Synthesis of ?-aminosuccinimide-containing peptides in Fmoc-based SPPS

Alpha- and beta- aspartyl peptide ester formation via aspartimide ring opening

The undesirable reaction of aspartimide formation has been proved to occur under both acid and base conditions in solid-phase peptide synthesis and is dependent on the beta-carboxyl protecting group, the acid or base used during the synthesis, as well as the peptide sequence. The hydrolysis of aspartimide-containing peptides, especially during HPLC purification, yields a mixture of alpha- and beta-aspartyl peptides that can not be purified easily. A previous study demonstrated that treatment of aspartimide-containing peptides with methanol in the presence of 2% diisopropylethylamine in solution leads to alpha- and beta-aspartyl peptide methyl esters. Taking advantage of these results and aiming at elucidating the optimal conditions for aspartimide ring opening, the effect of different types and concentrations of alcohols (primary and secondary) and bases (diisopropylethylamine, collidine, 4-pyrrolidinopyridine, 1-methyl-2-pyrrolidone, piperidine and KCN) was tested at various temperatures and reaction times. The best results were obtained with a combination of a primary alcohol and diisopropylethylamine, while aspartimide ring opening by secondary alcohols occurred only at high temperatures. The optimal conditions were also applied to solid-phase peptide synthesis.

The metal binding properties of the CCCH motif of the 50S ribosomal protein L36 from Thermus thermophilus

The TthL36 protein of the 50S ribosomal proteins from Thermus thermophilus has been found to contain the rare C(Xaa)2C(Xaa)12C(Xaa)4H (CCCH) sequence motif, a zinc finger binding motif, which for other zinc finger proteins is known to cleave RNA hairpins. In order to investigate the metal-binding properties of this T. thermophilus TthL36 protein, the core 26-mer polypeptide containing this CCCH motif was prepared by solid-phase peptide synthesis methods using Fmoc chemistry, purified by preparative RP-HPLC and characterized by circular dichroism, high-performance capillary zone electrophoresis and electrospray ionization mass spectrometry. Reaction of the acetamidomethyl (Acm)-protected polypeptide with iodine under acidic conditions resulted in the formation of the fully de-protected polypeptide. Of interest, the results demonstrate that the standard Acm-deprotection method with the synthetic TthL36 polypeptide using mercuric acetate in the presence of a large excess of 2-mercaptoethanol resulted in preferential formation of a very stable mercuro-polypeptide complex. The properties of the Acm-deprotected polypeptide in the presence of different metal ions were also investigated by spectroscopic methods. The results confirm that this TthL36 polypeptide containing the CCCH motif binds metal ions with different affinities, namely in the order Co(II)>Hg(II)>Zn(II).

Tpx-1 is a component of the outer dense fibers and acrosome of rat spermatozoa

Previously we reported the cloning of a member of the cysteine-rich secretory protein family, tpx-1, from a testis expression library using an outer dense fiber (ODF)-specific antiserum. Using immunohistochemical and immunoelectron microscopic techniques and Western blotting of purified sperm tail components, we have determined that tpx-1 exists as 25 and 27 kDa proteins in two components of rat spermatid: the ODFs and the acrosome. Tpx-1 mRNA is first expressed in the late pachytene spermatocytes, but the production of these tpx-1 proteins is translationally delayed for 4-5 days before being incorporated into the developing sperm acrosome, surrounding the elongating and condensing spermatid nucleus. Concurrent with sperm head formation, tpx-1 protein was incorporated into the developing sperm tail, and specifically the ODFs. The tpx-1 protein was seen within structures resembling granulated bodies in the cytoplasmic lobe of elongating spermatids and was incorporated subsequently into the growing tail in a manner consistent with ODF development. In addition, tpx-1 protein was localized at the ultrastructural level of the connecting piece of the neck and longitudinal columns of the fibrous sheath, suggesting common protein components in these cytoskeletal structures. As such, tpx-1 may have functional significance in the processes of sperm head development and tail function.

Direct characterisation by electrospray ionisation mass spectroscopy of mercuro-polypeptide complexes after deprotection of acetamidomethyl groups from protected cysteine residues of synthetic polypeptides

In this paper, we describe a rapid procedure to characterise the products generated in the presence of mercuric salts following removal of the acetamidomethyl (Acm)-protecting group from cysteine residues of synthetic polypeptides prepared by solid-phase peptide synthesis (SPPS) methods. In particular, electrospray ionisation mass spectrometry (ESI-MS) procedures have been employed to characterise the mercuro-polypeptide products related to the ribosomal L36 protein isolated from the bacterium Thermus thermophilus. The results demonstrate that very stable mercuro-polypeptide complexes can form under standard conditions of deprotection involving Hg(2+) salts in the presence of a reductant such as beta-mercaptoethanol. Metal ion exchange effects involving other divalent metal ions, such as Co(2+) or Zn(2+), can also be monitored by similar procedures, thus permitting the relative affinity and selectivity for metal ion-polypeptide interactions to be qualitatively assessed. Since the Thermus thermophilus ribosomal L36 protein contains a putative zinc finger binding CCCH motif, these procedures enable the formation of metal-ion complexes of synthetic polypeptides related to this structural motif to be directly examined.