β-Peptides: Ac

Dec 29, 2009 - Patrick S. Walsh , Ryoji Kusaka , Evan G. Buchanan , William H. James III , Brian F. Fisher , Samuel H. Gellman , and Timothy S. Zwier...
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J. Phys. Chem. A 2010, 114, 1581–1591

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Laser Spectroscopy of Conformationally Constrained r/β-Peptides: Ac-ACPC-Phe-NHMe and Ac-Phe-ACPC-NHMe† William H. James III,‡ Esteban E. Baquero,‡,§ Soo Hyuk Choi,|,⊥ Samuel H. Gellman,*,| and Timothy S. Zwier*,‡ Department of Chemistry, Purdue UniVersity, 560 OVal DriVe, West Lafayette, Indiana 47907-2084, and Department of Chemistry, UniVersity of Wisconsin, Madison, Wisconsin 53706 ReceiVed: September 21, 2009; ReVised Manuscript ReceiVed: NoVember 17, 2009

Single-conformation ultraviolet and infrared spectra have been recorded under the isolated molecule conditions of a supersonic expansion for three conformationally constrained R/β-peptides, Ac-L-Phe-ACPC-NHMe (rLβACPC), Ac-ACPC-L-Phe-NHMe (βACPCrL), and Ac-ACPC-D-Phe-NHMe (βACPCrD). These three molecules are close analogues of the hAla-containing R/β-peptide counterparts Ac-L-Phe-β3-hAla-NHMe, Ac-β3-hAlaL-Phe-NHMe, and Ac-β3-hAla-D-Phe-NHMe, which have been studied recently by James et al. (J. Am. Chem. Soc. 2009, 131, 6574). Incorporation of the β-amino acid trans-2-aminocyclopentanecarboxylic acid (ACPC) constrains the β-peptide backbone via the cyclopentane ring, producing clear changes in the conformational preferences relative to the unconstrained analogues. The conformational control is manifested most obviously in the complete absence of C6 H-bonded rings, which were dominant in the unconstrained R/β-peptides. The most stable C6 ring structure (C6a) in the absence of the ACPC ring cannot be formed in its presence, while a secondary C6 ring (C6b) has its energy destabilized by ∼20 kJ/mol. In rLβACPC, the preference for C5 structures in the N-terminal position, combined with the strong preference for C8 structures in the β-peptide subunit, leads to the observation of two C5/C8 bifurcated double ring conformers. Both C8/C7 sequential double rings and C11 single rings are observed in βACPCrL and βACPCrD. Here, the ACPC ring selectively stabilizes the C8a ring over other possible C8 structures. Finally, the combined evidence from IR and UV spectra lead to tentative assignments for diastereomeric pairs, exhibiting small but understandable shifts in the IR and UV spectra induced by the change in chirality at the R-peptide chiral center. I. Introduction Protein function frequently depends upon the conformation adopted by the polypeptide backbone. Recent years have seen many efforts to extrapolate from the biological precedent to create new types of oligomers that display conformational and ultimately functional similarities to proteins.1-8 Systems containing β-amino acid residues, either exclusively (“β-peptides”) or in combination with R-residues (“R/β-peptides”), have received a great deal of attention in this regard.2,3,9-12 As the study of these new “foldamers” has matured, it has become increasingly important to acquire fundamental insight regarding the conformational propensities of the new backbones. The work reported here is motivated by this goal. The protein building blocks, R-amino acids, are highly flexible. One attraction of β-amino acids as foldamer constituents is the prospect of introducing distinct conformational bias at the level of individual residues, which can lead to strong and specific folding propensities in β- and R/β-peptides.2,13 This goal has been commonly pursued via incorporation of the CR-Cβ bond of a β-amino acid into a ring, as illustrated by trans-2aminocyclopentanecarboxylic acid (ACPC) (Scheme 1).2,3,9,14 This type of cyclic β-residue retains the H-bond donor (NsH) †

Part of the “W. Carl Lineberger Festschrift”. * To whom correspondence should be addressed. Purdue University. § Present address: Process Analytical, The Dow Chemical Company, 2301 N. Brazosport Blvd. B-1463, Freeport, TX 77541. | University of Wisconsin. ⊥ Present address: Department of Chemistry, Northwestern University, Evanston, IL 60208-3113. ‡

SCHEME 1: Chemical Structures of the β-Amino Acid trans-2-Aminocyclopentanecarboxylic Acid (ACPC) (Left) and r-Amino Acid Proline

and acceptor (CdO) sites along the backbone, which is important because the most common regular secondary structures in proteins and in peptidic foldamers are associated with specific backbone H-bonding patterns. Use of cyclic β-residues can strongly enhance helical folding among both β- and R/βpeptides, relative to analogues containing acyclic β-residues.2,14-16 In contrast, the only proteinogenic R-amino acid residue with a cyclic constraint, proline (Scheme 1), lacks an H-bond donor site when incorporated into a polypeptide chain, and proline residues are well-known to destabilize R-helical secondary structure relative to other R-residues.17-19 Extensive structural analysis shows that R/β-peptides containing various proportions and patterns of R- and β-residues can adopt a number of helical conformations with distinctive intramolecular H-bonding patterns.2,9,14,20 Crystallographic data indicate two helices to be particularly common, one containing CdO(i) · · · H-N(i + 3) H-bonds and the other containing CdO(i) · · · H-N(i + 4) H-bonds.9 Both of these helical secondary structures are promoted by use of ACPC in β-residue positions. These two backbone H-bond patterns are observed

10.1021/jp9090975  2010 American Chemical Society Published on Web 12/29/2009

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SCHEME 2: Unconstrained r/β-Peptides Ac-β3-hAla-L-Phe-NHMe (βrL), Ac-β3-hAla-D-Phe-NHMe (βrD), and Ac-L-Phe-β3-hAla-NHMe (rβL) Studied in Ref 12

