<sup>15</sup>N‐nmr spectroscopy. 33. Assignments of isotactic and syndiotactic sequences in poly(<scp>D</scp>,<scp>L</scp>‐amino acids)

Kricheldorf, Hans R.; Hull, William E.

doi:10.1002/bip.360210210

Cited by 14 publications

(7 citation statements)

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“…41,42) and dimethylformamide (Nos. 13,20,27), whereas the lowest values were observed in 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 Tos methylene chloride (e. g. Nos. 9,35,37,45).…”

Section: Pr-nh-chr-co-o-co-oc2h Amentioning

confidence: 98%

“…Model reactions of Hashimoto and Imanishi16) and I5N NMR spectroscopic investigations of NCA polymerizations of themselves 13) suggest that chiral neighbours of the reactive chain end (second to last monomeric unit) influence the stereoselectivity. In the case of polymerizations, this sort of neighbouring residue effects is called "penultimate effect".…”

Section: Stereoselectivities Of Tripeptide Synthesesmentioning

confidence: 99%

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¹³C NMR sequence analysis, 21. Stereoselectivity of oligopeptide syntheses

Kricheldorf

Mang

1982

Makromol. Chem.

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Various diastereomeric N-protected dipeptide methyl esters were synthesized from N-protected racemic amino acids and racemic amino acid methyl esters. Derivatives of alanine, valine, leucine, and phenylalanine were condensed in various solvents including optically active solvents such as 0-acetyl lactic acid ethyl ester, N-acetyl lactic acid ethyl ester, or N-acetyl L-alanine methyl ester. The N-protected amino acids were activated by means of 2-ethoxy-lethoxycarbonyl-l,2-dihydroquinoline (EEDQ), dicyclohexylcarbodiimide and Cchlorothiophenol, l,l-carbonyldiimidazole, 1 -diethoxyphosphino-l,3,4-triazole, chloroacetonitrile, and isobutyl chloroformate. The reaction mixtures containing four stereoisomers were analyzed by means of 13C NMR spectroscopy with respect to the mole ratios of the diastereomeric dipeptides. The stereoselectivity of all experiments was low. The number of experiments favoring the formation of L-L (D-D) sequences exceeded by far the number of those favoring L-D (D-L) sequences. Tripeptide syntheses, conducted under the reaction conditions of various dipeptide syntheses, revealed that the stereoselectivity is influenced by chiral neighboring groups. However, these "penultimate effects" are not stronger than solvent effects. merization of D,L-amino acid NCA would yield technically useful polypeptide fibers'O). Several authors have claimed to have found highly stereoselective D,L-NCA polymerizations' -4), and the helical secondary structure of the growing peptide chain has been considered to be responsible for the stereoselectivity. Yet, for both hypotheses convincing experimental evidence is lacking8, 9 ) , If the stereoselectivity is not caused by a helical secondary structure, it must depend on other factors, such as chemical structure and conformation of the reactants. This means, that any peptide synthesis from racemic amino acids may in principle be more or less stereoselective. Chemical structure and conformation are also the main factors that govern the stereochemical course of enzymatic peptide syntheses or hydrolyses. Hence, a comparison of stereoselectivity of enzymatic and chemical peptide syntheses might also be of interest.In two previous papers, we have demonstrated by means of 15N NMR spectro-scopy8~ 'I) that most syntheses of Boc-D,L-Ala-D,L-Aia-oMe and Boc-D,L-V~~-D,L-V~~-OMe are partially stereoselective. The present work was aimed to provide a broader bases for conclusions of general validity. Results and DiscussionThe analytical procedure The condensation of an N-protected D,L-amino acid with a D,L-amino acid ester may lead to a mixture of four stereoisomers (Scheme I). Since the mole ratio of the diastereomeric dipeptides L-L/L-D or D-D/D-L represents the stereoselectivity of the synthesis, an analytical method was required which allows a routine and accurate differentiation between the diastereomers. 13C NMR spectroscopy met these requirements best, because it is more sensitive to stereoisomerism than 'H NMR spectroscopy and because a good signal-to-noise ratio is more easi...

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Section: Pr-nh-chr-co-o-co-oc2h Amentioning

confidence: 98%

Section: Stereoselectivities Of Tripeptide Synthesesmentioning

confidence: 99%

¹³C NMR sequence analysis, 21. Stereoselectivity of oligopeptide syntheses

Kricheldorf

Mang

1982

Makromol. Chem.

