Langmuir 1995,11, 1675-1679
1675
Monolayer Formation and Molecular Orientation of Various Helical Peptides at the Air/Water Interface Katsuhiko Fujita, Shunsaku Kimura, and Yukio Imanishi" Division of Material Chemistry, Faculty of Engineering, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto, Japan 606-01
Emiko Okamura and Junzo Umemura Institute for Chemical Research, Kyoto University, Gokasho, Uji, Japan 611 Received September 19, 1994. I n Final Form: February 1, 1995@ Ten kinds of helical peptides were synthesized to study interaction between helices in the peptide monolayer formed at the airlwater interface. Five of those have a repeating sequence of Ala-Aib and different end groups, other two a repeating sequence of Lys(Z)-Aib,and the rest a repeating sequence of Leu-Aib. All of the peptides take helical conformation in ethanol as confirmed by CD spectroscopy. The surface pressure-area isotherms of the peptide monolayers showed an inflection at the surface area corresponding to the cross section along the helix axis, where the monolayers are considered to collapse. The monolayers were transferred onto substrates and subjectedto IR transmission and reflection absorption spectroscopy. By comparison of amide I and I1 absorbances, the peptides seemed to lie on the substrates. It was concluded that the peptides in the monolayer orient their helical axis parallel to the airlwater interface.
Introduction Hydrophobic helical peptides are one of the secondary units constructing the core region ofglobular or membrane proteins.1,2 Assembly of helices frequently occurs in natural protein^.^,^ Studies on the interaction between helices and factors governing the molecular packing of helices are important in understanding the formation of tertiary structure and/or the stability of protein struct ~ r e . The ~ , ~monolayer technique is a useful means for the studies, because it detects intermolecular forces operating in a two-dimensional array of molecules and the pressure-area (n-A)isotherm provides information on the molecular p a ~ k i n g . ~ , ~ We have studied the monolayer properties of the hydrophobic helical peptides spread a t the airlwater interface. The helical hexadecapeptide, Boc-(Ala-Aib)8OMe (BAlGM), forms a monolayer a t the interface with a parallel orientation of the helix axis to the i n t e r f a ~ e , ~ which is revealed by the comparison of the n-A isotherm with the molecular structure determined by X-ray analysis.l0 The monolayers of BA16M and some of its derivatives with different terminal groups showed a phase transition from a liquid state to a solid state as the surface pressure i n c r e a ~ e d . ~ JThe l fluorescence microscopic investigation revealed that these molecules form a twoAbstract published in Advance ACS Abstracts, April 15,1995. (1)Fermi, G.; Perutz, M. F.Atlas ofMolecular Structures in Biology. 2. Haemoglobin and Myoglobin; Clarendon Press: Oxford, 1981. (2) Michel, H.J . Mol. Biol. 1982,158,567. (3)Branden, C.; Tooze, J. Introduction to Protein Structure; Garland: New York, 1991. (4) Alberts, B.; Bray, D.; Lewis, J.;Raff, M.; Roberts, K.; Watson, J. D. Molecular Biology of the Cell, 2nd ed.; Garland Publishing: New York and London, 1989;Chapter 3. ( 5 ) Rao, S. T.; Rossmann, M. G. J . Mol. Biol. 1973,76,241. (6)Ho,S.P.; DeGrado, W. F. J . A m . Chem. SOC.1987,109,6751. (7) Gaines, G. L., Jr. Insoluble Monolayers ut liquid interface; Interscience: New York, 1966. (8)Birdi, K.S. Lipid and biopolymer monolayers at liquid interfaces; Plenum Press: New York, 1989. (9)Fujita, K.;Kimura, S.; Imanishi, Y.; Rump, E.; Ringsdorf, H. Langmuir 1994,10, 2731. (10)Otoda, K.;Kitagawa, Y.; Kimura, S.;Imanishi, Y. Biopolymers ~1993,33,1337. (11)Fujita, K.; Kimura, S.; Imanishi, Y.; Rump, E.; Ringsdorf, H. J. A m . Chem. SOC.1994,116,2185. @
