Research interests

 

Peptides made of genetically coded amino acids are usually flexible molecules. As a result, they are difficult to crystallize, mainly because their hydrogen bond donors and acceptors (backbone N-H and C=O groups) can mutually interact in a variety of ways, thus decreasing the probability that molecules self-assemble into an ordered and periodic array in a crystalline solid. Conversely, peptides containing Cα-tetrasubstituted α-amino acids enjoy a much reduced conformational freedom (even at a limited main-chain length) and are more prone to yield single crystals. The nice thing with these molecules is that what you see in the crystals is also the conformation largely prevailing in solution. Therefore, X-ray crystallography is a powerful tool suitable to characterize at atomic resolution the 3D-architecture of these peptides, from which functionally relevant information can be extracted.

The low reactivity of Ca-tetrasubstituted a-amino acids complicates their use in peptide synthesis, but on the other hand has allowed the isolation and even the crystallographic characterization of a number of otherwise elusive activated compounds and intermediates, thus allowing a deeper understanding of peptide chemistry through crystallography.

Recent years have witnessed significant improvements on X-ray diffraction instrumentation, as well as the development of crystallographic software able to solve ab initio structures of increasing size and complexity. Nevertheless, getting the structure out of a peptide crystal is often a far from trivial task. Many crystallization attempts might be required before, in the favorable cases, a crystal of adequate size and quality can be obtained. In some instances, in order to collect decent data from otherwise poorly diffracting crystals, we have been forced to exploit the brilliance of synchrotron radiation. In other cases, the large size of the asymmetric unit poses significant challenges to the structure solution process. Sometimes,  the occurrence of disorder makes the refinement troublesome. Tenaciousness, experience, and ingenuity may help in  overcoming most of these difficulties, but even these struggles, when crowned by success, are part of a crystallographer’s delight. 

The X-ray diffraction structures of peptides, beside being informative in terms of 3D-architecture  and properties, in someone’s view may also possess a sort of aesthetical appeal (see Family Album). At least, this opinion is usually shared by those who synthesized the molecules.

 

 

Research Projects

à Crystallographic characterization of peptides based on non-coded amino acids

à Crystal structure of peptide antibiotics and related compounds

à Crystallographic characterization of peptide-based molecular scaffolds and supramolecular constructs

 

   Marco Crisma

Bio Organic Chemistry group

Number of peptide crystal structures per year in the Cambridge Crystallographic Database.

RED: with at least one Ca-tetrasubstituted a-amino acid residue in the sequence

GREEN: without any Ca-tetrasubstituted a-amino acid