Translocation across the membrane into cells is possible for certain peptides and proteins. To investigate the structural requirements of β‐peptides for this process, a series of fluorescein‐labeled β‐peptides has been synthesized and their cellular uptake into 3T3 mouse fibroblast cells determined by fluorescence microscopy (see picture). Polycationic β‐peptides have been shown to internalize into cells, and accumulation in the cytosol and nucleus is observed.

Dedicated to Professor Piero Salvadori, University of Pisa, on the occasion of his 70th birthday Whereas a-peptides are rapidly degraded in vivo and in vitro by a multitude of peptidases, substrates constructed entirely of or incorporating homologated a-amino acid (i.e., b-amino acid) units exhibit a superior stability profile. Efforts made so far to proteolytically hydrolyze a bÀb peptide bond have not proved fruitful; a study aimed at breaching this proteolytic stability is discussed here. A series of such bonds have been designed with side-chain groups similar in relative positions (constitution) and three-dimensional arrangements (configuration) as found about a-peptidic amide bonds. Increasing the prospect for degradation would permit the tuning of b-peptide stability; here, however, no cleavage was observed (1, 2, 4 ± 6, Table 1). Peptides comprised of a-and b-amino acids (mixed a,b-peptides, 8 ± 11) are expected to benefit from both recognition by a natural receptor and a high level of proteolytic stability, ideal characteristics of pharmacologically active compounds. b 3 -Peptides containing a-amino acid moieties at the N-terminus are degraded, albeit slowly, by several peptidases. Of particular interest is the ability of pronase to cleave an aÀb peptide bond, namely that of aAlaÀb 3 hAla. Significantly, successful hydrolysis is independent of the configuration of the b-amino acid. Some of the a,b-peptides discussed here are being investigated for their binding affinities to class I MHC proteins. The computer-programming steps required to prepare a,b-peptides on an automated peptide synthesizer are presented.

In view of the important role arginine plays in living organisms as the free amino acid and, especially, as a residue in peptides and proteins, the homologous beta-homoarginines are central in our investigations of beta-peptides (Fig. 1). The preparation of beta2-homoarginine derivatives suitably protected for solution- or solid-phase peptide syntheses is described with full experimental detail (9 and 12 in Scheme 1). The readily available Fmoc-beta3 hArg(Boc)2-OH is used for manual solid-phase synthesis of beta3-oligoarginines (on Rink amide or Rink amide AM resin) either by single amino acid coupling (Scheme 3) or, much better, by dimer-fragment coupling (Scheme 4). In this way, beta3-oligoarginine amides composed of 4, 6, 7, 8, and 10 residues, both with and without fluorescein labelling, were synthesized (Schemes 2-4), purified by preparative HPLC and identified by high-resolution mass spectrometry. The free amino acids (R)- and (S)-H-beta2 hArg-OH and (S)-H-beta3 hArg-OH were tested for their ability to function as substrates for NO synthase (iNOS); the beta3-oligoarginine amides (5, 6, and 7 residues) were tested for antibacterial (against six pathogens) and hemolytic (against rat and human erythrocytes) activities. All test results were negative: none of the free beta-homoarginines induced NO formation (Fig. 3), and there was no lysis of erythrocytes (concentrations up to 100 microM; Table 1), and no significant antibiotic activity (MIC > or = 64 microg/ml; Table 2). Cell-penetration studies with the fluorescence-labelled, peptidase-resistant beta3-oligoarginine amides were carried out with HeLa cells and human foreskin keratinocytes (HFKs). The results obtained with fluorescence microscopy are: i) the longer-chain beta-oligoarginine amides (8 and 10 residues; Figs. 4-6) enter the cells and end up in the nuclei, especially in the nucleoli, irrespective of temperature (37 degrees and 4 degrees with HFKs) or pretreatment with NaN3 (with HFKs), indicating a non-endocytotic and non-energy-dependent uptake mechanism; ii) the beta-tetraarginine derivative occupies the cell surface but does not enter the cells (with HeLa); iii) the cell-growth rate of the HFKs is not affected by a 1-microM concentration of the fluorescence-labelled beta-octaarginine amide (Fig. 7), i.e., there is no antiproliferative effect. In vivo experiments with mouse skin and the beta-octaarginine derivative show migration of the beta-peptide throughout the epidermis (Fig. 8). As a contribution to understanding the mechanism, we have also studied the behavior of fluorescence-labelled beta-octa- and beta-decaarginine amides (TFA salts) towards giant unilamellar vesicles (GUVs) built of neutral (POPC) or anionic (POPC/POPG mixtures) phospholipids: the beta-oligoarginine amides bind tightly to the surface of anionic GUVs but do not penetrate the lipid bilayer (Fig. 9) as they do with living cells. In contrast, a beta-heptapeptide FL-22, which had been used as a negative control sample for the cell-penetration experiments, entered the GUVs of ne...

Herrn Professor Rolf Huisgen zum 85. Geburtstag gewidmetThe NMR-solution structure of an a-heptapeptide with a central Aib residue was investigated in order to verify that, in contrast to b-peptides, short a-peptides do not form a helical structures in MeOH. Although the central Aib residue was found to induce a bend in the experimentally determined structure, no secondary structure typical for longer a-peptides or proteins was found. A b 2 /b 3 -nonapeptide with polar, positively charged side chains was subjected to NMR analysis in MeOH and H 2 O. Whereas, in MeOH, it folds into a 10/12-helix very similar to the structure determined for a corresponding b 2 /b 3 -nonapeptide with only aliphatic side chains, no dominant conformation could be determined in H 2 O. Finally, the NMR analysis of a b 3 -icosapeptide containing the side chains of all 20 proteinogenic amino acids in MeOH is described. It revealed that this 20mer folds into a 3 14 -helix over its whole length forming six full turns, the longest 3 14 -helix found so far. Together, our findings confirm that, in contrast to a-peptides, b-peptides not only form helices with just six residues, but also form helices that are longer than helical sections usually observed in proteins or natural peptides. The higher helix-forming propensity of long b-peptides is attributed to the conformation-stabilizing effect of the staggered ethane sections in b-peptides which outweighs the detrimental effect of the increasing macrodipole.

Two different strategies have been employed for the synthesis of Fmoc-protected beta(3)-homoarginine; the Arndt-Eistert homologation of alpha-arginine and the guanidinylation of beta(3)-homoornithine. Solid-phase beta-peptide synthesis was used for the preparation of beta-heptapeptide 1, which was designed to form a helix stabilized by electrostatic interactions through positively (beta(3)hArg) and negatively charged (beta(3)hGlu) amino acid residues. CD measurements and corresponding NMR investigations in MeOH and aqueous solutions do indeed show that the beta-peptidic 3(14)-helix can be stabilized by salt-bridge formation.