0743-746319512411-1675$09.00/0
dimensional crystal a t the airlwater interface and the morphology of the monolayers is different according to the end group of the helical peptide.12 In the present study, ten kinds ofhelical peptides (Figure 1)were newly synthesized to study interaction between helices in the peptide monolayer formed at the airlwater interface. All of the peptides have 2-aminoisobutyric acid (Aib)residue. This residue is effectiveto induce the helical conformation, because of the steric hindrance around the a-carbon atom ofAib residues makes other conformations difficultto O C C U ~ . ~ Five ~ J ~ ofthose peptides have a common repeating sequence of Ala-Aib and various end groups having different hydrophobicities, two have a repeating sequence of Lys(Z)-Aib,and the rest a repeating sequence of Leu-Aib. The modification of terminal groups was designed so as to provide a primary amphiphilicity for the helix, where the hydrophilic and hydrophobic regions are segregated along the primary structure. Some biologically active peptide hormones possess a primary amphiphilicity to promote distribution to phospholipid bilayer membrane with a parallel orientation to the bilayer normal.15J6The orientation of hydrophobic peptides spread a t the airlwater interface may be changed by the primary amphiphilicity. In this respect, the ammonium groups in HAlGFB, HKZlGM, and HL16B and the trisarcosine sequence in HS3L16B a t the N-terminal should be effective to give a primary amphiphilicity. These peptides were spread a t the airlwater interface to measure their n-A isotherms and transferred onto suitable substrates for spectroscopic analysis.
Materials and Methods Abbreviations. Aib, 2-aminoisobutyric acid; AdmAlGB, C ~ O H I ~ C O - ( A ~ ~ - A ~ ~ ) ~AdmAlGOH, -OCH~C~H CloH15CO-(Ala~; Aib)s-OH;BAlGM, Boc-(Ala-Aib)s-OCHs;BKZlGM, Boc-(Lys(Z)Aib)s-OCHa;BLlGB, Boc-(Leu-Aib)s-OCH&eH5;BMAlGB, Boc(12)Fujita, K.;Kimura, S.; Imanishi, Y.; Rump, E.; Ringsdorf, H. Langmuir 1995,11, 253. (13)Burgess, A. W.; Leach, S. J. Biopolymers 1973,12, 2599. (14)Karle, I. L.; Balaram, P. Biochemistry 1990,29,6747. (15)Schwyzer, R.; Erne, D.; Rolka, K. Helv. Chim. Acta 1986,69, 1789. (16) Erne, D.; Sargent,F.; Schwyzer,R. Biochemistry 1985,24,4261.
0 1995 American Chemical Society
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1676 Langmuir, Vol. 11, No. 5, 1995
to the C terminal of Boc-(Ala-Aib)T-OHwith DCC and HOBt, followed by an LH20 column and recrystallization from methanol to obtain BA16FB. HA16FB was prepared by removing Boc group from BA16FB with TFA, followed by recrystallization from F F methanol. BA16FB: TLC RPA) 0.46,RAB) 0.50. HA16FB: TLC RPA) 0.11, RPB) 0.10. B o c - ( A I a - A i b ) g o G F BA16FB . . BKZ16M and HKZ16M. BKZ16M was prepared by the same TFA H - ( L e u - A i b g o m HL16B procedure as BA16M from Boc-Lys(Z) and Aib-OMe.lO The TFA H - ( A I P - A i b ~ o ~HA'6FB F r r recrystallization was carried out in a methanoVisopropy1 ether Boc-McAla-Pro-MeAla-Aib-(Ala-Aib~~ HCI H-Sar3.