Conformational study of a salivary proline-rich protein repeat sequence

Salivary proline-rich proteins have a repetitive primary structure particularly rich in the amino acids proline, glutamine and glycine. One of the biological roles of these proteins is to bind and precipitate polyphenols (vegetable tannins) present in the diet (e.g. tea, coffee, fruit, chocolate) neutralising their harmful actions which include nutritional loss, inhibition of gut enzymes and oesophageal cancer. Two peptides overlapping in sequence, corresponding to the mouse salivary proline-rich protein MP5 repeat sequence: QGPPPQGGPQQRPPQPGNQ and GPQQRPPQPGNQQGPPPQGGPQ have been synthesised and studied in H2O/(2H6)dimethyl sulphoxide (9:1, by vol.) using 1H-NMR spectroscopy. Low-temperature far-ultraviolet CD spectroscopy and NMR conformational parameters indicate that the peptides adopt an extended random coil conformation in solution. There is no evidence for a defined polyproline type II helix in the peptides, despite the high proline content. NMR data show that the trans-proline isomer predominates to at least 90%.