Synthesis and NMR Analysis in Solution of Oligo(3‐hydroxyalkanoic acid) Derivatives with the Side Chains of Alanine,Valine, and Leucine (β‐Depsides): Coming Full Circle from PHB to β‐Peptides to PHB

Oligomers of 3‐hydroxyalkanoic acids that contain two, three, and six residues with and without O‐terminal (tBu)Ph₂Si and C‐terminal PhCH₂ protection have been synthesized in such a way that the side chains on the oligoester backbone were those of the proteinogenic amino acids Ala (Me), Val (CHMe₂), and Leu (CH₂CHMe₂). The enantiomerically pure 3‐hydroxyalkanoates were obtained by Noyori hydrogenation of the corresponding 3‐oxo‐alkanoates with [Ru((R)‐binap)Cl₂](binap=2,2′bis(diphenylphosphanyl)‐1,1′‐binaphthalene)/H₂ (Scheme 1), and the coupling was achieved under the conditions (pyridine/(COCl)₂, CH₂Cl₂, −78°) previously employed for the synthesis of various oligo(3‐hydroxybutanoic acids) (Schemes 2 and 3). The Cotton effects in the CD spectra of the new oligoesters provided no hints about chiral conformation (cf. a helix) in MeOH, MeCN, octan‐1‐ol, or CF₃CH₂OH solutions (Figs. 1 and 2). Detailed NMR investigations in CDCl₃ solution (Figs. 3–6, and Tables 1–5) of the hexa(3‐hydroxyalkanoic acid) with the side chains of Val (HC), Ala (HB), Leu (HH), Val, Ala, Leu (from O‐ to C‐terminus; 3 ) gave, on the NMR time‐scale, no evidence for the presence of any significant amount of a 2₁‐ or a 3₁‐helical conformation, comparable to those identified in stretched fibers of poly[(R)‐3‐hydroxybutanoic acid], or in lamellar crystallites and in single crystals of linear and cyclic oligo[(R)‐3‐hydroxybutanoic acids], or in the corresponding β‐peptide(s) (the oligo(3‐aminoalkanoic acid) analogs; 1 – 3 ). Thus, the extremely high flexibility (averaged or ‘random‐coil' conformation) of the polyester chain (CO−O rotational barrier ca. 13 kcal/mol; no hydrogen bonding), as compared to polyamide chains (CO−NH barrier ca. 18 kcal/mol; hydrogen bonding) has been demonstrated once again. The possible importance of this structural flexibility, which goes along with amphiphilic properties, for the role of PHB in biology, in evolution, and in prebiotic chemistry is discussed. Structural similarities of natural potassium‐channeling proteins and complexes of oligo(3‐hydroxybutanoates) with Na⁺, K⁺, or Ba²⁺ are alluded to (Figs. 7–9).     

NMR‐Structural Investigations of a β3‐Dodecapeptide with Proteinogenic Side Chains in Methanol and in Aqueous Solutions

The structural properties of an all‐β³‐dodecapeptide with the sequence H‐β‐HLys(N^ε‐CO(CH₂)₃‐S Acm)‐β‐HPhe‐β‐HTyr‐β‐HLeu‐β‐HLys‐β‐HSer‐β‐HLys‐β‐HPhe‐β‐HSer‐β‐HVal‐β‐HLys‐β‐HAla‐OH ( 1 ) have been studied by two‐dimensional homonuclear ¹H‐NMR and by CD spectroscopy. In MeOH solution, high‐resolution NMR spectroscopy showed that the β‐dodecapeptide forms an (M)‐3₁₄‐helix, and the CD spectrum corresponds to the pattern expected for an (M)‐3₁₄‐helical secondary structure. In aqueous solution, however, the peptide adopts a predominantly extended conformation without regular secondary‐structure elements, which is in agreement with the absence of the characteristic trough near 215 nm in the CD spectrum. The NMR and CD measurements with solutions of 1 in MeOH containing 3M urea further indicated that the peptide retains the regular secondary structural elements under these conditions, whereas, after addition of 40% (v/v) H₂O to the MeOH solution, the large ¹H‐chemical‐shift dispersion indicative of a defined spatial peptide fold was lost. The β³‐dodecapeptide is – so far – the longest β‐peptide shown to adopt a regular (M)‐3₁₄‐helix conformation in an organic solvent. The observation that the structure of this long β³‐peptide is not maintained in aqueous solution indicates that the (M)‐3₁₄‐fold is primarily stabilized by short‐range interactions.     