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“…never exceeded 4 monomer units. [21][22][23][24][25] Note that the distribution of the Ile n-mers is not symmetric. In other words, the amount of [L(d 10 )-Ile] n is different from the amount of (D-Ile) n (n ϭ 3-5).…”

Section: Figurementioning

confidence: 99%

Liposome-assisted selective polycondensation of α-amino acids and peptides

Hitz

Luisi

2000

Biopolymers

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“…A major advantage of the NMR method is the chance to quantify the signal intensities and thereby the molar composition of complex secondary structures. Of course, the parameters used for such NMR measurements, in particular the S.C. "contact time",need be adjusted for correct intensity ratios of the signals under investigation (43,53,56 The results obtained with I3C-and partially also with I5N NMR CP/MAS spectroscopy (57),9ave a better insight into the relationship between nature of an amino acid, polymerization mechanism and crystal growth of the resulting polypeptide. The oligopeptides precipitated in the first stage of the polymerization continue for steric reasons their chain growth only slowly.…”

Section: ) Secondary Structure and Crystal Growthmentioning

confidence: 99%

NMR spectroscopic investigation on polymerization mechanisms of amino acid NCAS

Kricheldorf

1986

Makromolekulare Chemie. Macromolecular Symposia

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Polymerizations of α‐aminoacid N‐carboxyanhydrides (α‐NCAs) can be initiated by nucleophilic or basic reagents. Depending on the nature of the reagent, nucleophilic attack at the carbonyl group C‐5 or deprotonation of the N‐H group may be the first reaction step. 1H NMR spectroscopy is best suited to detect incorporation of initiator fragments. In the case of primary aliphatic amines quantitative endgroup analyses may even allow the calculation of the average degree of polymerization (DP). In the case of base‐initiated polymerization the formation of N‐acyl‐NCA endgroups or the incorporation of the basic initiator is detectable. Furthermore, the role of electrophilic co‐catalysts, such as isocyanates or N‐acyl‐NCAs, can be elucidated. 13C‐ and 15N NMR spectroscopy are better suited than 1H NMR spectroscopy for stereosequence analyses of poly(D,L‐amino acid)s. The results of both methods agree in that primary amine‐initiated polymerizations of D,L‐NCAs yield nearly random stereosequences. Trialkylamine initiated polymerizations of α‐helix forming D,L‐NCAs favour the formation of isotactic blocks, whereas in the case of non‐helicogenic NCAs (e.g. D,L‐Val‐NCA) syndiotactic blocks are preferentially formed. 15N NMR spectroscopy also enables the sequence analysis of binary copolypeptides, and in favourable cases even ternary copolypeptides can be analyzed. 13C NMR crosspolarization/magic angle spinning (CP/MAS) spectroscopy of solid polypeptides enables the qualitative and quantitative determination of their secondary structure. Such investigations revealed that primary amine and tertiary amine‐initiated polymerizations may yield different ratios of crystallites built up by β‐sheets or α‐helices and thereby bimodal molecular weight distributions (MWDs).

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¹⁵N‐nmr spectroscopy. 33. Assignments of isotactic and syndiotactic sequences in poly(D,L‐amino acids)

Cited by 14 publications

References 25 publications

¹³C NMR sequence analysis, 21. Stereoselectivity of oligopeptide syntheses

¹³C NMR sequence analysis, 21. Stereoselectivity of oligopeptide syntheses

Liposome-assisted selective polycondensation of α-amino acids and peptides

NMR spectroscopic investigation on polymerization mechanisms of amino acid NCAS

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15N‐nmr spectroscopy. 33. Assignments of isotactic and syndiotactic sequences in poly(D,L‐amino acids)

Cited by 14 publications

References 25 publications

13C NMR sequence analysis, 21. Stereoselectivity of oligopeptide syntheses

13C NMR sequence analysis, 21. Stereoselectivity of oligopeptide syntheses

Liposome-assisted selective polycondensation of α-amino acids and peptides

NMR spectroscopic investigation on polymerization mechanisms of amino acid NCAS

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¹⁵N‐nmr spectroscopy. 33. Assignments of isotactic and syndiotactic sequences in poly(D,L‐amino acids)

¹³C NMR sequence analysis, 21. Stereoselectivity of oligopeptide syntheses

¹³C NMR sequence analysis, 21. Stereoselectivity of oligopeptide syntheses