A pair of mutually-attractive but self-repulsive decapeptides, with alternating charged/neutral amino acid sequence patterns, was found to co-assemble into a viscoelastic material upon mixing at a low total peptide concentration of 0.25 wt%. Circular dichroism spectroscopy of individual decapeptide solutions revealed their random coil conformation. Transmission electron microscopy images showed the nanofibrillar network structure of the hydrogel. Dynamic rheological characterization revealed its high elasticity and shear-thinning nature. Furthermore, the co-assembled hydrogel was capable of rapid recoveries from repeated shear-induced breakdowns (compliance), a property desirable for designing injectable biomaterials for controlled drug delivery and tissue engineering applications. A systematic variation of the neutral amino acids in the sequence revealed some of the critical design principles involved in this novel class of biomaterials. Lowering the hydrophobicity of the neutral amino acids lowered the elastic modulus and the resilience of the assembled hydrogel, thereby providing a means to fine-tune material property. Replacement of neutral amino acids in the sequence with proline (a β-sheet breaker) impaired the ability of the peptides to co-assemble into a hydrogel.

No abstract

Much uncertainty and controversy exist regarding the estimation of the enthalpy, entropy, and free energy of overall translational and rotational motions of solute molecules in aqueous solutions, quantities that are crucial to the understanding of molecular association/recognition processes and structure-based drug design. A critique of the literature on this topic is given that leads to a classification of the various views. The major stumbling block to experimentally determining the translational/rotational enthalpy and entropy is the elimination of vibrational perturbations from the measured effects. A solution to this problem, based on a combination of energy equi-partition and enthalpy-entropy compensation, is proposed and subjected to verification. This method is then applied to analyze experimental data on the dissociation/unfolding of dimeric proteins. For one translational/rotational unit at 1 M standard state in aqueous solution, the results for enthalpy (H degrees (tr)), entropy (S degrees (tr)), and free energy (G degrees (tr)) are H (degrees) (tr) = 4.5 +/- 1.5RT, S (degrees) (tr) = 5 +/- 4R, and G (degrees) (tr) = 0 +/- 5RT. Therefore, the overall translational and rotational motions make negligible contribution to binding affinity (free energy) in aqueous solutions at 1 M standard state.

The solution structure of a thermostable cytochrome c-552 from a thermophilic hydrogen oxidizing bacterium Hydrogenobacter thermophilus was determined by proton nuclear magnetic resonance spectroscopy. Twenty structures were calculated by the X-PLOR program on the basis of 902 interproton distances, 21 hydrogen bonds, and 13 torsion angle constraints. The pairwise average root-mean-square deviation for the main chain heavy atoms was 0.91 +/- 0.11 A. The main chain folding of the cytochrome c-552 was almost the same as that of Pseudomonas aeruginosa cytochrome c-551 that has 59% sequence identity to the cytochrome c-552 but is less thermostable. We found several differences in local structures between the cytochromes c-552 and c-551. In the cytochrome c-552, aromatic-amino interactions were uniquely formed between Arg 35 and Tyr 32 and/or Tyr 41, the latter also having hydrophobic contacts with the side chains of Tyr 32, Ala 38, and Leu 42. Small hydrophobic cores were more tightly packed in the cytochrome c-552 because of the occupancies of Ala 5, Met 11, and Ile 76, each substituted by Phe 7, Val 13, and Val 78, respectively, in the cytochrome c-551. Some of these structural differences may contribute to the higher thermostability of the cytochrome c-552.

Getting FIT: A bispherical (19)F imaging tracer, (19)FIT, was designed and synthesized. (19)FIT is advantageous over perfluorocarbon-based (19)F imaging agents, as it is not retained in the organs and does not require complex formulation procedures. Imaging agents such as (19)FIT can lead to (19)F magnetic resonance imaging (MRI) playing an important role in drug therapy, analogous to the role played by (1)H MRI in disease diagnosis.

No abstract

The energetics of thermal denaturation of two isoforms of ribonuclease T1 (Gln25 and Lys25) in various solvents have been studied by differential scanning calorimetry. It has been shown that the thermal transition of both forms of RNase T1 is strongly affected by slow kinetics, which cause an apparent deviation of the transition from a simple two-state model. By decreasing the heating rate or increasing the transition temperature, the denaturation of RNase approaches an equilibrium two-state transition. This permits determination of the thermodynamic parameters characterizing unfolding of the native structure. These thermodynamic parameters were correlated with the structural features of protein. Analysis of different contributions to the stability of RNase T1 shows that van der Waals interactions and hydrogen bonding are the major contributors to the conformational stability of the protein.

We have previously developed shear-responsive hydrogels assembled from mutually attractive but self-repulsive oligopeptide modules. To explore the mechanism of material assembly, we designed and synthesized a quartet of oligopeptides. The peptide quartet contains two positively charged modules 10 and 15 amino acid residues long and two negatively charged modules 10 and 15 amino acid residues long. Each positive module can pair with one negative module and hence there are four potential pairings, two of equal chain length (blunt-ended pairs) and two of unequal chain length (sticky-ended pairs). Viscoelastic properties and structural features of the hydrogels assembled from these oligo-peptide pairs were evaluated by dynamic rheometry and small-angle X-ray scattering (SAXS) techniques, respectively. It was found that the sticky ends provide no advantage in terms of mechanical properties of the hydrogels. Instead, the hydrogel assembled from the 10:10 peptide pair forms the strongest gel. These results are consistent with a cross-β structural model in which β-strands are stacked against each other in a parallel fashion to form nanofibers, the axes of which are perpendicular to individual β-strands. The optimal chain length of oligopeptide modules for such nanofiber network assembly is around 10 amino acid residues.