(Lcu.AibMoa HS3L16B (1:l (v/v)) mixture. BKZ16M was treated with TFA to obtain BMA16B HKZ16M. BKZ16M: TLC RAA) 0.59, RAB) 0.91. HKZ16M: TLC RPA) 0.38, RPB) 0.42. Figure 1. Molecular structures ofhelical peptides synthesized in t h e present study. BLlGB, HLlGB, and HS3L16B. BL16B was prepared by the same procedure as Boc-(Ala-Aib)s-OBzlwith minor modifications. BL16B was treated with TFA to remove Boc group to MeAla-Pro-MeAla-Aib-(Ala-Aib)s-OCH~C6H5; Boc, tert-butylobtain HL16B. Boc-Sar3-OH was synthesized by a stepwise oxycarbonyl; CD, circular dichroism; DCC, dicyclohexylcarboelongation as reported before.22 HL16B was connected to Bocdiimide; HKZlGM, TFA H-(Lys(Z)-Aib)8-0CH3; HLlGB, Sara-OH, followed by purification by an LH20 column to obtain TFA H-(Leu-Aib)8-OCHzC,jH5;HOBt, 1-hydroxybenzotriazole; HS3L16B, TFA H - S ~ ~ ~ - ( L ~ U - A ~ ~ ) MeAla,N-methyl~ - O C H Z C ~ H ~ ; BS3L16B. BS3L16B was treated with TFA to obtain HS3L16B. BL16B: TLC RAA) 0.50, RPB) 0.89; HL16B: TLC RAA) 0.30, L-alanine; RAS, reflection absorption spectroscopy; Sar, sarcosine RAB) 0.62; HS3L16B: TLC RAA) 0.38, RPB) 0.65. (N-methylglycine); TFA, trifluoroacetic acid; x-A isotherm, Method. BLlGB, HLlGB, and HS3L16B were dissolved in surface pressure-area isotherm. chloroform and other peptide samples were dissolved in chloPeptide Preparation. Boc amino acids were purchased from roform/methanol(9/1(v/v)) mixture at concentrations of (1.5-3) Kokusan Chemical Works, Ltd., Japan. HOBt, DCC, and TFA x M. The n-A isotherm was recorded at a constant rate were purchased from Peptide Institute, Inc., Japan. All the or reducing area with a Langmuir trough having the surface peptides used in the present study were prepared by the area of 900 em2. The peptide solution was spread on the aqueous conventional liquid-phase synthesis using as coupling reagents phase by using a microsyringe, and equilibrated for 15min before DCC and HOBt.I7 Thin-layer chromatography (TLC) was compression. Double-distilled water was used for the subphase. performed on silica-gel plate 60 F254,0.2 mm thick (Merk),with For IR transmission and reflection absorption spectra (RAS),23324 the solvent system (A) chloroforndmethanol/aceticacid (90:10:3 the monolayer was transferred onto a CaF2 plate (2.5 cm (v/v/v)) or (B) ethyl acetate/methanol(85:15 (v/v)). Peptides on diameter) or a slide glass coated with silver (2 x 5 cm) a t a the plate were visualized by sprayingninhydrine18 oro-t01idine.l~ constant surface pressure. The transfer rates were nearly unity. Benzyl ester of Aib was prepared from Aib (Tokyo Chemical The IR spectra were recorded by a Nicolet Model 710 Fourier Industry Co. Ltd., Tokyo) and p-toluenesulfonic acid (Tos-OH) transform infrared spectrophotometer equipped with an MCT monohydrate suspended in a benzyl alcoholitoluene (1:3 (v/v)) detector with resolution of 4 cm-l. For FL4 measurements, a mixture. Tos-OH Aib-OCHzC&~(Tos Aib-OBzl) was obtained Harrick Model RMA-1DGNRA reflection attachment was used as a white crystal after the conventional synthetic procedure.20 a t an incident angle of 85",with a Hitachi wire-grid polarizer. AdmA16B and AdmA16OH. Boc-(Ala-Aib)s-OBzlwas synCircular dichroism (CD) spectra of peptides in ethanoliwater thesized by the method reported p r e v i o ~ s l y .The ~ benzyl ester (9515 (v/v)) and of the films transferred onto quartz plates (1.2 was removed by the catalytic hydrogenation in a 2-methyl-lx 2.5 cm) were measured at room temperature on a JASCO 5-600 propanol solution with