The N-terminal 1-34 fragment of paratyroid hormone (PTH) is fully active in vitro and in vivo and it can reproduce all biological responses characteristic of the native intact PTH. Recently, helicity-enhancing substitutions in PTH(1-11) and PTH(1-14) have yielded potent analogues. To further investigate the role of a-helicity on biological potency, in the present work we synthesised and conformationally and biologically characterised seven PTH(1-11) analogues containing sterically hindered and helix-promoting Ca-tetrasubstituted amino acids, such as a-amino isobutyric acid (Aib), a-methyl Valine (aMeVal) and amino cyclopentane carboxylic acid (Ac5c). CD and NMR experiments and molecular dynamics calculations demonstrate that the substitution with Ca-tetrasubstituted amino acids led to the enhancement of the helical conformation. In TFE/water solutions, analogue VII, used as reference, adopts a stable a-helical segment spanning the sequence from Ile5 to His9. Analogues I - VI show a higher preference for the helical structure which comprises the sequence 2-9