among R-peptides as well, where they correspond, respectively, to the 310-helix and the R-helix. Among pure β-peptides a helix containing CdO(i) · · · H-N(i + 3) H-bonds (“12-helix”) is strongly favored by ACPC residues.15,21 Conformational analysis of R/β-peptides via crystallography or two-dimensional NMR has provided considerable insight regarding structural preferences, but a great deal of complementary information can be obtained by investigating the inherent local conformational preferences of small R/β-peptides isolated in the gas phase. Information gleaned from such analysis is not affected by solvation or crystal packing effects. Recently, we have examined a series of three di-R/β-peptides (Scheme 2) under isolated molecule conditions cooled in a supersonic expansion, using intramolecular amide-amide H-bonds, detected via infrared spectroscopy, as structural reporters.12 The three molecules included a diastereomeric pair, Ac-β3-hAla-LPhe-NHMe (βrL) and Ac-β3-hAla-D-Phe-NHMe (βrD), and a close analogue of the former in which the order of the R- and β-subunits was switched (Ac-L-Phe-β3-hAla-NHMe, rLβ). The aromatic side chain of the phenylalanine residues enabled us to use resonant two-photon ionization (R2PI) spectroscopy to record mass-resolved ultraviolet spectra. Conformation-specific UV and IR spectra were then recorded using ultraviolet hole burning (UVHB) and resonant ion-dip infrared (RIDIR) doubleresonance spectroscopies, respectively. Comparison of the resulting conformation-specific spectra with the predictions of quantum chemical calculations led to firm assignments for the H-bonding “architectures” of the conformational isomers. The possible structures could be divided into conformational families containing different H-bonded rings of varying size and type, including single rings (e.g., C11), sequential double rings (e.g., C8/C7eq or C6/C5), or bifurcated double rings (e.g., C5/C8). Representative examples of such structures relevant to the present study are shown in Figure 1. In our study of βrL, βrD, and rLβ a total of 18 conformational isomers were observed, six for each di-R/β-peptide.12 Among them were representative examples of several conformational families, for which unique spectral signatures could be deduced. In βrL and its diastereomer βrD, C8/C7eq sequential double-ring structures were highly populated under jet-cooled conditions. Three sets of βrL/βrD diastereomeric conformational pairs were identified in which the peptide backbone remained the same, but the chirality change could be compensated for by reorienting the phenyl ring (that is, a T g- for the chromophore position in Figure 1) leading to near-identical UV and IR spectra. There were clear energetic differences involving some conformational families observed for βrL and βrD; for example, a C6/C5 double-ring was highly populated in βrL, while a C11

Figure 1. Top: Chemical structures of the three cyclically constrained R/β-peptides considered in this paper Ac-L-Phe-ACPC-NHMe (rLβACPC), Ac-ACPC-L-Phe-NHMe (βACPCrL), and Ac-ACPC-D-PheNHMe (βACPCrD). Middle: Representative optimized geometries at the DFT M05-2X/6-31+G(d) level of theory of the experimentally observed conformational families. H-bonds are identified with red, dashed lines. Bottom (Left): Chemical structure of the ACPC residue with dihedral angles labeled. The dihedral angle values for the three ACPC ring puckering geometries identified (p-, p+, p′), and representative optimized geometries at the DFT M05-2X/6-31+G(d) level of theory for the dominant ring puckering pair (p-/p+). Bottom (Right): The three possible positions for the aromatic chromophore anti, gauche +, and gauche - (a, g+, g-).

structure was preferred in βrD. When the order of the R- and β-residues was swapped (rβL), C5/C8 and C5/C6 conformations were favored over the C7eq/C8 alternatives, apparently because of stabilization provided by an NH · · · π interaction made possible by the C5 ring. We concluded that, as in the corresponding R-peptides,22-24 the presence of a Phe side chain can dictate the conformational preferences of the H-bonded rings involving this subunit, directing the formation of C5 structures when Phe occurs at the N-terminus but C7eq structures when Phe occurs elsewhere in the peptide chain. Here, we report on the conformational preferences of the constrained R/β-peptides Ac-ACPC-L-Phe-NHMe (βACPCrL), its diastereomer Ac-ACPC-D-Phe-NHMe (βACPCrD) and Ac-L-PheACPC-NHMe (rLβACPC) (Figure 1, top panel). These three methyl-capped di-R/β-peptides are the analogues of the previously studied βrL, βrD, and rLβ, respectively, in which the flexible β3-hAla residues have been replaced by a constrained (S,S)-ACPC residue. Expansion-cooled, isolated molecule conditions have been employed for these studies, using the same conformation-specific spectroscopic probes previously used.12 As we shall see, the conformational preferences of the ACPCcontaining R/β-peptides differ from those of their flexible counterparts. Most notably, we find that β-residue rigidification removes C5/C6 and C6/C5 structures from the accessible conformational families. This change in the energy landscape leads to conformer populations dominated by C5/C8 bifurcated double-ring structures in rLβACPC, while C8/C7 sequential double-ring and C11 single-ring structures dominate in βACPCrL and βACPCrD. The ACPC ring displays two different types of puckering (Figure 1) that at times leads to a doubling of the number of conformations present, modulating the UV and IR spectra only slightly in the process.

Conformational Preferences of R/β-Peptides II. Methods A. Experimental Methods. The syntheses of the three constrained R/β-peptides were carried out via procedures analogous to those reported previously.12,21 Characterization data for each molecule is provided in the Supporting Information. The experimental methods used to record conformationspecific infrared and ultraviolet spectra have been described in detail elsewhere.12,25-27 Those details specific to the present study are included here. The solid peptide samples were introduced into the gas phase by heating the sample in a stainless steel sample holder fixed directly before a pulsed valve. Operating temperatures of approximately 240-250 °C were necessary to obtain sufficient vapor pressure. In order to reduce the effects of thermal decomposition, the solid sample was held in a glass insert inside the stainless steel holder in order to minimize contact with the metal walls of the sample holder. A pulsed valve with 400 µm orifice diameter (Parker General Valve, Series 9) was used in forming the supersonic expansion. The samples were entrained in a 70%/30% neon/helium mixture with a backing pressure of 1.5 bar and expanded into vacuum, with typical flow rates of 0.15-0.30 bar · cm3/s. Resonant two-photon ionization (R2PI) spectroscopy was utilized to record the ultraviolet spectra of the R/β-peptides in the S0-S1 region of the phenylalanine side chain in a mass selective fashion. The samples were excited with the frequency doubled output of a Nd:YAG pumped tunable dye laser. Typical ultraviolet laser energies of 0.1-0.5 mJ/pulse were used. The ultraviolet light traversed the ionization region of a time-offlight mass spectrometer as a collimated beam (∼1 mm diameter). The resultant ions were detected via a microchannel plate detector fixed atop a 1 m long flight tube. Ultraviolet hole-burning spectroscopy (UVHB) was used to obtain conformation-specific electronic spectra. The UVHB spectra were recorded using a high-power hole-burning laser (0.5-0.75 mJ/pulse, 10 Hz) fixed on a transition of interest in the R2PI spectrum, while a probe laser (0.1-0.5 mJ/pulse, 20 Hz) was tuned through the R2PI spectrum. The hole burn and probe lasers were counterpropagated, spatially overlapped, and temporally separated by 200 ns, with the hole-burn laser preceding the probe. The integrated ion signal from the probe laser was monitored using active baseline subtraction in a gated integrator to observe the difference in ion signal due to the probe laser with the hole-burn laser “on” vs “off”. Probe laser transitions arising from the same ground state level as the holeburn transition produced depletions in the ion signal. Conformation-specific infrared spectra were recorded using the IR/UV double-resonance scheme resonant ion-dip infrared spectroscopy (RIDIRS). Tunable infrared radiation in the amide NH stretch spectral range (3200-3500 cm-1, 3-5 mJ/pulse) was obtained using an injection-seeded Nd:YAG pumped parametric converter (LaserVision), while an AgGaSe2 crystal was employed to extend the tuning range of the parametric converter to study the amide I (primarily CdO stretch) spectral region (1640-1740 cm-1, 180-210 µJ/pulse). In a manner similar to UVHB, the IR (10 Hz) and UV probe (20 Hz) were counterpropagated, spatially overlapped, and temporally separated by 200 ns, with the IR pulse preceding the UV probe pulse. The UV probe was fixed on a transition of interest in the R2PI spectrum, typically an S0-S1 origin transition identified via UVHB. The IR wavelength was tuned while monitoring the ion signal from the UV probe laser. IR transitions from the same ground state level as the probed UV transition produced depletions in the monitored ion signal. A gated integrator

