The structural preferences of peptides (and depsipeptides) from the achiral MeAib and Hib residues, and the chiral Iva, (alpha Me) Val, (alpha Me) Leu, and (alpha Me) Phe residues, as determined by conformational energy computations, x-ray diffraction analyses, and 1H-nmr and spectroscopic studies, are reviewed and compared with literature data on Aib-containing peptides. The results obtained indicate that helical structures are preferentially adopted by peptides rich in these alpha-amino acids methylated at the alpha-carbon. Intriguing experimental findings on the impact of the chirality of Iva, (alpha Me) Val, and (alpha Me) Phe residues on helix screw sense are illustrated.

Terminally blocked, isotactic molecular and crystal structures of the Z-trimer is folded in a type I11 /?-turn conhomopeptides from the sterically de-protected trimer, hexamer, heptamer and formation, the longest homopeptides manding a-methylvaline of general for-octamer have been determined by X-ray form well-developed, regular, rightIn the crystal state. while the ed 3,,-helices. The screw sense in the helix Bz, Ac; n = 2-8) have been prepared of the pBrBz-blocked octamer has been step-by-step in solution and fully characconfirmed to be right-handed by solidterized. The conformations preferred in state and solution CD spectroscopy. The possible exploitation of these peptide helices as rigid and precise molecular rulers solution (B-turn and right-handed 3,,-heis discussed.lix) have been assessed by FT-IR, 'HNMR and CD spectroscopy. The

(Trialkyl- and (triarylphosphine)silver(I) derivatives containing anionic tris(1H-pyrazol-1-yl)borates were prepared from AgX (X = CF3SO3 or NO3), PR3 (R = phenyl, benzyl, cyclohexyl, 2,4,6-trimethylphenyl, or o-, m-, and p-tolyl) or PMePh2, and M[HB(pz)3] [M = K or Na; pzH = pyrazole in general; in detail, pyrazole (PzH), 3,5-dimethylpyrazole (3,5-Me2PzH), or 4-bromopyrazole (4-BrPzH)] and characterized through analytical and spectral (IR; 1H, 13C, and 31P NMR) measurements. These air stable, light-sensitive complexes are not electrolytes in CH2Cl2 and acetone. In these solvents they decompose even with strict exclusion of oxygen and light. The solid-state structures show that the silver atom adopts a distorted tetrahedral geometry. Crystal data with Mo Kα (λ = 0.710 70 Å) at 293 K:  [HB(Pz)3]Ag(PPh3), C54H50Ag2B2N12P2, a = 15.082(2) Å, b = 19.728(2) Å, c = 9.432(1) Å, α = 94.8(1)°, β = 107.4(1)°, γ = 87.8(1)°, triclinic, P1̄, Z = 2; [HB(Pz)3]Ag[P(o-tolyl)3], C30H31AgBN6P, a = 14.461(2) Å, b = 14.461(2) Å, c = 30.473(2) Å, α = 90°, β = 90°, γ = 120°, trigonal, R3̄, Z = 6; [HB(Pz)3]AgP(Bz)3, C30H31AgBN6P, a = 12.183(2) Å, b = 18.596(2) Å, c = 13.804(2) Å, β = 100.8(1)°, monoclinic, P21/n, Z = 4; [HB(3,5-Me2Pz)3]Ag[P(p-tolyl)3], C36H41AgBN6P, a = 34.839(3) Å, b = 10.375(2) Å, c = 10.203(2) Å, β = 90.8(1)°, monoclinic, P21/n, Z = 4. Crystal data with Cu Kα (λ = 1.5418 Å):  [HB(4-BrPz)3]AgPPh3, C27H22AgBBr3N6P2, a = 10.306(2) Å, b = 11.425(2) Å, c = 13.571(2) Å, α = 90.3(1)°, β = 107.0(1)°, γ = 91.4(1)°, triclinic, P1̄, Z = 2. The anionic soft S3-donor hydrotris(3-methyl-2-thioxo-1-imidazolyl)borate (Tm) was synthesized as its potassium salt from the corresponding imidazole and KBH4 in high yield. The potassium complex KTm was used as ligand transfer agent in the preparation of the silver(I) derivatives [(Tm)Ag] n and (Tm)Ag(PPh3). The S-donor 1-methylimidazolyl-2-thione (Hmimt) reacts with [HB(3,5-Me2Pz)3]Ag(PPh3), yielding the derivative [HB(3,5-Me2Pz)3](Hmimt)AgPPh3 in which the [HB(3,5-Me2Pz)3]- acts likely as bidentate N2-donor.

We have synthesized by solution methods and fully characterized the Nα-blocked heptapeptide methylamide mBrBz-[L-Iva-L-(αMe)Val]2-L-(αMe)Phe-L-(αMe)Val-L-Iva-NHMe, fully based on conformationally constrained C α-methylated α-amino acids. An X-ray diffraction investigation of the N α-benzyloxycarbonylated analogue showed that in the crystal state both independent molecules (A and B) in the asymmetric unit of the peptide adopt a fully developed, regular, right-handed 310-helical structure, although molecule A would be slightly distorted at the C-terminal residue. Solution conformational analysis on the mBrBz-blocked peptide was carried out in CDCl3 by means of NMR spectroscopy. For structure determination we performed restrained molecular dynamics simulations in CDCl3 based on a search of the conformational space derived from a simulated annealing strategy. For this peptide the NMR observables can be described by a single backbone conformation, more specifically a rigid 310-helix spanning the amino acid sequence from residue 1 to residue 6. The C-terminal methylamido NH group seems to be involved simultaneously in two H-bonds (with the preceding i − 3 and i − 4 carbonyl groups). Although in this peptide model there are no distinct NOE distances for discriminating 310- versus α-helix conformation, the sum of all NMR-derived restraints clearly results in a 310-helical structure. Convergence from different starting structures (including an α-helix) into a 310-helix was observed.