NGO ‐ Eine dritte Kraft?

Zu einer Dachorganisation von Institutionen und Nichtregierungsorgansiationen, die sich für den Erhalt der Umwelt einsetzen, wurde in den letzten 50 Jahren die „International Union for Conservation of Nature”︁. Ihr Generaldirektor erläutert vor Rio+10, welche Rollen die „Dritte Kraft”︁, die Politik, der Markt und die chemische Industrie spielen.     

Synthesis and characterization of semifluorinated polymers and block copolymers

The goal of the investigation presented here was to evaluate the influence of semifluorinated side chains on the bulk structure and the surface properties of polysulfones with different chain structure. Thus, segmented block copolymers consisting of polysulfone and semifluorinated aromatic polyester segments as well as polysulfones having semifluorinated side chains randomly distributed over the polymer backbone were synthesized and characterized. Oxydecylperfluorodecyl side chains were used because of their strong tendency for self-organization. The influence of the chain architecture on the self-organization as well as on the surface properties, particularly the wetting behavior, was examined. It could be shown that despite of the higher self-organizing tendency of block copolymers the surface properties of both polymer types are comparable and depend only on the concentration of side chains.     

Synthesen und Kristallstrukturen neuer heterobimetallischer Tantal‐Münzmetall‐Chalkogenido‐Cluster

Syntheses and Crystal Structures of Novel Heterobimetallic Tantalum Coin Metal Chalcogenido Clusters In the presence of phosphine the thiotantalats (Et₄N)₄[Ta₆S₁₇] · 3MeCN reacts with copper to give a number of new heterobimetallic tantalum copper chalcogenide clusters. These clusters show metal chalcogenide units some of which here already known from the chemistry of vanadium and niobium. New Ta—M‐chalcogenide clusters could also be synthesised by reaction of TaCl₅ and silylated chalcogen reagents with copper or silver salts in presence of phosphine. Such examples are: [Ta₂Cu₂S₄Cl₂(PMe₃)₆] · DMF ( 1 ), (Et₄N)[Ta₃Cu₅S₈Cl₅(PMe₃)₆] · 2MeCN ( 2 ), (Et₄N)[Ta₉Cu₁₀S₂₄Cl₈(PMe₃)₁₄] · 2MeCN ( 3 ), [Ta₄Cu₁₂Cl₈S₁₂(PMe₃)₁₂] ( 4 ), (Et₄N)[Ta₂Cu₆S₆Cl₅(PPh₃)₆] · 5MeCN ( 5 ), (Et₄N)[Ta₂Cu₆S₆Cl₅(PPh₂Me)₆] · 2MeCN ( 6 ), (Et₄N)[Ta₂Cu₆S₆Cl₅(P^tBu₂Cl)₆] · MeCN ( 7 ) [Ta₂Cu₂S₄Br₄(PPh₃)₂(MeCN)₂] · MeCN ( 8 ), [Cu(PMe₃)₄]₂[Ta₂Cu₆S₆(SCN)₆(PMe₃)₆] · 4MeCN ( 9 ), [TaCu₅S₄Cl₂(dppm)₄] · DMF ( 10 ), [Ta₂Cu₂Se₄(SCN)₂(PMe₃)₆] ( 11 ), [Cu(PMe₃)₄]₂[Ta₂Cu₆Se₆(SCN)₆(PMe₃)₆] · 4MeCN ( 12 ), [TaCu₄Se₄(PⁿPr₃)₆][TaCl₆] ( 13 ), [Ta₂Ag₂Se₄Cl₂(PMe₃)₆] · MeCN ( 14 ), _∞[TaAg₃Se₄(PMe₃)₃] ( 15 ). The structures of these compounds were obtained by X‐ray single crystal structure analysis.