Aggregation-induced red shift of the Cotton effect of mitochondrial structural protein

Investigating the effect of pH on the aggregation of two surfactant-like octapeptides

The aggregation of two de novo designed surfactant-like peptides, with sequences Ac-IIKKEENN-OH (P1) and Ac-IIEENNDD-OH (P2), has been studied in aqueous solution at various pH values using titration, Circular dichroism (CD), Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). It was found that aggregates of P1 and P2 both display 5 pK(a)'s, some of which differ considerably from the tabulated values for those amino acids. In particular, a lysine of P1 titrated to 4.939 +/- 0.066. CD spectra of P1 were insensitive to pH, while CD spectra of P2 demonstrated a random coil-to-beta-sheet transition as pH was decreased. AFM images confirmed that P1 aggregates were spheres at all pH values and ranged in size from 3 to 20 nm. On the other hand, P2 aggregates were twisted ribbons below pH 4 but spheres less than 10 nm above pH 4. In addition, AFM images demonstrated the partial breakup of these twisted ribbons upon elevating the pH from 3.03 to 3.46 and the virtual disappearance of ribbons at pH 3.82. FTIR spectra of P2 indicate a structural transition from random coil to beta-sheet as pH was decreased. The role of backbone hydrogen bonding as well as charge is discussed.

Modification of gelation kinetics in bioactive peptide amphiphiles

Peptide amphiphiles (PAs) previously designed in our laboratory are known to self-assemble into nanofibers that exhibit bioactivity both in vitro and in vivo. Self-assembly can be triggered by charge neutralization or salt-mediated screening of charged residues in their peptide sequences, and the resulting nanofibers can form macroscopic gels at concentrations as low as 0.5% by weight. Controlling the kinetics of gelation while retaining the bioactivity of nanofibers could be critical in tailoring these materials for specific clinical applications. We report here on a series of PAs with different rates of gelation resulting from changes in their peptide sequence without changing the bioactive segment. The pre-existence of hydrogen-bonded aggregates in the solution state of more hydrophobic PAs appears to accelerate gelation kinetics. Mutation of the peptide sequence to include more hydrophilic and bulky amino acids suppresses formation of these nuclei and effectively slows down gelation through self-assembly of the nanofiber network. The ability to modify gelation kinetics in self-assembling systems without disrupting bioactivity could be important for injectable therapies in regenerative medicine.

Aggregation of antifreeze glycoprotein fraction 8 and its effect on antifreeze activity

Antifreeze glycoproteins (AFGPs) have many potential applications ranging from the cryopreservation and hypothermic storage of tissues and organs to the preservation of various frozen food products. Since supplying native AFGP for these applications is a labor-intensive and costly process, the rational design and synthesis of functional AFGP analogues is a very attractive alternative. While structure-function studies have implicated specific structural motifs as essential for antifreeze activity in AFGP, the relationship between solution conformation and antifreeze activity is poorly understood. Toward this end, we have analyzed AFGP8 in aqueous solutions using dynamic light scattering (DLS) and circular dichroism (CD). Our results indicate that AFGP8 forms discrete aggregates in solution. These aggregates are predominantly composed of dimers that form at solution concentrations greater than 20 mM. CD spectroscopy indicates that the preferred solution conformation of AFGP8 is consistent with that of random coil. However, significant beta-sheet and alpha-helix character is observed in more concentrated solutions, indicating that these glycopeptides are highly flexible in solution. Aggregation appears to have a minimal effect on the overall solution conformation. Thermal hysteresis (TH) activity of the aggregated solutions is much higher than that of less concentrated solutions that do not form aggregates. While cooperative functioning between lower and higher molecular weight AFGPs has been reported, this is the first instance where cooperative functioning in lower molecular weight AFGPs has been observed.

Proposed knobs-into-holes packing for several membrane proteins

We investigated the possible side chain/side chain interactions of four potential transmembrane proteins. The basic assumptions are that the proteins are alpha-helical, and that the proteins aggregate with knobs-into-holes packing. It was found that these four proteins can be assembled into stereochemically feasible bundles of alpha-helices with hydrophobic exteriors and with hydrogen bonds between the side chains of one alpha-helix and the side chains of its knobs-into-holes packed neighbors.

Elastin--proteoglycan interaction. Conformational changes of alpha-elastin induced by the interaction

The interaction between alpha-elastin and a connective tissue proteoglycan was followed by optical density measurements and circular dichroism spectroscopy. It was found that interaction takes place at pH values below the isoelectric point of elastin with the formation of a complex coacervate. CD spectra demonstrated conformational changes of alpha-elastin caused by the interaction and resulting in an increase in the content of helical structure. This finding suggests the possibility of the involvement of proteoglycans in the molecular organization of elastin.

Biological membrane structure, I. The protein crystal model for membranes

A geometric model for the arrangement of phospholipid and protein in biological membrane systems has been proposed. The essential principle underlying this model is that when membrane proteins polymerize, the points of contact between proteins are few, and cavities lined with predominantly nonpolar amino acids are formed. Phospholipid molecules become oriented with the fatty chains inserted into the cavities while the polar heads remain on the surface of the membrane. This orientation applies to both faces of the membrane continuum. All the lipid known to be present in membranes can be accommodated in this manner. The body of evidence supporting this model has been presented.

Optical activity of biological membranes: scattering effects and protein conformation

THE FOLLOWING QUESTION HAS RECENTLY ARISEN IN THE LITERATURE CONCERNING THE INTERPRETATION OF THE OPTICAL ACTIVITY OF BIOLOGICAL MEMBRANES: do the characteristic spectral distortions observed for diverse membrane systems reflect some common and unique aspect of membrane architecture or are they the result of scattering effects owing to the particulate nature of membranous systems? We have confirmed the latter interpretation on the basis of the following experimental observations: (a) red blood cell membranes give a normal circular dichroism spectrum when scattering is reduced and (b) nonaggregated, nonmembranous helical proteins give distorted membranelike spectra when scattering is introduced. An improved estimate of secondary structure on the basis of undistorted spectra results in about 50 per cent alpha-helix for red blood cell membrane protein. In addition we conclude that the distortions in optical activity spectra offer no evidence in support of various proposed membrane models.

Biophysical contributions to membrane structure

Applications of physical techniques to studies of membrane structure have increased greatly in the past few years and they have begun to provide more precise structural parameters for membranes and also some indication of the physical states of the molecular constituents in the membranes. Direct measurements of membrane features have been made by electron microscopy and by X-ray diffraction methods. Spectroscopic techniques especially infrared absorption, nuclear magnetic resonance absorption and optical rotatory dispersion and circular dichroism measurements have provided additional data relating to the physical states of the molecular components in the membranes.