The effect of sterically demanding groups at proline residues on the conformational stability of the collagen triple helix was examined. The thermal stabilities (T-m and Delta G) of eight different triple helices derived from collagen model peptides with (4R)- or (4S)-configured amidoprolines bearing either methyl or bulkier tertbutyl groups in the Xaa or Yaa position were determined and served as a relative measure for the conformational stability of the corresponding collagen triple helices. The results show that sterically demanding substituents are tolerated in the collagen triple helix when they are attached to (4R)-configured amidoprolines in the Xaa position or to (4S)-configured amidoprolines in the Yaa position. Structural studies in which the preferred conformation of (4R)- or (4S)-configured amidoproline were overlaid with the Pro and Hyp residues within a crystal structure of collagen revealed that the sterically demanding groups point to the outside of these two triple helices and thereby do not interfere with the formation of the triple helix. In all of the other examined collagen derivatives with lower stability of the triple helices, the acetyl or pivaloyl residues point toward the inside of the triple helix and clash with a residue of the neighboring strand. The results also revealed that unfavorable steric dispositions affect the conformational stability of the collagen triple helix more than unfavorable ring puckers of the proline residues. The results are useful for the design of functionalized collagen based materials.