Polypeptides spin-lattice relaxation times

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

C Nmr investigations of proteins and/or polypeptide systems can be classified into three general categories (1) 1 3C chemical shifts and spin lattice relaxation times of native and specifically labelled or 13C-enriched proteins and subunits (2) helix-coil transitions of polypeptides and (3) binding studies, protein-substrate interactions and complex formation. 

Stoichiometric complexes of poly(L-lysine) cations and dodecyl sulfate anions were investigated by Ponomarenko et al. [51,52]. The complexes are prepared by the same method used in the preparation of poly(y,L-glutamic acid)-alkyltrimethylammonium complexes. The secondary stmcture adopted by the polypeptide in these complexes was found to be very susceptible to environmental conditions. In chloroform solution containing 1 to 2% (v/v) TFA the polypeptide chain is in the a-helical con formation whereas a disordered state is present at TFA concentrations above 5%. It was interesting to observe that a noticeable decrease in the spin-lattice relaxation time accompanied the helix-coil... 

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