Conformational studies on sequential polypeptides Part VI. Structural investigations on (Pro-Leu-Gly)10, (Pro-Leu-Gly)n and (Leu-Pro-Gly)n

Stereochemical restrictions on the occurrence of amino acid residues in the collagen structure

The primary structure of collagen is characterized by the repeating tripeptide sequence (Gly-R2-R3)n. The results of theoretical studies, carried out using contact criteria to compute the stereochemically allowed orientations for various side chains at locations 2 and 3, are reported here. It is found that side chains with only gamma-atoms, as in valine, serine and threonine, or with only one delta-methyl group, as in isoleucine, can occur equally well at locations 2 and 3, as is actually the case in collagen. Side chains with two Cdelta-atoms, as in leucine and phenylalanine, can also be accommodated at both positions. However, if they occur as R3, their freedom of orientation is severely restricted in the presence of a proline residue as R2 in a neighbouring chain. If water molecules bound to the chains of the triple helix are assumed to be present, then location 3 is virtually impossible for leucine and phenylalanine residues. Location 2 is, however, unaffected, and their presence as R2 can help to shield the water molecules from disturbance by the solvent medium. This may be the reason for the preferential occurrence of Leu and Phe residues in location 2 in the collagen triplets, although the polypeptides (Gly-Pro-Leu)n and (Gly-Pro-Phe)n form collagen-like structures.

Collagen mimetics

Collagen peptidomimetics have been synthesized as an alternative to natural collagen. The incorporation of unnatural residues such as peptoids in the collagen sequences can demonstrate potent and specific biological activity and enhance the biostability against enzymatic degradation. Furthermore, the use of achiral peptoids simplifies synthetic strategies by reducing racemization problems. The peptoid residue N-isobutylglycine (Nleu) has been successfully incorporated into a series of collagen mimetics composed of Gly-Pro-Nleu, Gly-Nleu-Pro, and Gly-Nleu-Nleu. The discovery of template-assembled collagen mimetics and metal binding ability has laid the foundation for new opportunities in the design of novel collagen mimetic complexes. The review summarizes the synthesis and integrated biophysical analyses of the structures of these collagen mimetics. Solid phase segment condensation techniques have been utilized for the synthesis of the single chain and template-assembled analogues. The characterization of the collagen-like structures has been established by temperature-dependent optical rotation measurements. CD, NMR spectroscopy, and molecular modelling simulations.

Sequence-specific liquid crystallinity of collagen model peptides. I. Transmission electron microscopy studies of interfacial collagen gels

The conformation, crystal structure and self-assembly behavior of three peptides with collagen-like repetitive sequences [(1) peptide GAPGPP: (Glu)(5)(Gly-Ala-Pro-Gly-Pro-Pro)(6)(Glu)(5); (2) peptide GVPGPP: (Glu)(5)(Gly-Val-Pro-Gly-Pro-Pro)(6)(Glu)(5); and (3) peptide GAPGPA: (Glu)(5)(Gly-Ala-Pro-Gly-Pro-Ala)(6)(Glu)(5)] were compared. The peptides were characterized using transmission electron microscopy, electron diffraction, environmental scanning electron microscopy, and Fourier transform ir spectroscopy in order to determine how the molecular geometry dictated by each sequence affects the spontaneous generation of long-range ordered structures. Samples of each peptide, at ambient temperature and at 5 degrees C, were examined as films dried from aqueous solution, air-water interfacial films, and chloroform-water interfacial films. Peptide GAPGPP prepared at 5 degrees C and dried from bulk solution was found to have a collagen-like triple-helical structure. A sinusoidally textured gel, suggestive of cholesteric behavior was observed for peptides GAPGPP and GVPGPP at the aqueous chloroform interface at 5 degrees C. Peptide GAPGPA also formed a gel, but less reproducibly and the sinusoidal texture was not as well defined. The periodicities of the sinusoidal textures were reproducibly 10 microm for peptide GAPGPP, 7 microm for peptide GVPGPP, and 6 microm for peptide GAPGPA. The differences in the periodicity of the banded structure and in the crystallization behavior of the three peptides is attributed to differences in the symmetry of the preferred packing arrangement for each peptide, as evidenced by electron diffraction from crystallites that coexist with the sinusoidal gel. These differences are believed to be a measure of the effective symmetry and shape of the molecular cross section.