J. Phys. Chem. A, Vol. 114, No. 3, 2010 1583 operating in active baseline subtraction mode was utilized to record the conformation-specific IR spectra. B. Computational Methods. In order to identify low-lying conformational minima, we carried out an initial screening for possible conformational minima using the Amber force field28 within the MACROMODEL suite of programs.29 The random search of conformational space resulted in approximately 150 starting structures within a 50 kJ/mol energy window for each of the three molecules. The starting structures obtained from the MACROMODEL search were then used as input structures for further screening with density functional theory using the B3LYP functional30,31 and 6-31+G(d) basis set. The results of B3LYP calculations provided a basis for tentative conformational assignments through comparison with harmonic vibrational frequencies. Since standard DFT B3LYP calculations do not properly account for dispersion,32-34 the tentatively assigned structures, as well as a selected subset of other structures, were reoptimized using Truhlar’s M05-2X density functional35-37 with the 6-31+G(d) basis set. The options scf ) tight and an ultrafine gird were included with the M05-2X calculations. This functional was designed to better account for dispersive interactions; therefore, we refine our conformational assignments via comparison to M05-2X/6-31+G(d) harmonic vibrational frequencies and relative energies. A comparison of the vibrational frequencies and infrared intensities for a selected subset of the structures is included in Supporting Information.38 The two methods lead to the same conformational assignments. A subset of these calculations was carried out with the computational resources of GridChem.39,40 The GAUSSIAN 03 suite of programs was used for all calculations.41 III. Results A. Conformation-Specific Spectroscopy. 1. R2PI and UVHB Spectra. Figure 2a shows the R2PI and UVHB spectra of Ac-L-Phe-ACPC-NHMe (rLβACPC) in the S0-S1 origin region (37250-37600 cm-1) of the phenylalanine residue. The UVHB spectra reveal the presence of two conformational isomers in the supersonic expansion. The most intense transition, located at 37316 cm-1 (labeled A), was used as hole-burn transition for the UVHB spectrum shown below it. This spectrum accounted for most of the major transitions in the spectrum. However, close inspection of this spectrum with the R2PI spectrum above it identified several other minor transitions that did not burn with A. UVHB of the transition marked by an asterisk in spectrum B produced the UVHB spectrum shown there, with S0-S1 origin at 37366 cm-1. The three most intense transitions in the R2PI spectrum comprise a progression with 41 cm-1 spacing, all belonging to the dominant conformer A. The UVHB spectrum of conformer B also shows significant vibronic activity with a progression with 35 cm-1 spacing. This vibronic activity suggests an interaction in both conformers between the phenyl ring that serves as UV chromophore and the peptide backbone. The analogous R2PI and UVHB spectra of Ac-ACPC-L-PheNHMe (βACPCrL) are shown in Figure 2b. The R2PI spectrum (top trace) shows one dominant transition, labeled A, at 37588 cm-1, with minor transitions located at both higher and lower wavenumbers. From the UVHB spectra (bottom traces), three conformational isomers were identified, labeled A-C. All three spectra show a single transition assigned as the S0-S1 origin of each conformer. The strong ∆V ) 0 Franck-Condon activity reflects a small change in geometry upon electronic excitation. Transitions D and E did not burn out with any of the UVHB

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Figure 2. (a) Top trace: R2PI spectrum of rLβACPC. Lower traces: UVHB spectra of the two resolved conformers A and B of rLβACPC. (b) Top trace: R2PI spectrum of βACPCrL. Lower traces: UVHB spectra of the resolved conformers A, B, and D of βACPCrL. (c) Top trace: R2PI spectrum of βACPCrD. Lower traces: UVHB spectra of the resolved conformers I-VI of βACPCrD. Asterisks indicate the transition on which the HB laser was fixed.

TABLE 1: Experimental S0-S1 Origin and Amide NH Stretch Frequencies (cm-1) and R2PI Intensities for the Observed Conformers of βACPCrL, βACPCrD, and rLβACPCa conformer

intensity

H-bond family

specific structure

expt S1-S0 origin (cm-1)

expt NH stretches (cm-1)

A B C D E

strong weak weak weak weak

C8/C7 C11 C11 C8/C7 C8/C7

βACPC rL C8a/C7eq(g-,p+) C11(g+,p-) C11(g+,p+) unassigned C8a/C7eq(a)b

37588 37579 37575 37546 37618

3278, 3412, 3398, 3269, 3253,

3352, 3437, 3406, 3321, 3274,

3473 3466 3420, 3462 3463 3370, 3465

I II III IV V VI

strong strong medium weak weak weak

C8/C7 C8/C7 C8/C7 C8/C7 C11 C8/C7

βACPC rD C8a/C7eq(a,p-) C8a/C7eq(a,p+) C8a/C7eq(g-,p-) C8a/C7eq(g-)b C11(g+,p′) unassigned

37559 37564 37571 37588 37602 37632

3266, 3264, 3287, 3270, 3374, 3354,

3367, 3360, 3368, 3288, 3433, 3366,

3472 3463 3469 3343 3444 3463

A B

strong weak

C5/C8 C5/C8

rLβACPC C5/C8a(a,p+) C5/C8a(a,p-)

37316 37366

3381, 3441, 3454 3383, 3434, 3450

a Also included are the conformational family and, where possible, the specific structure assigned to each of the observed conformers, based on comparison with DFT M05-2X/6-31+G(d) calculations. b We hypothesize that there are two conformers present at the electronic origin positions of both conformers E and IV. The conformers are ascribed to p+/p- ACPC ring-puckering pairs. See text for further discussion.

