Kinetic and conformational studies on some partially synthetic ribonuclease S' analogues modified in position 8.

Syntheses are described of two S-peptide analogues where the arginyl residue in position 10 has been replaced by ornithine and the phenylalanine in position 8 has been substituted by the unnatural amino acids cyclohexylalanine or p-fluorophenylalanine. In order to regenerate the arginyl residue, which is present in position 10 in the natural sequence, the S-peptide analogues belonging to the [Orn10]-series are transformed into the corresponding guanidinated derivatives by treatment with O-methylisourea. 1∈, 7∈, 10δ triguan-[Cha8, Orn10]-, 1∈, 7∈, 10δ-triguan-[pF-Phe8, Orn10]- and 1∈, 7∈, 10δ-triguan-[Tyr8, Orn10]-S-peptides were prepared. The ability to bind to and activate the S-protein of the synthetic S-peptide analogues, before and after guanidination, was tested by exploring their capacity to generate ribonuclease activity using RNA and C > p as substrates. The affinity of the different peptides for the S-protein in the absence of substrate was evaluated by difference spectroscopy.