palladium carbon. The Boc group was CD spectropolarimeter. An optical cell of 0.2 cm path length removed by treatment with trifluoroacetic acid (TFA). TFA was used for the peptide solution at the concentration of 1.5 x H-(Ala-Aib)B-OBzl or TFA H-(Ala-Aib)e-OH was dissolved in M, and six pieces of the plates were fixed in a cell holder pyridine and reacted with 5-fold mole of 1-adamantancarbonyl in series with an interval of 1mm thick spacer. The intensity chloride (Tokyo Kasei, Japan) a t room temperature for 24 h, of the Cotton effect was normalized for the mole number of followed by gel filtration with a Sephadex LH-20 (Pharmacia, peptides in a n unit area. Sweden) column by using methanol as eluent. The major fraction was collected and concentrated, followed by recrystallization from methanol to obtain AdmA16B or AdmAlGOH, respectively: Results AdmA16B TLC RAA) 0.60, RAB) 0.81; AdmA16OH TLC RAA) CD spectra of AdmAlGB, BMAlGB, BKZlGM, and 0.43, RAB) 0.58. BL16B in a n ethanouwater (95/5 (v/v))mixture are shown BMA16B. tert-Butvloxvcarbonvl N-methvl-L-alanine (BocMeAla) was synthesized from Boc-ka and ci-hI.21 Boc-MeAla in Figure 2. All the peptides possess a double-minimum and Aib-OBzl were coupled in a dimethylformamide solution, pattern of spectrum that is characteristic of a-helical followed by purification with a silica-gel column. The product structure.25 The helix content of the peptides was about dipeptide was treated with 4 N HC1 in a 1,4-dioxane solution to 70%,and the helical structure was not seriously influenced remove the Boc group. Boc-Pro was connected to the free N by modification of the N or C terminal. terminal ofthe dipeptide to obtain a tripeptide. After purification The n-A isotherms of AdmAlGB, AdmAlGOH, and by a n LH20 column and removal of the Boc group, Boc-MeAla BMA16B a t 20 "C are shown in Figure 3, and those of was connected to the N terminal of the tripeptide. The tetBA16FB and HA16FB in Figure 4. The inflection point, rapeptide was purified by an LH20 column. The tetrapeptide which represents the maximum value of the area elastic was hydrogenated as described above to obtain Boc-MeAla-ProMeAla-Aib-OH (BM40H). BM40H was coupled with TFA modulus, -A(dn/dA), appeared in the n-A isotherms H-(Ala-Aib)e-OBzl with DCC and HOBt. After purifying by a n around 260 A2/molecule(Figures 3 and 4). The irregular LH20 column, BMA16B was recrystallized from a methanoli bumping and the decreasing of the surface pressure a t a isopropyl ether (3:l (v/v)) mixture. TLC RLA) 0.65, RAB) 0.85. constant surface area were observed a t smaller areas than BA16FB and HA16FB. Aib-OCH&Fs (Aib-FB) was synthat a t the inflection point, indicating a collapse of the thesized from Aib and pentafluorobenzyl alcohol (PFB) (Tokyo monolayer. The area a t the inflection point of the n-A Kasei, Japan) using p-toluenesulfonic acid as a catalyst. Bocisotherms is very close $0 the sectional area of BA16M Ala was coupled with Aib-FB to obtain a dipeptide. The dipeptide along the helix axis (239 A2),according to X-ray analysis.1° was treated with 4 N HCl in a 1,4-dioxane solution and connected (17)Konig, W.; Geiger, R. Chem. Ber. 1970,103, 788. (18)Pataki, G. Techniques ofThin-layer Chromatography in Amino Acid and Peptide Chemistry;Ann Arbor Sci. Inc.: Ann Arbor, MI, 1968. (19)Pataki, G.J. Chromatogr. 1963,12, 541. (20)Izumiya, N.;Makisumi, S.Nippon Kuguku Zusshi 1967,78,662. (21) Olsen, R. K. J. Org. Chem. 1970,35, 1912.