spectra, suggesting them as S0-S1 origins of additional conformers. However, the intensity of these bands was too weak to record high-quality UVHB spectra of them. Their identification as additional conformers was made on the basis of their RIDIR spectra (section 2). The combined data from UVHB and RIDIR spectroscopy thus identifies five conformers of AcACPC-L-Phe-NHMe in the supersonic expansion. The R2PI spectrum of Ac-ACPC-D-Phe-NHMe (βACPCrD) in the S0-S1 origin region (37500-37700 cm-1) is shown in Figure 2c (top trace). This spectrum, unlike those of the other two molecules, is quite congested, with at least five transitions of significant intensity. The UVHB spectra (lower traces) dissects the R2PI spectrum into subspectra due to six conformational isomers, three of which contribute more than one transition in the R2PI spectrum indicative of a peptide backbone-phenyl ring interaction. The frequencies of the S0-S1 origins and other key spectroscopic data for the observed conformers of all three molecules are summarized in Table 1. 2. RIDIR Spectra in the NH Stretch Region. The black traces in Figure 3 present conformation-specific infrared spectra

of rLβACPC (Figure 3a), βACPCrL (Figure 3b), and βACPCrD (Figure 3c) in the amide NH stretch region (3250-3530 cm-1). Our recent study of the unconstrained R/β-peptide analogues, Ac-β3-hAla-L-Phe-NHMe (βrL), Ac-β3-hAla-D-Phe-NHMe (βrD), and Ac-L-Phe-β3-hAla-NHMe (rLβ) led to firm structural assignments for the dominant conformers.12 These results provide important points of comparison that can be used to obtain initial structural assignments for the conformers of the ACPC-constrained analogues of interest here: rLβACPC with rLβ, βACPCrL with βrL, and βACPCrD with βrD. As a result, we have reproduced these scans as the red traces in Figure 3 to facilitate this comparison. B. Ac-L-Phe-ACPC-NHMe (rLβACPC). The RIDIR spectra of conformers A and B of rLβACPC (Figure 3a) are very similar to, but distinct from, one another, confirming that they arise from separate conformers. In both spectra, a single broad, intense infrared transition appears near 3380 cm-1, with two weaker, narrow transitions above 3400 cm-1. This pattern of transitions is consistent with a structural motif with one strong, intramolecular hydrogen bonded NH and two free or weakly bound NH groups.

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Figure 3. Conformation-specific RIDIR spectra for rLβACPC (left, black traces), βACPCrL (middle, black traces), and βACPCrD (right, black traces). The red traces are representative spectra of the unconstrained R/β-peptides reproduced from ref 12. They are used here to tentatively assign the conformational families represented in the spectra of the ACPC-constrained analogues (see text).

Comparison with RIDIR spectra of the two conformers of rLβ (red traces) shows best correspondence with the spectrum assigned to the C5/C8 conformer (upper red trace). In rLβ, the spectrum of the C5/C6 conformer (lower red trace) was unique in having a free NH stretch near 3500 cm-1 ascribable to the terminal NHMe of the β-peptide subunit.26,27 This frequency is nearly 50 cm-1 higher than any present in rLβACPC. In C5/C8 conformers on the other hand (upper red trace), the NH group not involved in either the C5 or C8 rings is the central amide NH (Figure 1), immediately adjacent to a branched alkyl carbon that drops the free NH stretch frequency to near 3450 cm-1. This is close to those observed in rLβACPC. Furthermore, the H-bonded NH stretch (3381 in A, 3383 in B) is at a frequency consistent with formation of a C8 H-bond based on previous studies of β-peptides.12,26,27 On this basis, we tentatively assign the two observed conformers of rLβACPC to C5/C8 structures. C. Ac-ACPC-L-Phe-NHMe (βACPCrL) and Ac-ACPC-DPhe-NHMe (βACPCrD). Figure 3b shows the conformationspecific infrared spectra for the three conformers of βACPCrL resolved via UVHB spectroscopy (A-C). Two additional conformers, labeled D and E, were also identified through their unique RIDIR spectra, with intensity in R2PI too weak to hole burn. The corresponding RIDIR spectra of the six conformers of βACPCrD are shown in Figure 3c. The 11 spectra generally fall into one of two groups. βACPCrL(A, D, and E) and βACPCrD(I-IV and VI), all possess two or more broad, amide NH stretch fundamentals with frequencies shifted below 3400 cm-1 associated with strong H-bonded NH groups, and one narrow amide NH stretch fundamental near 3470 cm-1, due to a free NH. On the other hand, βACPCrL(B and C) and βACPCrD(V) all display a single relatively weak H-bonded NH stretch near 3400 cm-1 identified by its slight broadening and two narrow NH stretch fundamentals in the 3400-3450 cm-1 region associated with free or weakly bound NH groups. Once again, comparison with the spectra of selected conformers of βrL and βrD is instructive. In those cases, the observed conformers could be grouped into C8/C7 double-ring, C6/C5 double-ring, and C11 single-ring families, serving as likely candidates in the present work. Of these possibilities, the C8/

C7 double-ring structures had both H-bonded NH stretch fundamentals shifted below 3400 cm-1, with three representative examples shown as red traces in panels b and c of Figure 3. By comparison the C6/C5 double rings had H-bonded NH stretch fundamentals in the 3400-3450 cm-1 range (e.g., bottom red trace of Figure 3a). In fact, of the 12 observed conformers of βrL and βrD, only the C8/C7 structures had two H-bonds strong enough to shift both NH stretch fundamentals below 3400 cm-1. As a result, on this basis alone, we tentatively assign conformers A, D, and E of βACPCrL and I-IV and VI of βACPCrD to C8/ C7 sequential double-ring structures (Table 1). At the same time, the H-bonded NH stretch frequencies of the C8/C7 conformers vary quite considerably from one conformer to the next. In fact, in conformer IV, we have been unable to clearly detect a free amide NH stretch transition, but instead observe three resolved IR transitions below 3400 cm-1. As we shall see, it is not possible to form a structure with all three NH groups in strong H-bonds, pointing to the barely visible depletion at 3463 cm-1 in IV as the likely “missing” free amide NH stretch. As a result, the doublet present in E and IV near 3250 cm-1 must arise from another source, a point to which we will return later. Note that βACPCrD(VI) has a spectrum very similar to that of βrD(A′) (upper red trace), a conformer that was assigned to a C8/C7eq structure with an unusually weak C8 H-bond associated with an atypical C8 ring. The remaining three conformers, βACPCrL(B, C) and βACPCrD(V) have NH stretch spectral patterns very similar to one another, and closely analogous to βrD(B) (bottom red trace of Figure 3c, which was assigned previously to a C11 singlering structure. As a result, we assign the three conformers in this group as C11 structures. Finally, we note that the proposed tentative assignments are consistent with previous deductions12 regarding the dependence of the S0-S1 origin transition frequency on the local structure about the Phe side chain. In rLβACPC, the two C5/C8 structures both have S0-S1 origin transitions near 37300 cm-1, in keeping with the local C5 peptide backbone immediately surrounding the Phe side chain. At the same time, all 11 conformers of βACPCrL and βACPCrD, have S0-S1 origin transitions above 37500 cm-1, as anticipated for C8/C7 or C11 structures.

