Determination of statherin N-terminal peptide conformation on hydroxyapatite crystals

Proteins play an important role in inorganic crystal engineering during the development and growth of hard tissues such as bone and teeth. Although ma ny of these proteins have been studied in the liquid state, there is little direct information describing molecular recognition at the protein-crystal interface. Here we have used C-13 solid-state NMR (SSNMR) techniques to in vestigate the conformation of an N-terminal peptide of salivary statherin b oth free and adsorbed on hydroxyapatite (HAP) crystals. The torsion angle p hi was determined at three positions along the backbone of the phosphorylat ed N-terminal 15 amino acid peptide fragment (DpSpSEEKFLRRIGRFG) by measuri ng distances between the backbone carbonyls carbons in the indicated adjace nt amino acids using dipolar recoupling with a windowless sequence (DRAWS). Global secondary structure was determined by measuring the dipolar couplin g between the C-13 backbone carbonyl and the backbone N-15 in the i --> i 4 residues (DpSpSEEKFLRRIGRFG) using rotational echo double resonance (RED OR). Peptides singly labeled at amino acids pS(3), L-8, and G(12) were used for relaxation and line width measurements. The peptides adsorbed to the H AP surface have an average phi of -85 degrees at the N-terminus (pSpS), -60 degrees in the middle (FL) and -73 degrees near the C-terminus (IG). The a verage phi angle measured at the pSpS position and the observed high confor mational dispersion suggest a random coil conformation at this position. Ho wever, the FL position displays an average phi that indicates significant a lpha-helical content, and the long time points in the DRAWS experiment fit best to a relatively narrow distribution of phi that falls within the prote in data bank alpha-helical conformational space. REDOR measurements confirm the presence of helical content, where the distance across the LG hydrogen bond of the adsorbed peptide has been found to be 5.0 Angstrom. The phi an gle measured at the IG position falls at the upper end of the protein data bank alpha-helical distribution, with a best fit to a relatively broad phi distribution that is consistent with a distribution of alpha-helix and more extended backbone conformation. These results thus support a structural mo del where the N-terminus is disordered, potentially to maximize interaction s between the HAP surface and the negatively charged side chains found in t his region, the middle portion is largely alpha-helical, and the C-terminus has a more extended conformation (or a mixture of helix and extended confo rmations).