(22)Sugihara, T.; Imanishi, Y . ;Higashimura, T. Biopolymers 1975, 14, 723.
(23) Greenler, R. G. J. Chem. Phys. 1966,44, 310. (24)Okamura, E.;Umemura, J.;Takenaka, T. Can. J. Chem. 1991, 69,1691. (25)Holzwarth, G.; Doty, P. J.Am. Chem. Soc. 1966,87, 218.
Langmuir, Vol. 11, No. 5, 1995 1677
Helical Peptide Monolayer
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200 400 600 Molecular area (Azimolecule) Figure 4. Surface pressure-area isotherms of BA16FB (-) andHA16FB (- - -)ondouble-distilledwater at 20 "C. Amound is indicated with an arrow. -
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h
E 2 30 E 2
v
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g20 a, 0
-
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7 0 -
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Molecular area (A2imolecule) Figure 5. Surface pressure-area isotherms of BKZ16M (-) and HKZ16M (- - -) on double-distilled water at 20 "C.
200 400 6 00 Molecular area (Az/molecule) Figure 3. The surface pressure-area isotherms of AdmA16B (-), AdmA16OH (- - -), and BMA16B (- -) on double-distilled water at 20 "C. A mound is indicated with an arrow. This observation indicates that the five peptides take the a-helical structure at the aidwater interface with a parallel orientation to the interface. The n-A isotherms of BA16FB and HA16FB (Figure 4) are quite similar to those of Boc-(Ala-Aib)s-OBzland TFA H-(Ala-Aib)8-OBzl,12respectively. Substitution of the benzyl group with the pentafluorobenzyl group did not affect the molecular orientation and packing of the monolayer. The E-A isotherms of BKZ16M and HKZ16M (Figure 5) show a quite clear inflection around 430 AVmolecule. This inflection represents a collapse of the monolayer, because of the surface pressure bumps irregularly and decreases when the surface area is kept a t a constant value which is smaller than that a t the inflection point. The inflection point of BLIGB, HLlGB, and HS3L16B appears around 310-320 A2/molecule in the isotherm
0 0
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200 400 600 Molecular area (A2/molecule) Figure 6. Surface pressure-area isotherms of BL16B (- - -1, HBL16B (-), and HS3L16B (- -) on double-distilledwater at 20 "C. A mound is indicated with an arrow. (Figure 6), which is also ascribed to a collapse of the monolayer. These values of the surface area represent the sectional area ofeach helix, indicating that the peptides
1678 Langmuir, Vol. 11, No. 5, 1995
Fujita et al.
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-?