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Figure 4. Black traces: Experimental RIDIR spectra of rLβACPC in the amide NH spectral region. Stick spectra in red: The three best-fit sets of calculated harmonic vibrational frequencies (scaled 0.94) and infrared intensities, at the DFT M05-2X/6-31+G(d) level of theory. The relative energies are zero-point corrected energies for the indicated structures at the same level of theory.

D. Calculated Structures and Refined Conformational Assignments. Armed with these tentative assignments, we turn now to a more quantitative comparison with the calculated frequencies and infrared intensities of particular conformational isomers in order to see whether further refinements to the conformational assignments can be made. The fully optimized, low-energy structures for rLβACPC, βACPCrL, and βACPCrD could be divided into conformational families based on their peptide backbone structures. In rLβACPC, low-energy structures were dominated by members of the C5/ C8 conformational family, but representative examples of C7/ C8 and C11 structures were also present, with lowest energies 7 and 12.5 kJ/mol above the global minimum, respectively. In βACPCrL and βACPCrD, all low-energy structures in the first 15 kJ/mol are either C8/C7 double-ring or C11 single-ring structures. The most stable C8/C7 and C11 structures are nearly equal in energy in βACPCrL, while in the βACPCrD diastereomer, the C8/C7 structures are calculated to be more stable than the lowest-energy C11 conformation by about 8 kJ/mol. Notably absent are C6/C5 or C5/C6 structures, which were among the dominant conformers of βrL and rβL. The calculated minima show that the dominant C6 ring type, C6a, is not formed when the ACPC residue is employed. Minima corresponding to minor C6 ring types are possible, but structures that incorporate them have energies ∼20 kJ/mol above the global minimum, presumably due to steric effects imposed by the cyclopentane ring. Furthermore, while C5/C8 structures were important low-energy conformers in rLβACPC, the change in ordering of R- and β-residues produces C8/C5 structures that are high in energy, since they can only be formed by having two CdO groups accepting a H-bond from a single NH, which is strongly energetically disfavored. Close inspection of the conformations present indicate that in almost all cases, a given peptide backbone structure can be formed with two nearly isoenergetic ACPC conformations involving ring puckering at the δ carbon, as shown in Figure 1. These two ring puckering structures are labeled p+ or p- depending on the sign of χ2. In addition, as in the unconstrained analogues, the phenyl ring can take up any one of three positions relative to the peptide backbone, labeled (g+), (g-), or (a) to denote their position relative to the adjacent amide N (Figure 1).12,22 1. Ac-L-Phe-ACPC-NHMe (rLβACPC). Figure 4 compares the experimental RIDIR spectra (top traces) of the two conformers, A and B, of rLβACPC with scaled (0.94), harmonic vibrational frequencies and infrared intensities (lower traces, displayed as

James et al. sticks) for three low-energy C5/C8 conformers at the DFT M052X/6-31+G(d) level of theory. While all three computed C5/ C8 spectra reproduce the general pattern of NH stretch transitions, the relative intensities and spacing of the two higherfrequency NH stretch transitions are better matched by the top two stick spectra, which are also the lowest energy conformers at this level of theory. Both are C5/C8a(a) structures which differ only in the ring puckering coordinate (p+ vs p-, Figure 1). The different ring puckering geometries produce small shifts in the frequencies of all three NH stretch fundamentals, with a lower frequency C8 NH stretch associated with higher frequency NH · · · π NH stretch transition, and vica versa. These differences arise because the puckering of the cyclopentane ring modulates the position of the amide groups in the peptide backbone by small amounts (∆rH · · · O ) 0.08 Å). Note that the C5/C8 structures have, in addition to the two NH · · · OdC H-bonds, an NH · · · π interaction involving the central amide NH. This is one of the factors that stabilizes the C5/C8 bifurcated ring structure relative to C7/C8 or C11 structures. At the DFT M05-2X/6-31+G(d) level of theory, the lowest energy C7/C8 and C11 structures are 7 and 13 kJ/mol higher in energy, respectively. Finally, it is worth stressing again that C5/C6 conformations, which compete with C5/C8 structures in the unconstrained rLβ analogue, are not observed in rLβACPC due to a steric constraint imposed by the ACPC ring on C6 structures, strongly destabilizing them. In summary, the entire population of rLβACPC resides in a single C5/C8 peptide conformation (C5/C8a(a)) but is spread over the two ring-puckering structures. In the ultraviolet spectrum (Figure 2a), the ACPC ring puckering pair produce S0-S1 origins that differ in frequency by 50 cm-1, reflecting the presence of an NH · · · π interaction whose strength is modulated by the ring puckering (Table 1). Strong Franck-Condon activity in a low-frequency vibration of 41/35 cm-1 is also consistent with this NH · · · π interaction, which also shows up clearly in the amide NH stretch spectrum by shifting its fundamental below that of the C5 NH group to 3441/3434 cm-1. 2. Ac-ACPC-L-Phe-NHMe (βACPCrL). Figure 5 compares the experimental amide NH stretch spectra of βACPCrL with calculated stick spectra associated with the indicated low-energy C8/C7 (left, middle) and C11 (right) conformations. The close correspondence confirms the tentative assignments made in the preceding sections of conformers A, D, and E to C8/C7 and B/C to C11 structures. The six calculated C8/C7 structures come in three p+/p- ACPC pairs, all of which utilize the most stable C8a ring but differing in the type of C7 ring, with both C7ax and C7eq structures calculated to be low in energy. This is different than in βrL, where C7eq structures were energetically preferred.12 It is not clear whether this is a deficiency of the calculations or a real difference associated with the presence of the ACPC ring. The calculated spectra show differences in the C8 and C7 H-bonded NH stretch transitions that reflect subtle changes in the strength of the amide-amide H bonds in these sequential double-ring structures. However, the match between experiment and calculation is insufficient to make firm assignments to specific structures on that basis alone. The fact that there are three experimentally observed C8/C7 conformers but twice that number of plausible calculated spectra suggests that we are observing only one member of each p+/p- ACPC pair in each case. For conformer A, we have recorded a RIDIR spectrum in the amide I (primarily CdO stretch) spectral region, shown in Figure 6. While all candidate structures show a pattern of CdO stretch transitions consistent with experiment, the spacing and

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Figure 5. Black traces: Experimental RIDIR spectra of βACPCrL in the amide NH spectral region. Stick spectra in red: Best-fit sets of calculated harmonic vibrational frequencies (scaled 0.94) and infrared intensities, at the DFT M05-2X/6-31+G(d) level of theory. The relative energies are zero-point corrected energies for the indicated structures at the same level of theory.

doublet. The calculated splitting (16 cm-1), is close to that observed experimentally (20 cm-1). At the same time, the calculated splittings of the other amide NH stretch transitions are shifted by much smaller amounts (1 and 7 cm-1) that are not resolved in the experimental spectrum.

