Spectroscopic analysis of [Trp3]-beta-casomorphin analogs. Comparative structure conformation-activity studies

A series of [3-tryptophan]-beta-casomorphin-5([Trp3]-beta-CM-5) analogs were investigated by circular dichroism (CD) and fluorescence spectroscopy to explore their structure-conformation properties in solution. In addition, the comparative opioid activities of these compounds were evaluated using the in vitro guinea pig ileum (GPI) and mouse vas deferens (MVD) assays. Specifically, the pentapeptide sequence of [Trp3]-beta-CM-5, H-Tyr-Pro-Trp-Pro-Gly-OH (I) was modified at Pro-2 and Pro-4 by D-Pro substitutions to provide two diastereometric analogs, [Trp3-D-Pro-4]-beta-CM-5 (II) and [D-Pro2,4,Trp3]-beta-CM-5 (III). In the GPI and MVD assays, beta-CM-5 effected IC50 values of 1.3 microM and 8.9 microM, respectively, which confirmed its known mu/delta-selectivity on these two peripheral opioid receptor subtypes. The potencies of compounds I, II, and III were 0.2, 2.0, and less than 0.005 relative to beta-CM-5 on the GPI assay. Compounds I and II exhibited pronounced mu/delta-selectivities (greater than 18.9- and 12.4-fold respectively), whereas compound III was essentially inactive in both the GPI and MVD assays. CD studies of beta-CM-5 and its [Trp3]-beta-CM-5 analogs showed striking differences in their near-UV and far-UV spectra in aqueous or organic solvents. In the far UV CD spectra, weak (20%) alpha-helicity (maximum at 193 nm and minima at 208 and 222 nm) for beta-CM-5 was obtained in trifluoroethanol (TFE); however, none of the [Trp3]-beta-CM-5 analogs showed such CD bands. Of potential relevance to gamma-turn or C7 secondary structure was the observation of a strong negative band at 245 nm for compounds II and III which was not solvent-dependent in H2O or TFE, whereas compound I showed this CD band exclusively in TFE. In the near-UV CD at 275 nm (Trp electronic transition), the relative order of intensities of this band were determined for the [Trp3]-beta-CM-5 compounds to be II greater than I greater than III, which was identical to their relative biological potencies in both the GPI and MVD assays. Fluorescence energy transfer (FET) experiments of compounds I-III provided the intramolecular distances (r) between their Tyr (donor) to Trp (acceptor) side-chains, by the Förster method, and were as follows: [Trp3]-beta-CM-5, r = 10.6 A; [Trp3, D-Pro4]-beta-CM-5, r = 9.6 A; and [D-Pro2,4,Trp3]-beta-CM-5, r = 11.0 A.(ABSTRACT TRUNCATED AT 400 WORDS)