Two series of combinatorial tripeptide libraries were constructed, based on an antioxidative peptide isolated from a soybean protein hydrolysate. One was a library of 108 peptides containing either His or Tyr residues. Another was a library of 114 peptides related to Pro-His-His, which had been identified as an active core of the antioxidative peptide. The antioxidative properties of these libraries were examined by several methods, such as the antioxidative activity against the peroxidation of linoleic acid, the reducing activity, the radical scavenging activity, and the peroxynitrite scavenging activity. Two Tyr-containg tripeptides showed higher activities than those of two His-containing tripeptides in the peroxidation of linoleic acid. Tyr-His-Tyr showed a strong synergistic effects with phenolic antioxidants. However, the tripeptide had only marginal reducing activity and a moderate peroxynitrite scavenging activity. Cysteine-containing tripeptides showed the strong peroxynitrite scavenging activity. Change of either the N-terminus or C-terminus of Pro-His-His to other amino acid residues did not significantly alter their antioxidative activity. Tripeptides containing Trp or Tyr residues at the C-terminus had strong radical scavenging activities, but very weak peroxynitrite scavenging activity. The present results allow us to understand why protein digests have such a variety of antioxidative properties.

Recent research from a variety of fields has revealed numerous biological roles for glycoconjugates. In order to investigate the functions of such molecules, extensive efforts have been directed toward the synthesis of natural and modified structures.2 To this end, the efficiency of oligosaccharide synthesis has been improved dramatically due to the development of new glycosylation reactions using a wide range of leaving groups and mild activating conditions. A trend utilizing the concept of chemoselective glycosylation has emerged.3 Such strategies take advantage of the differential chemical reactivities of glycosyl donors which can be controlled by protecting groups (ether-vs ester-type) and leaving groups.The most straightforward method among these is the direct use of glycosylation products as donors for the next coupling reaction, thereby negating the need for additional steps in the further manipulation of the anomeric center after each g l y~~~y l a t i o n .~~-~~~~~~J~~~~~~~-~~~ However, the length of the resulting sugar chain has been limited by the number of available leaving groups andor protecting groups.To overcome the limitation of existing strategies, we investigated the possibility of using two sets of chemically distinct (orthogonal) glycosyl donors and activation conditions (Scheme 1). The criteria for this concept to be practical are that (1) X should be unaffected under condition b required to activate the other donor (i.e., Y), and vice versa, and (2) both X and Y should remain compatible with subsequent manipulations of temporary protecting groups. For this orthogonal strategy, we (activalmr X) I A c , e o ycondition (activates ( b ) V)after nglycasylatiin cycles selected the phenylthio group for X and fluoride for Y as the leaving groups, and NIS-TfOH (or AgOTf)4,5 (condition a) and Cp2HfC12-AgC1046 (condition b) as promoters, respectively. For an initial attempt to demonstrate the feasibility of the strategy, N-phthaloyl (Phth) protected glucosamine (GlcN) derivatives were chosen as the monosaccharide units. This decision was based solely on the assumption that any stereochemical ambiguity could be eliminated by the strong 1,2-trans directing nature of the NPhth g r~u p .~~,~ However, it is to be stressed that the basic principle should be applicable to a wide variety of oligosaccharide structures. In addition, the biological significance of p-1,4 linked oligomers of glucosamine (e.g., chitin) is well recognized.* Also, the hydroxyl group at the C-4 position of GlcN is known to be relatively unreactive.2f Therefore, the construction of this type of oligosaccharide is a challenging task? The required GlcN derivatives 3-6 were synthesized according to the procedure described for closely related compounds.1°In order to assess the orthogonality of the above-mentioned combination of reactions, we examined glycosylations using 111J2 and 213J4 as donors. Thus, thioglycoside 1 was reacted (4) (a) Konradsson, P.; Mootoo, D. R.; McDevCt, R. E.; Fraser-Reid, B. J. Chem. SOC., Chem. Commun. 1990, 270-...

In view of the possible effects of the sphingoid base on protein kinases, and the fact that the sphingoid bases used in previous studies were not chemically well-defined, we have studied the effects of chemically well-defined sphingosines and their derivatives on kinase activity. Both (4E)-D- and (4E)-L-erythro-sphingenine showed a weak inhibitory effect, and (4E)-L-threo-sphingenine had a moderate inhibitory effect. In contrast, (4E)-N,N-dimethyl-D-erythro-sphingenine and the sphingosine preparation from a commercial source showed a strong inhibitory effect on PK-C in A431 cells as well as on purified PK-C. Synthetic (4E)-D-erythro-sphingenine and several samples of natural sphingosine inhibited v-src or c-src tyrosine kinase activity measured with polyglutamate-tyrosine (4:1) as substrate. N-Acetylated or N-methylated sphingosines did not inhibit src kinase activity, but rather produced a consistent 1.5-2-fold stimulation of such activity.

No abstract