Figure 6. Black trace: Experimental RIDIR spectra of βACPCrL (A) in the amide I region (primarily CdO stretch). Stick spectra in red: Best-fit sets of calculated harmonic vibrational frequencies (scaled 0.96) and infrared intensities, at the DFT M05-2X/6-31+G(d) level of theory. The relative energies are zero-point corrected energies for the indicated structures at the same level of theory.

relative intensity pattern is best matched by the C8a/C7eq(g-, p-) structure at the bottom. As a result, we tentatively assign conformer A to the C8a/C7eq(g-, p-) structure. This would relegate conformer D to a C8a/C7ax structure with its axial C7 ring, consistent with the red shift in the S0-S1 origin position for this conformer (Figure 2). Evidence for the presence of p+/p- ACPC pairs is also apparent in the RIDIR spectrum of conformer E (Figure 5, middle), which shows a doublet near 3250 cm-1 split by about 20 cm-1. The calculations shown in the stick spectra below are those for the C8a/C7eq(a, p() pair, which differ most notably in the lowest frequency amide NH stretch, where the doublet appears in the experimental spectrum. We postulate, then, that the UV transition used as monitor transition in recording the RIDIR spectrum of conformer E has unresolved contributions from both members of the ACPC pair, producing the observed

Finally, the spectra of conformers B and C are also well reproduced by an ACPC ring puckering pair of C11(g+, p() structures. Each spectrum contains a single hydrogen bonded amide NH stretch located close to 3400 cm-1, a stabilizing NH · · · π interaction which lowers the frequency of the interior amide NH to approximately 3430 cm-1, and a free amide NH above 3460 cm-1, all of which are indicative of a C11 H-bonded ring. In order to achieve the amide NH · · · π interaction, the phenyl chromophore must be located in the (g+) orientation, thus ascribing both conformers to C11(g+) structural motifs. As Figure 2a shows, in the ultraviolet, these conformers have S0-S1 origins separated by only 4 cm-1, an amount commensurate with the anticipated small changes in the Phe environment induced by ACPC ring puckering in a C11 single ring structure. 3. Ac-ACPC-D-Phe-NHMe (βACPCrD). The top panel of Figure 7 compares the experimental RIDIR spectra for the six conformers of βACPCrD with calculated spectra consistent with them. As anticipated based on comparison with βrD, conformers I-IV and VI all are fit well as C8/C7 sequential double ring structures. Once again, it is difficult based on the NH stretch IR data alone to make a firm conformational assignment to a single structure. The two lowest energy C8/C7 conformers form an ACPC pair, and match well with spectra I-III. On the basis of the small separation (5 cm-1) in the S0-S1 origin positions in the UV (Figure 2c, Table 1), we tentatively assign conformers I and II to the lowest energy C8/C7eq(a, p() ACPC pair. The large difference in intensity of the two conformers in the R2PI spectrum would seem out of keeping with their close calculated energies, although relative intensities in the R2PI spectrum can sometimes be misleading.42 It is also possible that the barrier to interconversion of the ACPC ring puckering pair is small enough that cooling between them occurs readily in the expansion. The doublet at low frequency in the spectrum of

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Figure 7. Black traces: Experimental RIDIR spectra of βACPCrD in the amide NH spectral region. Stick spectra in red: Best-fit sets of calculated harmonic vibrational frequencies (scaled 0.94) and infrared intensities at the DFT M05-2X/6-31+G(d) level of theory. The relative energies are zero-point corrected energies for the indicated structures at the same level of theory.

conformer IV could again be accounted for as a second ACPC pair unresolved in the UV, differing in this lowest frequency NH stretch. The final three traces in the left-hand panel of Figure 7 display the experimental and two calculated best-fit spectra of conformer VI. The calculated spectra have a reduced splitting between the two H-bonded NH stretch fundamentals, indicative of weak C8 H-bond, very similar in appearance to that observed in the A/A′ diastereomeric pair of the unconstrained Ac-β3-hAla-PheNHMe.12 Both of the two best-fit structures take up C8 H-bonded rings (C8c and C8b′) that weaken the C8 H-bond and shift it to higher frequency, as observed. It is also noteworthy that in forming these unusual C8 ring, the ACPC ring must take on a higher-energy structure with dihedrals different from those in p+ or p-. For this reason, the structure labels in Figure 7 include a p′ designation. Both these structures are also higher in energy than the other C8/C7 structures by some 10-15 kJ/mol, consistent with the small intensity in the R2PI spectrum (Figure 2c). The right-hand panel of Figure 7 compares the RIDIR spectrum of conformer V with two best-fit calculated spectra, both of which are C11 structures, as anticipated based on comparison with βrD. The experimental spectrum is unusual in having a “free” amide NH stretch at 3444 cm-1, shifted almost 20 cm-1 below its frequency in the βACPCrL analogues. The structure that best accounts for this shift down in frequency is C11(g+, p′), with its unusual ACPC ring puckering geometry (p′). As a result, we tentatively assign conformer V to this structure. IV. Discussion In the present study, the single-conformation UV and IR spectral signatures of two conformers of rLβACPC, five of βACPCrL, and six of βACPCrD have been characterized. In rLβACPC, the ACPC ring locks in a single C5/C8 structure, with population residing in a pair of conformers differing only in ACPC ring puckering (C5/C8a(a, p()). Three of the five conformers of βACPCrL are C8/C7 sequential double-ring structures, two of which form a p+/p- ACPC ring-puckering pair, while the remaining two conformers have both been assigned to C11(g+, p() structures that also differ only in the

Figure 8. R2PI spectra of βACPCrL (top trace) and βACPCrD (lower trace). ACPC ring-puckering pairs are designated with green tie lines. Diastereomeric pairs are indicated by blue and red arrows, with the line color indicative of the direction of the frequency shift in S0-S1 origin position in going from βACPCrL to βACPCrD. See text for further discussion.