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Wavelength (nm)
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Figure 7. CD spectra of AdmA16B (- - -1, AdmA16OH (- -1,
BKZ16M (- -), and BL16B (-1 monolayer transferred onto quartz plates at surface pressures smaller by 2 mN/m than those at inflection points. take a parallel orientation to the interface irrespective of the nature of end groups and amino acid residues. Amound is observed in the x-A isotherms ofAdmAlGB, BAlGFB, HLlGB, and HS3L16B a t the initial part of the sharp increase of surface pressure (indicated by arrow in Figures 3 , 4 , and 6). The mound was observed similarly in the case of BA16M and its d e r i v a t i v e ~ . ~ItJ was ~ shown by a fluorescence microscopy that the densely packed solid domain appeared a t the top of the mound.lZ The mound is, therefore, explained in terms of phase transition of the monolayer from a liquid state to a solid state. Although any mound was not observed in the isotherm of BLlGB, the steep slope indicates the occurrence of a solid state of the monolayer. On the other hand, monolayers of AdmAlGOH, BMAlGB, HAlGFB, BKZlGM, and HKZ16M are considered to stay in a liquid state, because the slope of the isotherm is gentle and the collapse pressure is relatively low. The monolayers were transferred onto quartz plates a t a constant surface pressure which is smaller by 2 mN/m than that a t the inflection point, and CD spectra of the monolayers were measured (Figure 7). The intensity of the Cotton effect a t 222 nm changed by 15%upon rotation of the quartz plates, probably due to linear dichroism.26 CD spectra are the same among the peptides, AdmAlGB, AdmAlGOH, BKZlGM, and BLlGB, showing a-helical c o n f o r m a t i ~ n .No ~ ~significant difference was observed in the CD spectra between AdmA16B in a solid state and AdmA16OH in a liquid state, indicating that the different monolayer states between AdmA16B andAdmA16OH are not due to conformational difference. RAS and IR transmission spectra of AdmA16B are shown in Figure 8. The monolayers were transferred onto Ag and CaFz plates, respectively, a t 17 mN/m. The absorbance ratio of amide I band a t 1660 cm-l and amide I1 band a t 1550 cm-l was significantly different in the two spectra. Notably, amide I1 band a t 1550cm-l was stronger than amide I band a t 1660 cm-l in the RAS. This result is explained in terms of orientation of peptide molecules on the substrates. The absorption having a transition moment perpendicular to the optical path axis, which is parallel to the substrate surface, appears strongly in the transmission spectrum. On the other hand, the absorption having a transition moment perpendicular to the substrate surface absorbs the p-polarized light, which produces a n elictric field parallel to the incidence plane, intensively in the RAS (Chart 1, The transition moment of amide (26) Shindo, Y.; Ohmi, Y. J . Am. Chem. SOC.1985,107, 91.
I
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Wavenumber / cm-1 Figure 8. RAS and IR transmission spectra of AdmA16B
monolayer transferred onto a slide glass coated with silver and a CaFz plate, respectively, at a surface pressure of 17 mN/m. Incident angle of RAS was 85". Chart 1. Illustration of Polarized Light with the Monolayer Plane in the RAS and Transmittance IR Measurements (Top) and the Transition Moments of Amide I and Amide I1 Bands in a Helical Peptide (Bottom) p-polarized light
,+ I
+$-,
s-polarized light
RAS
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transmittance
/7 the transition moment of amide I1
the transition moment of amide I
I absorption lies appreciably in parallel to the helix axis and that of amide I1 absorption lies perpendicular to the helix axis (Chart 1,bottom).27 Consequently, the helical peptide is concluded to have the helix axis oriented perpendicularly to the normal of substrate and, therefore, in parallel to the aidwater interface. The monolayers of BKZ16M and BL16B were also transferred onto a CaFz plate a t 11 and 18 mN/m, respectively, which are smaller by 2 mN/m that those at the inflection point, and IR transmission spectra were measured (Figure 9). The absorption of amide I band is strong, and the absorption wavenumber is the same as that of AdmA16B and is characteristic of helical structure.2s Therefore, BKZ16M and BL16B are also concluded to take the a-helical structure at the interface having their helix axis oriented parallel to the interface. Discussion The n-A isotherms of the peptides spread on water are classified into following three groups: (i) a steep slope, BL16B; (ii) a steep slope with inflection (or a mound) a t the initial part of the sharp increase of surface pressure, ~~
(27) Krimm, S.; Bandeker, J. Adu. Protein Chem. 1986,38, 181. (28) Kennedy, D. F.; Chrisma, M.; Toniolo, C.; Chapman, D. Biochemistry 1991,30, 6541.
