Table 1.

Binding affinity of the studied compounds.

	Compound	IC50 ± S.E.M. [nM] 1
No.		μOR 2
1	TAPP	5.1 ± 3.5
Backbone Expansion without Changing the Spatial Positioning of the Side-Chain
8	[(R)-β2hTyr1]-TAPP*	77.6 ± 2.4
9	[(R)-β2h-m-Tyr1]-TAPP*	48.9 ± 2.3
10	[(S)-β2hAla2]-TAPP	> 1000
11	[(R)-β2hPhe3]-TAPP	95.4 ± 2.5
12	[(R)-β2hPhe4]-TAPP	1.9 ± 2.4
Backbone Expansion with Changing the Spatial Positioning of the Side-Chain
13	[(S)-β2hTyr1]-TAPP *	338.8 ± 5.1
14	[(S)-β2h-m-Tyr1]-TAPP*	11.2 ± 6.3
15	[(R)-β2hAla2]-TAPP	954.9 ± 2.9
16	[(S)-β2hPhe3]-TAPP	15.5 ± 2.5
17	[(S)-β2hPhe4]-TAPP	7.8 ± 4.0

¹ IC₅₀ ± S.E.M. Half-maximal inhibitory concentration ± standard error of the mean. Mean of three determinations in duplicate. ² Radioligand: 0.5 nM [³H]DAMGO.