ACPC ring puckering coordinate. Finally, five of the six conformers of βACPCrD are C8/C7 double rings, with a single C11 structure as a minor conformer. A. Diastereomer-Specific Effects. One of the more striking conclusions of the present study can immediately be drawn from a cursory inspection of Figure 8, which compares the R2PI spectra of the βACPCrL and βACPCrD diastereomers. These spectra are quite unique, with little direct overlap in the main bands of the two diastereomers. Thus, the change of chirality at a single center within the backbone appears to cause a significant change in the conformational ensemble for these molecules that in turn produces changes in the local environment of the Phe side chain that shift the UV absorptions of the conformers. Our previous study of the diastereomers βrL and βrD revealed the presence of three diastereomeric pairs in which the two members of the pair, one from each diastereomer,

Conformational Preferences of R/β-Peptides showed nearly identical UV and IR spectra, indicating that the two had the same local environment for the Phe residue and the same intramolecular H-bonding in the peptide backbone.12 Other conformers did not show this spectroscopic pairing because the chirality change had a substantial effect on the relative energies and local environment of the Phe ring. In seeking to identify diastereomeric pairs, one must keep careful track of the Phe position. When the Phe ring is in the g+ position, the change in chirality of the R-amino acid (L vs D) retains the g+ phenyl ring position (Figure 1). However, when the phenyl ring is in the a or g- position, the chirality change must be compensated for by an interchange of these positions (a T g-) to retain the same Phe environment. Thus, diastereomeric pairs are formed by structures with the same peptide backbone that either retain the g+ position or swap the a and g- positions. Visual inspection of the UVHB scans of βACPCrL and βACPCrD shows no conformer pairs appearing at identical UV wavelengths, indicating that, in the presence of the ACPC ring, a change in chirality of the R-amino acid always produces a shift in the S0-S1 origin position of the phenyl ring. Nevertheless, the expectation is that diastereomeric pairs (i.e., similar conformations adopted by each of the two diastereomers) may still be present and we can be guided to their presence by close similarities in the NH stretch infrared spectra. Furthermore, if there are cases where assignments in βACPCrL and βACPCrD already involve a diastereomeric pair, they can help us decipher the kinds of spectral shifts to look for in the UV. The most straightforward diastereomeric pair to assign is that associated with the C11(g+) structures. In βACPCrL, conformers B and C are both C11(g+) structures differing in the ACPC ring puckering coordinate, while in βACPCrD, the single C11 structure (conformer V) is also assigned to C11(g+). Thus, this diastereomeric pair have S0-S1 origin positions shifted from one another by 23 cm-1, with βACPCrD blue shifted from its diastereomeric equivalent in βACPCrL. This blue shift arises because conformer V of βACPCrD experiences a stronger NH · · · π interaction than do conformers B/C of βACPCrL. It is well-known that such polar XH · · · π interactions blue shift the S0-S1 origin of the phenyl ring, as occurs in complexes of benzene with HCl (+125 cm-1)43 or H2O (+55 cm-1).44 This change in the strength of the NH · · · π interaction was noted previously in comparing the amide NH stretch spectra of βACPCrL and βACPCrD (section 3). Using similar arguments, βACPCrL(A) has already been tentatively assigned to C8a/C7eq(g-, p+) and conformer I of βACPCrD to its diastereomeric equivalent, C8a/C7eq(a, p-). As shown in Figure 8, these two are the most intense transitions in their respective R2PI spectra, again consistent with assignment as a diastereomeric pair. It remains, then, to rationalize the observed shift in the S0-S1 origin of βACPCrD(I) to the red of that from βACPCrL(A) by 29 cm-1. This shift can be attributed to an interaction of a CH of the ACPC cyclopentane ring with the π-cloud of the phenyl ring when the chromophore is in position (a), as it is in C8a/C7eq(a, p-) assigned to conformer I (Figure 9, right). This interaction is not possible in its diastereomeric equivalent, C8a/C7eq(g-, p+), assigned to βACPCrL(A) (Figure 9, left). A small red shift is anticipated for a phenyl ring interacting with a nonpolar alkyl CH group (e.g., the S0-S1 origin of the benzene-C2H6 complex is -47 cm-1 from benzene).45 Additional evidence supporting the assignment of the A/I diastereomeric pair is found in the vibronic spectra of the two conformers. The UVHB spectrum of βACPCrD(I) exhibits low-frequency vibronic activity due to a change in geometry

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Figure 9. Optimized geometries at the DFT M05-2X/6-31+G(d) level of theory for the βACPCrL(A)/βACPCrD(I) diastereomeric pair. The structure ascribed to conformer I of βACPCrD exhibits a CH-aromatic π-cloud interaction, while such an interaction is absent in the corresponding structure ascribed to conformer A of βACPCrL.

upon electronic excitation, which may be the result of the CH-π interaction, while the corresponding spectrum of βACPCrL(A) shows no such low-frequency vibronic activity, indicative of the absence of such an interaction. Finally, conformers E and IV have single-conformation IR spectra that are close analogues of one another, with an S0-S1 origin shift to the red in IV by 35 cm-1. Due to the small intensity of IV, we are unable to clearly detect the anticipated vibronic activity. Nevertheless, we make a tentative assignment of these two conformers to a C8/C7 diastereomeric pair (a T g-), despite being unable to assign the two to a specific C8/C7 conformation. B. Conformational Control via a Ring-Constrained Residue. A main motivation of the present study has been to understand in some detail the effects of introduction of the ACPC residue on the local conformational preferences of these model R/β-peptides. Here we return to this fundamental question, using the unconstrained R/β-peptides rLβ, βrL, and βrD as our primary points of comparison. Several deductions can be drawn from this comparison. First, the introduction of the ACPC ring retains and even strengthens certain conformational preferences, most notably in the C5/C8 structures that appear as dominant conformers in rLβ and rLβACPC, and the C8/C7 conformers that are in high abundance in βrL/βrD and βACPCrL/βACPCrD. The preference for C5 over C7 rings when the R-peptide unit is at the N-terminus is also retained,12,22 dictated in large measure by the added presence of an amide NH · · · π interaction made possible by the C5 R-peptide subunit. A second general deduction is that conformers involving a C6 H-bonded ring are entirely absent in the presence of the ACPC ring. This trend is perhaps the most obvious and important local structural effect of the ACPC ring, since in the unconstrained R/β-peptide analogues, C6/C5 and C5/C6 structures are among the lowest in energy,12 and in homogeneous β3-peptides, C6 single-ring and C6/C6 double-ring structures are in high abundance.26,27 Figure 10 shows close-up views of the C6a and C6b rings in an unconstrained Ala-containing β-peptide subunit (Figure 10, top) and compares this local conformation to the only stable C6 ring structures with the ACPC ring present (Figure 10, bottom). Note that the ACPC C6 structures are C6b structures, which are less stable than C6a in the absence of the ACPC ring, but are even higher in energy with ACPC present by some 20 kJ/mol. Furthermore, it is apparently not possible to form the C6a ring structure in the presence of the ACPC ring, even though this structure is the most stable C6 ring in the absence of ACPC.

