Disordered fatty-acid chains in piperazinium myristate and palmitate.

(I) Piperazinium n-tetradecanoate (myristate), C4H12N2(2+).2C14H27O2-, Mr = 542.85, m.p. = 368.9 K, triclinic, P1, a = 5.681 (1), b = 7.454 (1), c = 20.676 (3) A, alpha = 84.59 (1), beta = 86.37 (1), gamma = 81.73 (1) degrees, V = 861.5 A3, Z = 1, T = 296 K, Dm = 1.05, Dx = 1.05 g cm-3, F(000) = 304, Mo K alpha, lambda = 0.7107 A, mu = 0.723 cm-1, R = 0.062 for 2198 independent reflections with I greater than sigma (I). (II) Piperazinium n-hexadecanoate (palmitate), C4H12N2(2+).2C16H31O2-, Mr = 598.95, m.p. = 370.2 K, triclinic, P1, a = 5.678 (1), b = 7.472 (1), c = 22.916 (3) A, alpha = 84.64 (1), beta = 89.82 (1), gamma = 81.32 (1) degrees, V = 956.8 A3, Z = 1, T = 296 K, Dm = 1.04, Dx = 1.04 g cm-3, F(000) = 336, Mo K alpha, lambda = 0.7107 A, mu = 0.708 cm-1, R = 0.052 for 1931 independent reflections with I greater than sigma (I). These crystal structures, together with the previously reported n-decanoate and n-dodecanoate (laurate) salts, form an isostructural series. The piperazinium cations, which have a crystallographic centre of symmetry, are in the chair conformation. Each cation forms a salt bridge to four different anions with N ... O distances ranging from 2.67 to 2.70 A. The alkanoate chains are bent with torsion angles of 78.6 degrees (I) and 79.7 degrees (II) at the C3-C4 bond. The longer sections of the alkanoate chains are close-packed together in an antiparallel array.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding characterization of the iron transport receptor from the outer membrane of Escherichia coli (FepA): differentiation between FepA and FecA.

The dissociation constants for the binding of ferric enterobactin with FepA and FecA are quantitated with displacement experiments. It is found that Kd for FepA is 12 times lower than the one for FecA. This indicates that FepA is an high-affinity receptor while FecA binds ferric enterobactin with a lower affinity. Monoclonal antibodies specific for binding epitopes of FepA inhibit the binding of ferric enterobactin with purified FepA. These same antibodies do not inhibit the binding of ferric enterobactin with purified FecA. This indicates that the binding epitopes in FecA and FepA are different.

Molecular structures of CPF-dipeptides, inhibitors for the binding of CD4 with gp120.

The dipeptides N-carbomethoxycarbonyl-prolyl-phenylalanyl-benzyl esters (CPFs) play a significant role in the prevention of HIV-1 virus infection by interacting with the glycoprotein gp120, one of the envelope proteins of the HIV-1 virus. Of the four possible isomers of CPF, (L-pro-L-phe), (L-pro-D-phe), (D-pro-L-phe) and (D-pro-D-phe), herein denoted LL etc., the crystal structures of LL, stereoisomeric LD and the racemic mixture of LD/DL have been determined. All three peptides crystallize in the orthorhombic system and they all have similar cell dimensions: (i) LL, P2(1)2(1)2(1), a = 13.699(2), b = 25.893(5), c = 6.155(1) A, Z = 4, Dcale = 1.333 g cm-3, R = 0.070 for 1247 observed reflections; (ii) LD, P2(1)2(1)2(1), a = 11.663(2), b = 26.557(2), c = 7.281(1) A, Z = 4, Dcalc = 1.290 g cm-3, R = 0.054 for 1918 observed reflections; (iii) LD/DL, Pbc2(1), a = 11.953(2), b = 24.208(8), c = 7.782(2) A, Z = 4, Dcalc = 1.292 g cm-3, R = 0.080 for 894 observed reflections. Both the enantiomeric LD and the LD in racemic LD/DL have a similar conformation, an extended peptide chain with phi 1 = -76, -73 degrees; psi 1 = 160, 158 degrees, psi 2 = 123, 131 degrees and psi 2 = -172, -167 degrees, while peptide LL adopts a bent conformation at the Phe residue, phi 1 = -69 degrees, psi 1 = 158 degrees, phi 2 = -60 degrees and psi 2 = -34 degrees.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization and preliminary X-ray analysis of ferric enterobactin receptor FepA, an integral membrane protein from Escherichia coli.

Diffraction-quality crystals have been obtained of the integral membrane protein ferric enterobactin receptor (FepA) from the outer membrane of Escherichia coli. Crystals were grown using the zwitterionic detergent lauryldimethylamine oxide (LDAO), the precipitants polyethylene glycol (PEG) 1000 and sodium chloride, and the additive heptane-1,2,3-triol; they have the symmetry of the orthorhomic space group C2221 with a = 112.2, b = 137.2 and c = 135. 4 A and diffract to 2.5 A resolution. The crystals were flash-cooled and a preliminary data set was collected at 103 K. The crystals are suitable for three-dimensional structure analysis.

Crystal structure of the outer membrane active transporter FepA from Escherichia coli.

Integral outer membrane receptors for iron chelates and vitamin B12 carry out specific ligand transport against a concentration gradient. Energy for active transport is obtained from the proton-motive force of the inner membrane through physical interaction with TonB-ExbB-ExbD, an inner membrane complex. Here we report the crystal structure of an active transport, outer membrane receptor at 2.4 A resolution. Two distinct functional domains are revealed: (i) a 22-stranded beta-barrel that spans the outer membrane and contains large extracellular loops which appear to function in ligand binding; and (ii) a globular N-terminal domain that folds into the barrel pore, inhibiting access to the periplasm and contributing two additional loops for potential ligand binding. These loops could provide a signaling pathway between the processes of ligand recognition and TonB-mediated transport. The blockage of the pore suggests that the N-terminal domain must undergo a conformational rearrangement to allow ligand transport into the periplasm.