Thermal Synthesis of Polypeptides from N-Butyloxycarbonyl Oligopeptides Containing Aspartyl Residue at C-Terminus

Hindawi International Journal of Polymer Science Volume 2017, Article ID 8364710, 16 pages https://doi.org/10.1155/2017/8364710 Research Article Thermal Synthesis of Polypeptides from N-Butyloxycarbonyl Oligopeptides Containing Aspartyl Residue at C-Terminus Toratane Munegumi1 and Takafumi Yamada2 1 Department of Science Education, Naruto University of Education, Naruto, Tokushima 772-8502, Japan Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-857, Japan 2 Correspondence should be addressed to Toratane Munegumi; Received 4 January 2017; Revised 22 May 2017; Accepted 12 June 2017; Published 30 July 2017 Academic Editor: Peng He Copyright © 2017 Toratane Munegumi and Takafumi Yamada. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The thermal reactions of amino acids have been investigated for pure organic synthesis, materials preparation in industry, and prebiotic chemistry. N-t-Butyloxycarbonyl aspartic acid (Boc-Asp) releases 2-butene and carbon dioxide upon heating without solvents. The resulting mixture of the free molten aspartic acid was dehydrated to give peptide bonds. This study describes the thermal reactions of N-t-butyloxycarbonyl peptides (Boc-Gly-L-Asp, Boc-L-Ala-L-Asp, Boc-L-Val-L-Asp, and Boc-Gly-Gly-L-Asp) having an aspartic residue at the carboxyl terminus. The peptides were deprotected upon heating at a constant temperature between 110 and 170∘ C for 1 to 24 h to afford polypeptides in which the average molecular weight reached 7800. 1. Introduction Polypeptides [1] have been well investigated as protein model compounds [1–8]. Numerous reports on the methodology for the synthesis of polypeptides have been published [1–14]. The N-carboxyl-𝛼-amino acid anhydride (NCA) method (1) [2, 3, 9, 10], polymerization of amino acids using active esters (2) [1, 4–7], solid-phase peptide synthesis (3) [8, 12], and the heating of amino acids (4) [13, 14] are typical examples. The NCA method (1) is suitable for making homopolypeptides and random copolypeptides but is not suitable for the synthesis of sequential copolyamino acid, which is more important for the build-up of functional polypeptides. Sequential polyamino acids have repetitive amino acid residues, in which the amino acid residues can be like -(Gly-Gly-Asp)𝑛-. The active ester method (2) [1, 4–7] and solid phase synthesis (3) [8] are more suitable for the synthesis of sequential copolyamino acid. However, the problems of methods (2) and (3) are a long reaction time and the use of much solvent. In contrast, the synthesis of polyamino acid by heating a derivative of the amino acid (4) [13, 14] requires neither long reaction time nor solvents. In previous papers [15, 16], we reported the synthesis of homopolypeptides [15] and random copolypeptides [16] upon heating of N-t-butyloxycarbonyl aspartic acid anhydride (Boc-Asp anhydride) and mixtures of Boc-L-Asp, anhydride, and Boc-Gly, Boc-L-Ala, or Boc-Val. In this paper, we report a trial for the synthesis of sequential copolypeptides by the heating of Boc-peptides instead of these anhydrides. As shown in Figures 1 and 2, Boc-peptides (5a–d) and Boc-L-Asp (5e) were prepared for heating under a stream of N2 . 