Collagen-based structures containing the peptoid residue N-isobutylglycine (Nleu): synthesis and biophysical studies of Gly-Pro-Nleu sequences by circular dichroism, ultraviolet absorbance, and optical rotation

A peptoid residue N-isobutylglycine (Nleu) was introduced as a proline surrogate in collagen-like triple helical structures. A series of single chain and template-assembled collagen-based peptide-peptoid structures composed of Gly-Pro-Nleu sequences were prepared by solid-phase segment condensation methods. Both a synthetic route in solution and a solid phase method were employed to couple the KTA (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid, also known as the Kemp triacid) based template, KTA-(Gly-OH)3, to peptide-peptoid chains. Biophysical studies using CD, uv absorbance, and optical rotation measurements demonstrated that these compounds form triple-helical structures when the chains are longer than critical lengths. Results from melting curve measurements indicated that the Gly-Pro-Nleu sequence is comparable to the Gly-Pro-Pro sequence in stabilizing a triple-helical conformation. The KTA-based template stabilized triple-helical structures as can be seen by the increased melting temperatures as compared to equivalent single chain molecules. In addition, the template reduced the minimum chain length necessary to form a triple helix from six to only three trimer repeats.

NMR and x-ray studies of collagen model peptides

The main structural component in collagen is the triple helix which is generally composed of the amino acid sequence repeat (X-Y-Gly)n with proline and hydroxyproline often present at positions X and Y. Non-globular, fibrillar proteins like most collagens are difficult to work with from a structural perspective. An alternative approach to collagen structural elucidation is to study considerably shorter fragments of the triple helix. To date, various triple helical model peptides such as (Pro-Pro-Gly)n and (Pro-Hyp-Gly)n have been investigated by various physical and spectroscopic techniques. The advent of easy solid phase peptide synthetic methodology and the development of multi-dimensional heteronuclear and high field NMR technologies have promoted significant advances in the structure elucidation of a number of triple helix peptides. Here, the main focus is to review and to address the current state of knowledge in the field of NMR and x-ray analysis of triple helical model peptides.

Synthesis and structural studies of a pentapeptide sequence of elastin. Poly (Val-Gly-Gly-Leu-Gly)

Poly (Val-Gly-Gly-Leu-Gly), a polypeptide mimicking the physico-chemical properties of the glycine-rich regions of elastin, has been synthesized and studied both in solution and in the aggregated state. By comparison, also the conformation of different "monomeric" units has been investigated. The polymer showed increased disorder with respect to the "monomers", the molecular conformation being accounted for by a more or less random collection of isolated beta-turns. Nevertheless, in the solid state the polymer is able to adopt supramolecular structures reminiscent of those found for elastin.

Conformational studies on polypeptide models of collagen. Poly(Gly-Pro-Val), poly(Gly-Pro-Met), poly(Gly-Val-Pro) and poly(Gly-Met-Pro)

The title polytripeptides were synthesized and studied experimentally, by circular dichroism, and theoretically, by quantum mechanical methods. With the exception of poly(Gly-Pro-Val), which was found to be essentially structureless in solution, the other polymers adopt folded conformations, most probably of type II beta-bends. Conclusions from theoretical studies were generally in agreement with the experimental results. In particular, it is noteworthy that the optimized (phi, psi) maps for poly(Gly-Pro-Met) showed the absolute minimum (phi = 60 degree, psi = 0 degrees) located inside the beta II bend space.

Experimental and theoretical conformational studies on polypeptide models of collagen. Poly(Gly-Pro-Ile) and poly(Gly-Ile-Pro)

The synthetic polytripeptides poly(Gly-Pro-Ile) and poly(Gly-Ile-Pro) were studied both experimentally (mainly by circular dichroism spectroscopy) and theoretically by quantum mechanical methods. Poly(Gly-Ile-Pro) adopts a collagen-like structure under favourable conditions while the isomeric poly(Gly-Pro-Ile) does not. Theoretical studies emphasize severe intrastrand interactions which limit the main chain conformations in the case of poly(Gly-Ile-Pro). On the other hand, both the side cahin and the backbone in poly(Gly-Pro-Ile) can take up many different local conformations. Therefore, it seems that the conformational behaviour of synthetic polytripeptides can be at least partially explained in terms of local interactions.

A theoretical study of the structures of (Gly-Pro-Leu)n and (Gly-Leu-Pro)n

Theoretical conformation studies have been carried out for the polytripeptides (Gly-Pro-Leu)n and (Gly-Leu-Pro)n and the Fourier transforms of the structures have been calculated. X-ray powder patterns of these polymers had indicated that both these polymers take up coiled-coil triple-helical structures, but in the case of (Gly-Pro-Leu)n it was not clear whether the triple helix is formed by three parallel polypeptide chains or by a single chain folding back on itself (Scatturin et al 1975). Our studies show that both the polytripeptides can take up stereochemically satisfactory triple-helical structures with three parallel chains. There is also very good agreement between the calculated intensity distribution and that of the observed X-ray pattern, in each case.