Conformational Preferences of RNase A C-Peptide Derivatives Containing a Highly Constrained Analogue of Phenylalanine

Abstract

Both enantiomers of a highly constrained derivative of phenylalanine, FiFi, were prepared in optically pure form. Studies were performed to elucidate the effects of substituting this amino acid for phenylalanine in RN-24, a derivative of the RNase A C-peptide. Thus RN-24, and the analogues 9 and 10, in which Phe-8 was replaced by each of the enantiomers of FiFi, were prepared. Comparative circular dichroism (CD) experiments indicated relative tendencies to adopt helical structures, and variable-temperature CD studies showed the relative ease with which these conformations were lost at elevated temperatures. These observations were rationalized via computer-assisted molecular modeling, which showed that the phenyl groups of (R,R)-FiFi in the peptidomimetic 9 can be accommodated via distortion of the helical conformations and that such distorted conformations persist as the temperature is increased. Conversely, intolerable contacts occur in an analogous conformation of the (S,S)-FiFi peptidomimetic 10 and these preclude helicity. Consistent with these observations, molecular dynamics studies of these peptidomimetics at 276 K indicate that helical conformations of 9 and RN-24 are observable under conditions in which the analogous conformation of 10 is lost. Overall, these studies demonstrate that cyclopropane amino acids can be used to enforce elements of secondary structure (albeit distorted) or to preclude them altogether.

Preferred 3D‐Structure of Peptides Rich in a Severely Conformationally Restricted Cyclopropane Analogue of Phenylalanine

“…Furthermore, the results of our solution conformational analysis, taken together with those extracted from the crystal-state study, also reported here, confirm earlier preliminary findings [20,[22][23][24]28] that c 3 diPhe has the ability to conform well to an ideal b-turn or a 3 10 /ahelix. This general 3D-structural tendency parallels those reported earlier for the prototypical Aib residue, Ac 3 c (I) and other side-chain cyclized, C a -tetrasubstituted a-amino acids with a larger ring size.…”

“…[28] It is worth noting that c 3 diPhe represents one of the first amino acids investigated so far which lacks asymmetry in the backbone (on the a-carbon atom) but possesses side-chain asymmetry (on each of the two bcarbon atoms). Previous examples include the atropoisomeric binaphthyl amino acid Bin, described by some of us (F.F.…”

“…[26,27] Interestingly, a single (R,R)-c 3 diPhe residue, positioned in the middle of an N-and C-blocked, all-S, 13-mer peptide, was shown to enforce helicity. [28] Conversely, (S,S)-c 3 diPhe precludes helicity in the diastereomeric peptide.…”

Cyclopropane Amino Acids That Mimic Twoχ1-Conformations of Phenylalanine

“…[25] In that synthesis, chirality was introduced by means of an asymmetric bishydroxylation [26,27] of trans-1,2-diphenylethene; the optically active diol produced was converted to a cyclopropane and then to the desired cyclopropane amino acid. The same approach would not be applicable to Boc-DiFi because bishydroxylation of 1,1-diphenylethene does not generate chirality.…”

“…The reaction mixture was concentrated after 30 min yielding a thick oil. The crude product was purified by flash chromatography with a gradient of 25 (3): Boc-cyclo-Phe-NHiPr was prepared as described above for Boc-Phe-NHiPr, but with (2S,3S)-Boc-cyclo-Phe-OH [29] (300 mg, 1.08 mmol) as the substrate and increasing the reaction time to 8 h to give 263 mg (64 %) of Boc-cyclo-Phe-NHiPr as a white powder. (5): Boc-FiFi-NHiPr was prepared as an intermediate by the following procedure.…”