2. Materials and Methods 2.1. Instrumentation. A nuclear magnetic resonance (NMR) (JEOL FX-100 NMR system (JEOL, Tokyo, Japan)) was used for the collection of 1 H-NMR spectra. A Hitachi model 260-50 infrared (IR) spectrophotometer (Hitachi, Tokyo, Japan) was used for the collection of IR spectra. A Hitachi 200-10 spectrophotometer was used for spectrophotometry measurements. A Jasco DIP-181 digital polarimeter (Jasco, Tokyo, Japan) was used for the measurement of the optical rotation of the peptide derivatives. A Hitachi 163 gas chromatograph 2 International Journal of Polymer Science R H H N C C OH H O n 1a–d C（3 R H （3 C C O C N C C OH H O C（3 O a–d Bo＝2 O NEＮ3 R H （3 C C O C N C C O N H O C（3 O n a–d O HONSu DCC a: n = 1, ２ = -H b: n = 1, ２ = -C（3 c: n = 1, ２ = -CH(＃（3 )2 d: n = 2, ２ = -H 1a: n = 1, ２ = -H (Gly) 1b: n = 1, ２ = -C（3 (L-Ala) 1c: n = 1, ２ = -CH(＃（3 )2 (L-Val) 1d: n = 2, ２ = -H (Gly-Gly) O C（3 a: n = 1, ２ = -H b: n = 1, ２ = -C（3 c: n = 1, ２ = -CH(＃（3 )2 d: n = 2, ２ = -H Figure 1: Preparation of N-t-butyloxycarbonyl-amino acid and peptide active esters. Boc2 O: di-tert-butyl dicarbonate; NEt3 : triethylamine; HONSu: N-hydroxysuccinimide; DCC: N,N 󸀠 -dicylohexylcarbodiimide. COOH COOH C（2 H H N C C OH H O 1e (L-Asp) 3a/NEＮ3 C（3 CN HOC（2 Ph TsOH H C（3 C（2 H H （3 C C O C N C C N C C OH H H O O C（3 O 5a (Boc-Gly-L-Asp) O C OC（2 Ph C（2 Ts／− （3 ．+ C C O C（2 Ph H O  COOH b–d NEＮ3 COOH Bo＝2 /NEＮ3 5a–e C（2 C（3 H （3 C C O C N C C OH H O C（3 O 5e (Boc-L-Asp) 110–170∘ C ．2 （2 /Pd-C C（3 R H （3 C C O C N C C H C（3 O O 5b–d C（2 H N C C OH H O n b: n = 1, ２ = -C（3 (Boc-L-Ala-L-Asp) c: n = 1, ２ = -CH(＃（3 )2 (Boc-L-Val-L-Asp) d: n = 2, ２ = -H (Boc-Gly-Gly-L-Asp) Polypeptides and related compounds Figure 2: Preparation of N-t-butyloxycarbonyl peptides containing aspartyl residue at C-terminus. TsOH: p-toluenesulfonic acid; TsO-: ptoluenesulfonate; Boc2 : di-t-butyl dicarbonate. equipped with a chiral glass capillary column Chirasil-Val [17, 18] was used for the separation of the enantiomeric derivatives of amino acids. For thermal analysis, we used a Shimadzu DT-40 thermal analyzer (Shimadzu, Kyoto, Japan). A Jasco Trirotar-V as the flow pump and a Jasco UVIDEC100-IV spectrophotometer as the detector were used for the HPLC system equipped with a gel permeation column G3000 PW (TSK, Yamaguchi, Japan). Analysis of the evolved gases from the thermal analyzer was performed with a Shimadzu GCMS-QP1000A. (DCC) was supplied by Watanabe Chemical Industries, Ltd. (Hiroshima, Japan). Palladium on charcoal was purchased from Nippon Engelhard Ltd. (now N.E. CHEMCAT, Tokyo, Japan). Trifluoroacetic anhydride and triethylamine (NEt3 ) were purchased from Tokyo Chemical Industries Co., Ltd. (Tokyo, Japan). Hydrochloric acid (6 M) for the hydrolysis of peptides and acetic acid were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). The 2-propanol solution containing 2.0 M hydrogen chloride was prepared by bubbling HCl into 2-propanol. 2.2. Materials 2.2.2. Preparation of Substrates 2.2.1. Starting Materials and Reagents for the Preparation of Substrate Peptide Derivatives. Glycine (1a), L-alanine (1b), and L-valine (1c) were supplied by Nippon Rika Co., Ltd. (Tokyo, Japan). Glycylglycine (1d) and di-t-butyl dicarbonate (Boc2 O) were purchased from Peptide Institute, Inc. (Minoh-shi, Osaka, Japan). L-Aspartic acid (1e) and Nhydroxysuccinimide (HONSu) were purchased from Nacalai Tesque (Kyoto, Japan). N,N 󸀠 -Dicyclohexylcarbodiimide (1) N-t-Butyloxycarbonyl Amino Acids (2a–d) Boc-Gly (2a). Glycine (7.51 g, 0.1 (...truncated)