H and c NMR study of phosphopeptides : I—Acetylphosphoserine and acetylphosphothreonine

The ¹H and ¹³C NMR spectra of acetylserine, acetylphosphoserine and acetylphosphothreonine have been measured and completely analysed as a function of pD. For pD> 4 the NMR parameters are only slightly pD dependent. The second titration step of the phosphate group (pK₂ = 6.5) is much more difficult to detect in AcPSer (AcPTher) than in PSer (PThr). The ionization of the phosphate is of minor direct influence on the NMR parameters, their comparatively much larger changes in PSer (PThr) are due to conformational changes caused by the electrostatic interaction between the amino and phosphate group. The conformation in which the atoms H-C-Cβ-O-P form a planar W-type arrangement predominates at all pD values in the studied region from 4 to 14 in contrast to the non-cetylated phosphoamino acids in which its percentage passes a minimum at pD 8.