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0040-4039(95)00053-4

Tetrahedron Letters, Vol. 36, No. 10, pp. 171%1718, 1995 Elsevier Science Ltd

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Solid Phase Synthesis of a Self Complementary (Antiparallel) Chiral Peptidic Nucleic Acid Strand.

Alessandro Lenzi, a Gianna Reginato, b Maurizio Taddeia, 1. and Elisabeth Trifilieff c aDipartimento di Chimica Organica "Ugo Schift", Universit~ di Firenze, Via G. Capponi 9,1-50121 Firenze, Italy,

bCentro CNR Chimica dei Composti Eterociclici e loro Applicazioni, Via G. Capponi 9, 1-50121 Firenze, Italy and CLaboratoire de Chimie Organique des Substances Naturelles, CNRS, Centre de Neurochimie, 5, rue Blaise Pascal,

67084 Strasboug Cedex, France

Abstract: The solid phase synthesis (Boc protocol) of a new chiral peptidic nucleic acid formed

exclusively by o~-amino acids is described. The product showed a defined T m suggesting the

possibility of a DNA-like self-aggregation in solution

In the preceding communication we described the preparation of N-Boc and N-Fmoc 0~-amino

acids having a nucleobase in the residue. 2 Using these products we decided to prepare a

polypeptide, having a self complementary primary structure homomorphous with DNA. This

compound was supposed to mimic DNA forming duplex with himself (if self complementary)

through the Watson and Crick base pairing. 3 The sequence choosed was the analogue of the

oligonucleotide ATATATAT, and has the structure H2N-LysAbyGlyTbyGlyAbyGlyTbyGlyAby-

GlyTbyGlyAbyGlyTbyGly-OH reported in Scheme 1.4

o y~..~.o o h. . l~ .o

o %o o. , ½o

N N O N

1.12 N v 3 NH2 O H2N

Scheme I

1717

1718

We choose glycine as the spacer to have a more simple structure where the chirality of the

spacer will not complicate the possible self-recognition of the oligomer. Lysine at the N-terminus

was chosen to increase the solubility of this relatively low polar structure in aqueous medium. The

peptide was synthesised manually by solid-phase methodology using Boc-chemistry, starting from

a Boc-Gly-Pam resin (0.1 mmol scale). Boc-Aby-OH, Boc-Gly-OH, Boc-Tby-OH and Boc-Lys(2-C1-

Z) were coupled using BOP 5 as the coupling agent in the presence of DIEA. The introduction of

Tby was done using 2 equiv excess of Boc-Tby in DMF/CH2C12, a coupling time of 30 min and

repeating the coupling two times. Aby was introduced using a mixture of DMSO/DMF as the

solvent, the Boc-Aby being insoluble in DMF. The peptide was cleaved from the resin by HF under

standard conditions and then purified by HPLC (on a reverse phase C-18 column, using

H20/CH3CN , 0.1% TFA as eluent). This purified peptide was characterized by Electrospray Mass

Spectrometry. The double charged (1156.2 Da), triple charged (771.6 Da) and quadruple charged

(579.1 Da) molecular ions clearly defined a molecular mass of 2312.34 Da (1; C94H122N46026: M +

calc = 2312,30). We measured the variation of absorbance versus temperature of this compound (1.5 10 -5 M) at

260 mn in 0.1 M NaC1, 10-2M sodium phosphate, 10-4M EDTA, pH 7 obtaining a T m value of

19°C, which suggest the formation of a duplex due to a self recognition of this molecule similar to

that of the DNA-DNA duplex. The thermal stability of this complex, relatively lower than that of

analogues PNA systems, 6 can be explained with a the smaller number of basis employed here.

Moreover the semi-rigid peptidic backbone may have some difficulties to adopt the double helical

structure in solution. Nevertheless we have demonstrated the possibility to prepare a real chiral

peptidic nucleic acid with a structural backbone analogous to natural peptides and that this product

shows some typical properties of nucleic acids. Further work directed towards the understanding

of the structure of such compounds in solution and the possibility of formation of hybrids with

DNA will be reported in due course. Acknowledgements. This work has been financially supported by C.N.R. (Rome) as a "Contributo di

Ricerca n. 93.03239.03". The authors thank also A .Garbesi and coll. (CNR, Bologna) and G. Spada

(University of Bologna)for the valuable suggestions and B. Innocenti (University of Florence )for the HPLC

analysis.

References and Notes. 1. Present address: Dipartimento di Chimica, Universith di Sassari, via Vienna 2, 1-07100,

Sassari, Italy 2. Lenzi, A.; Reginato G.; Taddei, M. Tetrahedron Lett., previous letter. 3. Egholm, M.; Buchardt, O.; Nielsen P.E.; Berg, R.H.J. Am. Chem. Soc. 1992, 114, 1895. 4. For the sinthesis of other peptides analogues to DNA see: Lewis, I. Tetrahedron Lett. 1993,

34, 5697. 5. Le-Ngyen, D.; Heitz, A.; Castro, B.J.J. Chem. Soc. Chem. Perkin Trans. I, 1987, 1915.

6. Wittung, P.; Nielsen, P.E.; Buchardt, O.; Egholm, M.; Nord~n, B. Nature, 1994, 368, 561.

(Received in UK 2 December 1994; accepted 6 January 1995)