C-13 Spin-Relaxation Study of Some Amino Acids, Their Phospho-Derivatives and Some Small Peptides

Abstract

Carbon C-13 spin-lattice relaxation times, T1, of Serine, Threonine, Phosphoserine and Phosphothreonine have been measured in D₂O at neutral pD. Changes in over-all motions under phosphorylation has been detected. Segmental motions are not affected by phosphorylation. With the aid of T1(C-13) values of other amino acids an empirical relationship between NT1 and molecular weights (Mw) is tested that seems valid also for small peptides. Such relationship allows to interpret results from T1 measurements with the aid of a simple spherical model.

In the course of a NMR study on amino acids Ser and Thr and their corresponding phospho derivatives, phosphoserine (PSer) and phosphothreonine (PThr) (1,2) the biological significance of which has been recently confirmed (3–5), we detected an influence of the phospho group on the over-all conformation of the side-chain.

Aim of the present letter is to investigate by the aid of T1(C-13) relaxation time the possibility of the phospho group to act like an ‘anchor’ on the whole molecule with a consequent decrease of over-all molecular rotational diffusion and/or segmental motions. By the aid of NT1(Cα) values of other amino acids we look also for a relationship that relates molecular weights (Kw) and experimental NT1 values similar to the one already found in smaller cyclic peptides (6).

Present T1(C-13) relaxation study on PSer, PThr, Ser and Thr in natural abundance in D₂O (0.5 M) was performed using the inversion-recovery method on a Varian XL-200-FT spectrometer.