate of protoporphyrin-lX diethyl ester, and below it the spectrum of cyanometmyoglobin. The fine features of these and other beautiful contact shift spectra have been discussed e l~ewhere ;~ our special interest lies in the resonances “ a ” and ‘‘ b” attributed, on the basis of their integrated intensities and temperature-dependent shifts, to the methyl groups and meso-protons respectively, of the protoporphyrin-IX macrocycle. The precise assignments of these resonances to particular