Helical Peptide Monolayer
Langmuir, Vol. 11, No. 5, 1995 1679
AdmA16B
BKZ16M
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Wavenumber / cm-1 Figure 9. IR transmission spectra of AdmAlGB, BKZlGM, and BL16B monolayer transferred onto a CaFz plate at surface pressures smaller by 2 mN/m than those at inflection points. AdmAlGB, BAlGFB, HLlGB, and HS3L16B; (iii) a gentle slope with a relatively low collapse pressure, AdmAlGOH, BMAlGB, HAlGFB, BKZlGM, and HKZ16M. The monolayers of class i and iii are in a solid and a liquid state, respectively, and in class ii monolayers are the phase transition from a liquid state to a solid state is observed upon compression. Since all the peptides take a-helical structure in the monolayer, the different behaviors of the isotherm are due to different molecular structures of the peptides. The isotherm of BL16B is similar to that of stearic acid,29 indicating that BL16B is easily packed into a twodimensional crystal. The cross section of the peptide a t the inflection point was compared with that of the molecular model.30 According to the theoretical model under the assumption that the side chains take a fully extended conformation, the longest distance from the center of the helix to the edge of the &methyl group of Leu residue and that from the center to the edge of the ,&methyl group of Aib residue is estimated to be 7.6 and 5.5 A, respectively. The length of the helix along the helix axis should be equivalent to that of BA16M (25.7 because both have the same secondary stTucture. The molecular area a t the inflection point (310 A2/molecule)or the area
A),
(29) Kajiyama, T.; Umemura, K.; Uchida, M.; Oishi, Y.; Takei, R. Bull. Chem. SOC.Jpn. 1989, 62, 3004. (30) (a) Momany, F.; McGuire, R. F.; Burgess, A. W.; Sheraga, H. A. J . Phys. Chem. 1975, 79, 2361. (b) Sisido, M. Private communication.
extrapolated to zero pressure of the steep slope (340 Azl molecule) in the n-A isotherm is in good agreement with the product ofthe helix length by the helix diameter taking the Leu and the Aib 9s the surface of one side of the helix and the other (13.1 A), and is significantly smaller than the product of the helix length by the helix diameter takiFg the Leu residue as the surface of the entire helix (15.2A). It is, therefore, indicated that the side chains of Leu residue of a helix penetrate into a groove of a n other helix, so that a n interdigitation state appears. The molecular packing of BL16B should be so tight due to interdigitation that a solid monolayer of BL16B is formed. A n ammonium group a t the N terminal of HL16B and HS3L16B might disturb the molecular packing of the peptide. However, this effect is overcome by the interdigitation effect upon compression, and the phase transition of the monolayer takes place. On the other hand, the cross s e c t i p a t the inflection point of BKZ16M and HKZ16M (430 AVmolecule),which was estimated from the CPK model, indicates that the side chains of Lys(Z)residue lie on the surface of the helix rod without interdigitation. The bulky side chains of Lys(Z) residue might sterically hinder the molecular packing, resulting in a liquid monolayer. AdmA16OH and BMA16B did not form a solid monolayer, although BA16OH and BA16B formed it with accompanying phase t r a n ~ i t i o n . AdmA16OH ~ has a hydrophobic adamantyl group a t the N terminal and a hydrophilic carboxyl group a t the C terminal. This structure is regarded a s a primary amphiphilicity, whose property is strengthen in AdmA16OH more than BA16OH due to the large end group. The peptides with a primary amphiphilicity tend to orient themselves parallel to the normal of lipid bilayer.31 As a consequence, the helix axis of AdmA16OH takes a n oblique orientation to the air1 water interface, resulting in a liquid monolayer. In the case of BMAlGB, three residues a t the N terminal have no amide protons, and the carbonyl groups form intramolecular hydrogen bonds with amide protons of the following residues. Therefore, the N terminal part of the peptide should be favorably located a t the air phase, resulting in a n oblique orientation of the helix against the airlwater interface and the liquid state monolayer. On the other hand, BA16B has free amide protons and carbonyl groups a t the N and the C terminal, respectively, which cause hydration of both termini of the helix, leading to the parallel orientation to the interface and formation of the solid-state monolayer. It is expected that peptides having a high degree of primary amphiphilicity will orient themselves perpendicularly to the interface. LA940749L (31) Otoda, K.; Kimura, S.; Imanishi, Y. Biochim.Biophys.Actu1993, 1150,1.