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James et al. where the effects of ring puckering on both the NH H-bonds and or Phe side chain are small, transitions due to the two conformers may not be resolved. In certain cases, we have indirect evidence that both ACPC puckering conformers contribute to the IR spectrum, indicating that they are unresolved in the UV. Alternatively, it may be the case that the barrier to interconversion between the ACPC minima may be low enough that population is effectively funneled to the energetically favored minimum during expansion cooling. V. Conclusions

Figure 10. Six-atom H-bonded cycles (C6) involving an intramolecular amide-amide NH · · · OdC H-bond of a β-peptide subunit. Top: Optimized C6a and C6b H-bonded structures of Ac-β3-hAla-NHMe with the corresponding dihedral angles listed for ready comparison. Bottom: Optimized C6b H-bonded structure of Ac-ACPC-NHMe with the corresponding dihedral angles listed. The C6a H-bonded cycle cannot form in Ac-ACPC-NHMe due to steric constraints imposed by the cyclopentane ring (see text for further discussion).

A third general deduction is that the absence of C6 rings resulting from the ACPC subunit is accompanied by an increased presence of conformers belonging to the C11 family. The competition between C6/C5 and C11 conformers was hypothesized in our previous work on the unconstrained analogues,12 with βrL preferring C6/C5 over C11, and βrD the reverse. In the current study, the ACPC ring removes C6 structures entirely, thus promoting C11 conformers, which now appear with significant abundance in both βACPCrL and βACPCrD. A fourth general deduction is that besides preventing C6 formation, the ACPC residue also locks in a particular C8 conformation. As one might imagine, as the H-bonded ring grows in size, more conformational variants of the H-bonded ring become possible, with varying dihedral angles along the backbone atoms. In the unconstrained β- and R/β-peptides, at least four C8 ring types were identified,12,26,27 labeled C8a-C8d. In the ACPC-containing R/β-peptide analogues studied here, 10 of the 13 observed conformers contain a C8 ring. Nine of these 10 structures contain the C8a ring. This striking preference for the C8a ring type was predicted by the computational study of a capped ACPC diamide by Wu and Wang.46 A final general deduction is that while the major effect of incorporation of the ACPC residue is a restriction in the local preferences of the β-peptide subunit to favor C8a structures, the ACPC ring itself has inherent conformational flexibility in the ring-puckering coordinate (Figure 1). In the absence of asymmetry in the surroundings, the two out-of-plane puckering minima would be equivalent; however, the rest of the R/βpeptide backbone and Phe substituent produce an asymmetry, effectively doubling the number of conformers that can be formed. These two forms typically do not alter the local conformation of the peptide backbone significantly, and puckering therefore generates a pair of conformers that is nearly isoenergetic. However, in some cases the ring puckering alters the NH · · · OdC distance and as a result modulates the frequency associated with the C8 amide NH stretch. The small energetic differences computed for ACPC puckering pairs would lead to a hypothesis that both members should be observed; however, only three ACPC puckering pairs were clearly resolved. In cases

The single-conformation UV and IR spectra for 13 conformers of Ac-L-Phe-ACPC-NHMe (rLβACPC) Ac-ACPC-L-Phe-NHMe (βACPCrL), and Ac-ACPC-D-Phe-NHMe (βACPCrD) were obtained and analyzed. Clear preferences for the C5/C8 (2 conformers), C8/C7 (10 conformers), and C11 (3 conformers) conformational families were found, while C6 H-bonded rings were notably absent. Furthermore, the conformational control accompanying the inclusion of the ACPC residue places restrictions on the type of C8 ring formed (C8a). The rigidity of the ACPC ring also led to more subtle shifts in the UV and IR spectra, even for diastereomeric pairs, modulating the strength of NH · · · π and NH · · · OdC H-bonds in response to the chirality change at the R-amino acid. Acknowledgment. W.H.J., E.E.B., and T.S.Z. acknowledge support for this research from the National Science Foundation (NSF-CHE0909619). S.H.G. and S.H.C. were supported by NSF Grant CHE-0848847; in addition, S.H.C. was supported in part by a fellowship from the Samsung Scholarship Foundation. Supporting Information Available: NMR and mass spectral data used to characterize the synthesized samples and table containing relative energies and harmonic vibrational frequencies for comparison. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173. (2) Horne, W. S.; Gellman, S. H. Acc. Chem. Res. 2008, 41, 1399. (3) Hayen, A.; Schmitt, M. A.; Ngassa, F. N.; Thomasson, K. A.; Gellman, S. H. Angew. Chem., Int. Ed. 2004, 43, 505. (4) Cheng, R. P.; Gellman, S. H.; DeGrado, W. F. Chem. ReV. 2001, 101, 3219. (5) Hecht, S.; Huc, I. Foldamers: Structure, Properties, and Applications; Wiley-VCH: Weinheim, 2007. (6) Seebach, D.; Matthews, J. L. Chem. Commun. 1997, 2015. (7) Seebach, D.; Beck, A. K.; Bierbaum, D. J. Chem. BiodiVersity 2004, 1, 1111. (8) Seebach, D.; Hook, D. F.; Glattli, A. Biopolymers 2006, 84, 23. (9) Choi, S. H.; Guzei, I. A.; Spencer, L. C.; Gellman, S. H. J. Am. Chem. Soc. 2008, 130, 6544. (10) Epand, R. F.; Schmitt, M. A.; Gellman, S. H.; Sen, A.; Auger, M.; Hughes, D. W.; Epand, R. M. Mol. Membr. Biol. 2005, 22, 457. (11) Horne, W. S.; Boersma, M. D.; Windsor, M. A.; Gellman, S. H. Angew. Chem., Int. Ed. 2008, 47, 2853. (12) James, W. H.; Baquero, E. E.; Shubert, V. A.; Choi, S. H.; Gellman, S. H.; Zwier, T. S. J. Am. Chem. Soc. 2009, 131, 6574. (13) Schmitt, M. A.; Choi, S. H.; Guzei, I. A.; Gellman, S. H. J. Am. Chem. Soc. 2006, 128, 4538. (14) Horne, W. S.; Price, J. L.; Gellman, S. H. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9151. (15) Appella, D. H.; Christianson, L. A.; Klein, D. A.; Powell, D. R.; Huang, X. L.; Barchi, J. J.; Gellman, S. H. Nature 1997, 387, 381. (16) Rathore, N.; Gellman, S. H.; de Pablo, J. J. Biophys. J. 2006, 91, 3425. (17) Woolfson, D. N.; Williams, D. H. FEBS Lett. 1990, 277, 185. (18) Rothe, M.; Theysohn, R.; Steffen, K. D.; Schneide., H; Zamani, M.; Kostrzew., M. Angew. Chem., Int. Ed. 1969, 8, 919. (19) Boussard, G.; Marraud, M.; Aubry, A. Biopolymers 1979, 18, 1297.

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